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Attached: Performance and selection of water-blocking filling grease for optical cables
With the continuous development and application of communication optical cables, the long-term reliability of optical cables has attracted more and more attention. During the long-term use of optical cables, due to the erosion of moisture and water, the transmission performance will decrease, affecting its normal use. In the production of optical cables, there are two main types of water-blocking materials for cable cores, one is water-blocking filling ointment, and the other is water-blocking tape and water-blocking yarn. During the production process, various water-blocking materials can be used to achieve the purpose of moisture-proofing and water-blocking according to the process and equipment conditions. As far as the current status of domestic optical cable production is concerned, the use of these two types of materials has become the two main means of filling and blocking water in the optical cable industry. Because optical cable water-blocking filling grease (hereinafter referred to as cable grease) can play a good role in waterproofing, moisture-proof, buffering and bonding during production, has good reliability and is easy to maintain, therefore, cable grease is widely used in optical cable production. . Cable paste is one of the key structural components in optical cables. In general optical cable structures, cable paste easily forms a tight layer with materials in contact with each other. Even if its impact is not seen in a short period of time, considering the mass transfer effect between materials, It will also have an impact over time. In addition to good physical-chemical stability, cable paste must also have good temperature stability, so its main performance indicators should be tested before selecting cable paste.
2. Relevant properties and selection principles of cable paste
The performance of cable paste is the basis for production, use and procurement. From the perspective of the use of optical cables, it has the following characteristics:
a. Moderate viscosity, strong thixotropy, and can be filled at room temperature;
b. Excellent temperature performance and long-term stability;
c. It has good compatibility with optical cable materials, low acid value, oxidation resistance, etc.;
d. Water-blocking cable paste that swells when exposed to water has fast water absorption speed, high water absorption rate and good water-blocking performance;
e. Good workmanship and quality stability.
The main properties of cable paste include: viscosity, cone penetration, oxidation induction period, oil separation, hydrogen evolution, compatibility, flash point, dropping point, acid value, evaporation, water absorption time, moisture content, resistance to copper, aluminum, and steel Corrosivity, color stability, etc.
The performance of cable paste has many effects on the performance of optical cables. For example, excessive hydrogen evolution will cause an increase in optical cable losses; poor compatibility with optical cable components will deform the optical cable components, reduce strength and elongation, and affect the service life of the optical cable; The oxidation induction period is too short, and the resistance to oxidation and decomposition is reduced, causing premature aging of the optical cable; the flash point is too low, making it flammable, etc. At the same time, there is also a correlation between various properties of cable paste. Often the improvement of one performance will be accompanied by the sacrifice of another performance. For example, in order to reduce oil separation and improve high temperature non-drip performance, it is hoped that the viscosity of the filling paste will be larger. , the yield stress is a little higher, but in order to improve the process performance of the optical fiber filling paste and ensure sufficient penetration at low temperatures to reduce the additional loss of the optical fiber, the viscosity of the filling paste is required to be smaller and the yield stress is correspondingly lower. Another example is to ensure that the optical cable can be used at higher temperatures, requiring a higher dropping point. However, if the dropping point is too high, it may be at the expense of reducing the cone penetration. Therefore, the characteristics of cable paste are the result of comprehensive optimization and cannot be measured by just one characteristic parameter. In the production process of optical cables, cable paste needs to be optimized according to customer requirements, the area of use of the optical cable, the direction of use of the optical cable, etc.
3. Main performance analysis and selection of cable paste
Nowadays, cold-filled grease is usually used in the production of optical cables, so it should have a certain degree of thixotropy, that is, when it is subjected to shear, the viscosity decreases as the shear rate increases. At a certain shear rate, as time increases, the shear stress decreases and the viscosity decreases. When a certain time is reached, the shear force no longer changes; after shearing is stopped, the viscosity begins to rise slowly again. This process Not completely reversible due to the breakage of granular bonds and changes in their molecular orientation. Therefore, choosing the appropriate viscosity is very important during the processing and long-term use of optical cables. Generally speaking, the viscosity should neither be too high nor too low, because during the processing process, the viscosity is expected to be smaller to facilitate filling into the cable core. When the viscosity is too small, the filling paste will easily be lost during the processing. The cable core cannot play a buffering role when it is stressed, and may cause water seepage in the cable core; the viscosity is too high, making it difficult to pump and fill during production, causing great losses to the equipment. At the same time, the grease cannot easily penetrate into the cable core, resulting in incomplete filling. , uneven. In production, it is also necessary to consider the viscosity-temperature properties of the grease. As the temperature increases, the viscosity of the cable core water-blocking filling grease decreases, and on the contrary, the viscosity increases. The performance of this system's viscosity changing with temperature changes is called the viscosity-temperature property of the system. . During the use of cable core water-blocking filling grease, we hope that the viscosity of the grease will change less with temperature, but the premise is that when the oil filling temperature is high, the viscosity of the grease meets the process requirements. Therefore, we believe that the viscosity of cable paste should be controlled within a certain range when meeting the requirements for optical cable processing and use. According to production practice and a large number of tests, for water-resistant ointment, when the shear rate is 50s-1 at 25°C, the viscosity is preferably 8000 to 12000mPa·s; for water-absorbing swelling ointment, when the shear rate is 50s-1 at 25°C When, the viscosity is preferably 7000~11000mPa·s.
2. Cone penetration
The cone penetration and viscosity indicators are responses to the thixotropic properties of the cable paste. The cone penetration reflects the yield stress of the cable paste when it is subjected to external forces and deforms. The viscosity represents the obstruction created between two relatively moving objects. Compared with the viscosity of the filling paste, the resistance to movement is a more intuitive and easy-to-detect indicator.
Penetration is a measure of the consistency of the grease. Its measurement method is to measure the depth of the standard cone (mass 150g) of the penetrometer plummeting vertically into the filling paste sample within 5 seconds under the specified temperature and load. The unit is 1/10mm, and the specific measurement method can be carried out according to ASTM D937. The greater the cone penetration, the less dense the filler paste will be, and vice versa.
If the cone penetration is too small, the cable paste will become thicker, which is not conducive to filling. If the cone penetration is too large, the cable paste consistency will decrease. During the processing, the cable paste will be easily lost, and the filling will not be full, which may cause water seepage in the cable core. Therefore, normal temperature is usually used in production. Taper penetration in the range of 300~450 1/10mm.
When testing the cone penetration, not only the normal temperature (25°C) test but also the low temperature (-40°C) test is required. For optical cables used in low temperature environments, the cable paste should have a low yield stress and sufficient strength at low temperatures. The softness enables the optical cable to still obtain good buffering protection at low temperatures. Therefore, the cable paste should be able to maintain a certain cone penetration at low temperatures. Since the low-temperature cone penetration mainly depends on the performance of the base oil, for low-temperature use, base oil with a low freezing point must be used. At a temperature of -30°C, mineral oil can be used; at -40°C, mineral oil and synthetic oil can be used. Mixed oil of oil; -50℃, synthetic oil is required; -60℃, only silicone oil can be used. When using synthetic oil, its glass transition temperature and crystallization temperature need to be lower than the temperature. The low-temperature cone penetration cannot be too small, otherwise it will affect the low-temperature flexibility of the optical cable, affect the transmission performance of the optical cable, and is not conducive to pumping and filling during production. In low-temperature production, a cone penetration in the range of 150 to 200 1/10mm is usually used.
3. Oxidative induction period (OIT)
The oxidation induction period indicates the ability of the filling paste to resist oxidative decomposition. It is usually measured using a differential thermal analyzer. Under the action of high-temperature oxygen, the cable paste undergoes a catalytic oxidation reaction to determine the thermal stability of the cable paste. The oxidation induction period (OIT) ) The longer the time, the better the oxidation stability. Cable paste will undergo varying degrees of oxidation reactions with the oxygen it comes into contact with at high temperatures and normal temperatures. Once free radicals are generated, a chain reaction will occur, accelerating the oxidation and generating a large amount of alcohols, aldehydes, acids, etc., which will react further. Polymers are generated, which increases the viscosity of the entire system and causes the cable paste to age. At the same time, the oxidation process also has hydrogen evolution, which affects the transmission performance of the optical cable. Therefore, antioxidants must be added to the cable paste during production to prevent the generation of free radicals. chain reaction.
The measurement of the oxidation induction period of optical cable water-blocking filling grease is an accelerated aging test. Based on the OIT measured at high temperature (190+0.5°C), the service life of the cable grease at room temperature can be estimated. According to YD/T 839.4-2000 standard, the OIT of cold application optical cable filling paste should be no less than 20 minutes to ensure normal service life at room temperature. The oxidation induction period can be measured by measuring the DSC curve of the cable paste sample on a differential thermal analyzer to obtain the OIT value.
When the temperature starts to rise from room temperature, the optical cable water-blocking filling paste sample undergoes endothermic melting at the melting temperature. Because the cable paste is a multi-component material, the melting points of each component are different, so the temperature range of the melting peak is wide. When the cable paste sample begins to oxidize, an exothermic reaction occurs, making it easier to read the OIT value on the DSC curve.
Through experiments, it was found that due to the heating effect of the cable paste during the production process, resulting in a small loss of antioxidants, the OIT value of the cable paste in the cable is about 5 to 8% lower than the raw material of the cable paste. The amount of loss of different raw materials is slightly different, so this should be fully taken into consideration when selecting cable paste.
4. Oil separation
There are many performance indicators of cable paste, among which oil separation directly affects the service life of cable paste. Cable paste is mainly composed of base oil and thickener. When the base oil separates from the paste, the proportion of thickener increases accordingly. If the oil separation is serious, the cone penetration of the cable paste will become smaller. At low temperatures, It becomes hard as time passes and the additional stress increases, thus affecting the transmission performance of the optical cable.
Since the cable paste itself is a colloidal dispersion system, its base oil and thickener are not bonded together through chemical bonds, but the oil is dispersed in the thickener through hydrogen bonds, intermolecular van der Waals forces and adsorption forces. Such a system It is unstable in itself. When the system is subjected to external force or the weight of the oil molecules squeezes each other and the force is greater than the adsorption force such as hydrogen bonding and van der Waals force, the oil will separate from the colloidal dispersion structure. The longer the time, the more separated oil will be. The more oil is produced, the more oil is separated and the speed of oil separation gradually slows down. Since a part of the oil is separated, the amount of thickener in the cable paste is relatively increased, and the ability to absorb oil is enhanced. Under certain conditions, the adsorption force of the thickener and the gravity of the oil molecules reach a balance, and no more oil is separated.
The cable paste used in production is usually stored for a short time and has a low oil separation value, but it cannot be said that its quality is good. Oil separation is closely related to the type and amount of additives, and the degree of dispersion in the base oil. Due to the thixotropic properties of the cable paste, if its grid structure is unstable, the amount of oil separation will gradually increase during the production and use of the cable paste. , seriously affecting product quality, so when selecting cable paste, the relationship between oil separation and time must be taken into consideration, and a cable paste with less oil separation over time must be selected, that is, a cable paste with a stable system must be selected.
5. Hydrogen evolution value
In the optical cable structure, if the hydrogen evolution value of the cable paste is large, hydrogen will penetrate and diffuse into the optical fiber, causing hydrogen loss of the optical fiber. In essence, the hydrogen loss of the optical fiber is the penetration and diffusion of hydrogen into the optical fiber glass, and into the glass. Defects react to cause an increase in optical fiber attenuation at some characteristic wavelengths. This process includes both physical and chemical processes. The physical process mainly refers to the diffusion process of hydrogen gas in the optical fiber glass. During this process, the hydrogen molecules do not react with the defects of the glass, so the hydrogen loss caused is only related to the absorption spectrum characteristics of the hydrogen molecules penetrating into the fiber core. . The size of this type of hydrogen loss is only related to the concentration of hydrogen molecules in the optical fiber. When the temperature is high, less hydrogen is dissolved into the optical fiber glass, and the resulting hydrogen loss is smaller. The process is also reversible. When there is no hydrogen atmosphere outside, the hydrogen in the optical fiber can leak out again, and the hydrogen loss is eliminated. The chemical process of hydrogen loss is that while hydrogen molecules diffuse in the glass, hydrogen will react with defects in the glass to form certain specific chemical bonds. The intrinsic vibrations or high-order vibration modes of these chemical bonds will also occur in the glass. Causes increased attenuation at characteristic wavelengths, and this hydrogen loss is permanent.
The test of hydrogen evolution is an accelerated simulation process of the normal hydrogen evolution process, which is measured using a gas chromatograph. By collecting the gas released from the cable paste at a certain temperature and after a certain period of time, the gas is analyzed to determine whether hydrogen is evolved. The specific operation is: put 100g sample into a clean and dry glass bottle, seal it with a stopper that can withstand high temperatures and will not produce hydrogen, put it in an oven and keep it at 80°C for 24 hours, and use a syringe to autoclave the sample after thermal aging. Take 1ml sample from the glass bottle and immediately inject it into the chromatograph. Inject the gas continuously for more than 2 times, and measure the peak height of hydrogen. During the experiment, in order to balance the volume in the glass bottle and make the sample gas uniform, the same volume of air should be added while taking the air, and after the needle is inserted into the bottle, it should be pushed and sucked repeatedly. After the measurement is completed, the The volume of space inside the bottle. According to the volume of space and the volume of added air, the total volume of gas generated by the sample is calculated to calculate the thermal aging hydrogen evolution value.
As the amount of hydrogen evolution from the cable paste increases, a certain hydrogen atmosphere will be maintained inside the optical cable. The physical and chemical processes of hydrogen loss in the optical fiber occur at the same time, causing the additional loss of the optical fiber to increase and reducing the transmission performance of the optical cable. Therefore, the hydrogen evolution value with a higher hydrogen evolution value should be used in production. For low cable paste, it is generally suitable to have hydrogen evolution (80℃, 24h) less than 0.1ul/g.
The compatibility of cable paste means that the long-term contact between cable paste and optical cable materials will not cause structural changes and deterioration of mechanical and electrical properties, that is, cable paste will not have obvious physical and chemical interactions with other materials. . Although compatibility is related to the chemical structure and chemical composition of the two materials under investigation, since the composition of cable paste is more complex than other materials in optical cables and has a certain fluidity, it can fully contact with other materials. Cable paste is critical when considering the compatibility of materials in fiber optic cables. Cable pastes with poor compatibility will swell and plasticize materials such as plastics and fibers, leading to the reorganization of their tissues and structures, increasing internal stress, easily causing cracking, and reducing tensile strength. Bad cable pastes can also cause metal composite belts to Lather or layer. In addition, the compatibility results of different cable pastes with different materials will be different and there is no certain regularity. Therefore, the compatibility is based on the performance changes of different cable pastes in long-term contact with different materials. The compatibility of cable paste is related to the life of the optical cable and is a particularly important performance. As long as the optical cable materials are in contact with the cable paste, their compatibility must be considered. Currently, these materials mainly include PBT (polybutylene terephthalate), PA (polyamide), and PP (polypropylene). , PE (polyethylene), steel-plastic composite tape, aluminum-plastic composite tape, steel wire, FRP, etc. The method of measuring compatibility is to make a certain sample of the relevant materials, infiltrate the cable paste with other materials, age it according to the specified time and temperature, and then measure the parameter changes of the material before and after aging, such as the change of the loose tube material. Quality changes, tensile properties (including yield strength, breaking strength, breaking elongation) changes and changes in the oxidation induction period; quality changes, tensile properties (including yield strength, breaking strength, breaking elongation) changes and oxidation of sheath materials Changes during the induction period; quality changes of steel-plastic composite tapes and aluminum-plastic composite tapes, and whether there is blistering or delamination.
Ensuring the compatibility of cable paste with other materials is closely related to the basic material and process quality of the cable paste. The reasonable formula and stable ingredients of the cable paste and the strict control of the production process are the prerequisites for ensuring the compatibility of the cable paste with other materials. Therefore, in production, we should choose cable paste with good basic materials, stable formula and process to ensure that its compatibility meets performance indicators.
7. Flash point: The cable paste should ensure that it does not spontaneously ignite within the entire temperature range of the optical cable test and application. The higher the flash point, the less likely the cable paste is to spontaneously ignite in the air. The flash point of cable paste depends on the quality of the base oil, light oil or A base oil containing more light components will have a lower flash point, and vice versa. The flash point of cable paste is also related to its volatility. If the flash point is low, the volatility will be high, which will affect the long-term performance of the filling paste. Therefore, cable paste with a flash point greater than 200°C should be selected during production.
8. Dropping point The dropping point is an indicator that reflects the thermal properties of the cable paste. The cable paste will soften as the temperature increases when heated under specified conditions. The temperature when a drop of oil drips from the mouth of the grease cup is the dropping point. The level of the dropping point indicates the degree to which the cable paste can be heated during use. Therefore, as long as the dropping point is higher than the maximum temperature for the production and use of the cable paste, the usage requirements can generally be met. An excessively high dropping point may sacrifice the low-temperature performance of the cable paste, resulting in reduced penetration, increased specific gravity, increased acid value, etc. Generally, the dropping point of the cable paste is greater than 150°C.
9. Acid value The acid value indicates the free acid content in the cable paste. The number of milligrams of potassium hydroxide required to neutralize 1g of cable paste is the acid value. The acid value is an indicator for predicting the corrosiveness of cable paste and reflects the impact on hydrogen evolution.
Since the cable paste components contain some acidic components, it is normal to have a certain acid value. However, if the acid value is too high, it means that the free acid content is too high, which will have an impact on the surrounding materials, such as oxidation, corrosion, etc. When the cable paste oxidizes and deteriorates during long-term storage and use, the acid value will also increase. Therefore, the change in the acid value can also be used as a measure of the oxidation stability of the cable paste. In production, the acid value of cable paste is generally required to be lower than 1.0mgKOH/g.
10. Evaporation: During the long-term use and storage of cable paste, the cable paste tends to dry out due to the volatilization of the base oil. Therefore, the amount of evaporation mainly depends on the properties of the base oil. After the cable paste evaporates for a long time, it will become thicker, the dropping point will decrease, and the acid value will increase, resulting in a decrease in cone penetration. Therefore, the evaporation amount should be as small as possible. In production, cable paste with a smaller evaporation amount should be selected. Usually the evaporation amount should be less than 1.0%.
11. Water absorption time. Judging from the development of cable paste, water-blocking cable paste has been increasingly used in production. Compared with the water-resistant cable paste, the paste contains water-absorbing substances. When moisture or water enters the cable core, the water-absorbing substances can quickly absorb water and expand to form a gel and block the gaps, thus avoiding the problems caused by ordinary water-resistant cable pastes. Water seepage in gaps caused by uneven filling. The water absorption time is the time it takes for the cable paste to dissociate into the water layer and convert the water into colloid. When the colloid is completely formed. The shorter the water absorption time, the faster the water absorption rate of the cable paste and the better the water blocking effect. Therefore, cable paste with a shorter water absorption time is usually used in production.
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