Optical Fiber Coatings
This is a continuation from the previous tutorial - relay trains and periscopes.
Introduction
It could be said that the modern era of fiber optics began in 1966, with the publication of the paper ‘‘Dielectric-Fibre Surface Waveguides for Optical Frequencies‘‘ by Dr. C. K. Kao and G. A. Hockham of Standard Telecommunications Laboratories Ltd. (STL).
This paper discussed the theory and potential use of optical fiber for communications. Dr. Kao believed that fiber loss could be reduced below 20 dB/km by eliminating metal impurities in the glass. Such attenuation would allow 1% of the light entering 1 km of this type of fiber to successfully reach the other end.
During the same period, Corning Glass Works researchers, R. Mauer, D. Keck, and P. Schultz, were experimenting with high-purity fused silica. They employed titanium as a dopant to produce a higher refractive index (RI) (1.466) fiber core, compared with the surrounding lower RI (1.4584) silica cladding.
They were able to draw fibers, at 1900°C, to a diameter of 100 μm. In 1970, ‘‘Method of Producing Optical Waveguide Fibers’’ was filed with the U.S. Patent Office and issued 3 years later as U.S. 3,711,262.
Before 1966, fibers had losses of roughly 1000 dB/km. The Corning team effort reduced this loss to only 16 dB/km, thus demonstrating the feasibility of optical fibers as a communication medium. Such fibers were said to be capable of carrying 65,000 times more information than copper wire.
This tutorial focuses on the development, science, and performance of protective coatings for optical fiber. The widespread deployment of high-bandwidth optical fiber would not have been feasible without the immediate protection afforded by ultraviolet (UV)-curable coatings.
Applied on fiber draw towers, at speeds approaching 60 mph, these coatings enable fibers to survive the rigors of proof testing, cabling, and installation. UV-curable coatings have also proven to be durable in the field, with millions of kilometers of fiber already in operation for more than 20 years.
Early History of Coatings for Optical Fiber
Although the inherent strength of pure silica is known to be nearly 14,000N/mm2, it soon became apparent, during the development of optical fibers, that some type of protective coating was required to shield the fibers from abrasion to preserve their strength.
The combination of moisture and stress causes microscopic flaws in the glass to propagate, resulting in fiber failure. Without protective coatings, optical fibers would never have been a practical alternative to copper for telecommunications.
According to Stevens and Keough, the prime requirements for optical fiber coatings were protection against microbending and static fatigue. This necessitated that cured coatings ‘‘be concentric about the fiber, be continuous over the length of application, be of constant thickness, be abrasion resistant and moisture retardant.’’
Suitable liquid coatings, consequently, had to be of a workable and stable viscosity (minimally 6 months), adhere to glass, have a relatively low surface tension, be free of particle contamination, have minimal hydrogen generation (\(10^{-2}-10^{-4}\) μl/g), and have fast cure speeds. In addition, it was expected that cured coatings would have stable modulus and adhesion properties over the 25-year operating life of the installed cable.
In the early 1970s, J. E. Ritter, Jr., explored the effectiveness of polymeric coatings in preventing the degradation of abraded soda-lime glass in the presence of moisture. His results indicated ‘‘acrylic, epoxy, and silicone coatings all significantly increased the short-term strength of abraded glass.’’ He believed that the coatings functioned as a diffusion barrier and limited the availability of water at the glass surface.
Wang et al. and Wei issued additional reports on the contributions of polymeric coatings towards providing resistance to moisture, in the early and mid-1980s. Wang et al. reported on the accelerating effects of water on the deterioration of fiber strength through fatigue and aging.
Wei further described how the combination of moisture and stress initiates crack propagation and ultimately reduces fiber strength and provided examples of how this was retarded by polymeric coatings.
Early coating materials used in the protection of optical fiber included two package systems, blocked urethanes, solvent-based lacquers, silicone rubbers, and UV radiation–curable epoxy acrylates. By the early 1980s, UV-curable coatings became the popular choice for protection of optical fibers, largely because of their rapid cure response and easily tailored properties.
In 1974, H. N. Vazirani of Bell Telephone Laboratories developed one of the first UV-curable coatings for optical fiber. U.S. Patent 4,099,837 describes that the ‘‘polymer coating comprises the polymerization product of a prepolymer mixture resulting from reacting acrylic acid with a 0.4 to 1.0 weight ratio mixture of aliphatic diglycidyl ether to aromatic diglycidyl ether, and further characterized in that polymerization product contains a UV sensitizer in order to cure said prepolymer mixture with ultraviolet light.’’
Additional claims cited 1,4-butanediol diglycidyl ether as the aliphatic ether and the diglycidyl ether of halogenated bisphenol A as the aromatic ether. The use of a silane or titanate coupling agent was also claimed.
The fiber coating, described earlier, was applied and cured as the fiber was drawn at a speed of 25 m/min. Physical properties of the polymer coating were reported as a Young’s modulus of 6000 psi and an elongation at break of approximately 20%.
The resulting coated fiber was evaluated and was found to have tensile strengths greater than 500,000 psi in 1-km gauge lengths and long-term strength retention properties in moist environments.
Schonhorn et al. also demonstrated that the interaction between the coating and the glass interface determines the ability of the coating to prevent degradation of fiber strength and that this is enhanced by the inclusion of silane coupling agents within the coating.
Evolution of Optical Fibers and Protective Coatings
Attenuation losses associated with optical fibers continued to decline during the 1970s: 4 dB/km (1975), 0.5 dB/km (1976), and 0.2 dB/km (1979). The latter value corresponds to 63% of a light signal reaching the end of a 10-km long fiber.
In 1982, Corning achieved an attenuation of 0.16 dB/km, on single-mode fibers transmitting at 1550 nm, representing a 100-fold improvement over the 1970 transmission of Mauer’s first low-loss fiber.
It has been reported that if the ocean had the same transparency as the glass in such low-loss fibers, one could see to the bottom of the Mariana Trench in the Pacific Ocean, a little more than 6 miles below sea level.
1. Coating Contributions to Microbending Minimization
Concurrent with the aforementioned improvement in fiber transmission properties, fiber coatings evolved from single-layer to dual-layer systems. In the early 1980s, the outer diameter for dual-layer systems was standardized to between 245 and 250 microns, while the outer diameter for the inner coating, contacting the glass, ranged from 190 to 210 microns.
The dual-layer coating system was designed to enhance protection for fibers against microbending-induced attenuation. This phenomenon is caused by microscopic departures from straightness in the waveguide axis.
Varying causes of microbending include longitudinal shrinkage of the fiber coating, poor drawing or cable manufacturing methods, or stresses imposed during cable installation.
D. Gloge first reported that microbending, losses could be reduced by shielding the fiber from outside forces by using a soft inner coating, having a modulus of 14,000 psi (~100 MPa), and an outer shell of a material having a modulus of 140,000 psi (~1000 MPa).
The inner primary coating is designed to act as a shock absorber, under the tougher outer layer, to minimize attenuation caused by microbending. It has a very low crosslink density and current primary coatings typically have a modulus between 0.5 and 3.0 MPa. It must adhere to the glass, yet strip cleanly from the glass, to facilitate splicing and connecting.
The outer primary coating, sometimes called the secondary coating, protects the primary coating against mechanical damage and acts as a barrier to lateral forces. It also serves as a barrier to moisture. It is a hard coating, having a high modulus and Tg, to facilitate good handling and durability.
It is generally fast curing, for ease of processing, and has good chemical resistance to solvents, cable filling gels, and moisture. The surface properties of the secondary coating must be carefully controlled to allow good adhesion of the ink used in color identification while allowing for good winding onto takeup spools. A schematic diagram of an optical fiber is shown in Fig. 4.1.
During the development of dual-layer coating systems, it was important to consider the modulus of the inner coating not only at room temperature but also at colder temperatures to which fibers could reasonably be exposed.
Viscoelastic coating materials are known to increase in modulus as temperature drops (i.e., they become stiffer). If these coatings also adhere tightly to glass, they can impose forces on the fiber that will produce microbending-induced signal attenuation.
Kuzushita et al. reported on the low-temperature modulus properties of nine coatings and correlated them with the added attenuation observed at -30°C for fibers coated with four resin types: polyester-polyol–type urethane acrylate, polyether-polyol–type urethane acrylate, polybutadiene acrylate, and UV-curable silicone.
The modulus of coating films at cold temperatures was initially determined by Instron tensile strength testing of films in temperature-controlled chambers. This process proved fairly time consuming, because of the time required for temperature equilibration between runs.
The development of dynamic mechanical analyzers gave rise to the exceedingly more efficient dynamic mechanical analysis (DMA) of UV-cured films. This nondestructive test allows for temperature sweeps at a chosen frequency to define a material’s modulus as a function of temperature.
The slope of a coating’s modulus curve as it changes from the glassy phase to the rubbery phase can, in part, determine the suitability of coatings for use at low temperatures. Sarkar et al. shared examples of DMA curves for soft RTV silicone and UV-cured urethane acrylate primary coatings and showed how they compared with the attenuation properties of fibers on which they were applied.
This reference also illustrates improved temperature-induced attenuation at 1300 nm for the UV-cured urethane acrylate coating, which exhibited a significantly lower modulus profile at lower temperatures.
2. Glass Fiber Fracture Mechanics and Coating Contributions to Fiber Strength Retention
Fiber fatigue is an important mechanical property of optical fibers. Pristine silica fibers have strengths of approximately 7 GPa at ambient condition. However, fibers can experience fatigue when subjected to lower stresses for long periods. Fiber fatigue is thought to occur by crack growth of existing flaws on the glass surface due to interaction between the Si-O bond and the moisture in the environment when the fiber is subjected to stress.
The coating is thought to contribute to fiber fatigue in that basic compounds present in the composition can accelerate glass corrosion. Conversely, acidic components have been shown to improve fatigue resistance. Skutnik et al. found that coatings with strong adhesion facilitated a greater n-value fatigue parameter for coated fibers. Supporting this is the observation that both n-value and adhesion generally increase with time after draw.
When measuring fatigue, it is typical to measure either static fatigue or dynamic fatigue. In static fatigue, a constant stress is applied to the fiber, and the time to failure is measured. In dynamic fatigue, the strength of the fiber is measured as a function of the applied stress rate.
There are two techniques for measuring dynamic fatigue: tension and two-point bending. In the tensile test, a fiber is gripped at each end and pulled in tension until it breaks. In the two-point bending test, fibers are bent between two faceplates that move toward each other at a controlled rate until the fibers break.
Gulati published one of the earliest papers that discussed test methods for measuring the tensile and bending strengths of optical fibers. He also calculated the required proof stress level to ensure fiber durability of at least 20 years when subjected to a known value of service stress.
Michalske and Bunker published a number of studies on the fracture mechanics of glass and glass fibers. Helfinstine published a very thorough review on delayed failure or sub-critical crack growth in glass.
Wang and Zupko found fiber strength retention to be a function of fiber wetting and adhesion by protective polymeric coatings. Fiber strength decreased when application viscosity was increased and wetting of the fiber was consequently decreased.
Sakaguchi et al. demonstrated that silane treatment stabilizes glass surfaces against water. The group evaluated the behavior of untreated, silicone-treated, and silane-treated glass in water at different temperatures. Infrared spectroscopy was used to monitor the interaction of water molecules with glass silanol groups.
Schlef et al. evaluated the performance of a number of UV-curable primary and secondary coatings via static fatigue, dynamic tensile testing, and proof testing and concluded that these coatings ‘‘retain the initial strength and fatigue resistance of optical fibers.’’
Dunn and Smith performed a variety of abrasion and static fatigue tests, demonstrating that the use of hard UV-cured secondary coatings yielded fibers with improved strength and handling characteristics, compared with silicone single coatings.
3. Durability of Fiber Optic Coatings
Long-term durability of protective coatings was considered to be of considerable importance because fibers installed into outside plant networks were expected to have a minimum service lifetime of 25 years.
D. R. Young provided ‘‘the first report on a long-term, long length static fatigue test in an outdoor, in-ground trough environment.’’ He reported that UV-cured acrylate composite protective coatings were shown to provide excellent protection against a variety of environments: 65°C air, high and low pH solutions, and temperature/humidity cycling of 150 days 65°C/98%relative humidity (RH) to 10°C/4% RH.
Weibull plots, which describe the probability of failure at a given stress for a given length, were provided for fibers exposed to temperature/humidity cycling and immersed in petroleum jelly, similar to what is used as loose tube filling compound.
In 1984, O. R. Cutler reported on the results of his durability testing on UV-curable coating films. In his study, Cutler exposed 75 μm thick films of a variety of commercial fiber coatings to temperatures of 38, 54, 88, 125, and 175°C, for up to 1 year.
He developed Arrhenius plots that showed, based on the time required to double the coating’s modulus, operating lifetimes of primary and secondary coatings could extend beyond 100 years at room temperature. Several coatings had similar calculated service lifetime when aged continuously at 54°C (130°F).
Nevins and Taylor issued their report on the effect of a variety of environmental conditions on the ‘‘three key characteristics of fiber optic waveguides which may be effected by environmental conditions: strength, attenuation and resistance to losses caused by microbending.’’
In addition to the conditions reported earlier by Young, the research team of Nevin and Taylor mentioned exposure to seawater, fungi, and abrasives. They also cited an experiment, Procustes, designed to correlate accelerated aging tests with actual long-term aging.
Simoff et al. studied the aging of a polyether urethane acrylate primary coating, both in films and on fiber, and correlated the changes in physical properties of the films with the stripping force required to remove the coating from the fiber.
In 1993, Chawla et al. reported on fiber optic coating durability, as was measured by weight changes and shifts in DMA modulus profiles. DMA provides insight into a material’s durability following exposure to a wide variety of environments such as hydrolytic, thermo-oxidative, and chemical exposure.
Comparison of DMAs before and after exposure to these conditions allows one to monitor changes in the material’s glass transition temperature profile and the material’s equilibrium modulus.
The equilibrium modulus region of the DMA curve is observed at the modulus plateau reached in the rubbery phase. This modulus can be related to the crosslink density of a cured coating’s network through the equation
\[\tag{4.1}\rho_0=E_0/6kT\]
where \(\rho_0\) represents crosslink density, \(k\) is the Boltzmann constant, and \(T\) is the temperature, in degrees-Kelvin.
Decreases in equilibrium modulus indicate a reduction of a coating network’s crosslink density through chain scission, and hence a weaker coating. Conversely, increases in the equilibrium modulus can signify embrittlement of the coating network through crosslinking. Chawla et al. demonstrated excellent durability of several primary coatings after 1 year of aging at 125°C.
The results of additional durability studies have been published and offer a deeper understanding of this subject.
Cabling of Optical Fibers
Early 250-μm outer diameter coated fibers were further protected from the potential hazards of the cabling process by placement in buffer tubes. The tubes had an inside diameter many times larger than the diameters of the enclosed fibers. Fiber length was designed to slightly exceed the length of the buffer tubes, to introduce fiber slack.
This slack was necessary to prevent tensile loading, which could compromise the optical transmission of the fiber. The buffer tube also contained water-blocking filling compound. The filling compound not only blocked water from reaching the fiber, but also provided a medium in which the fibers could freely move past each other during thermal expansions and contractions.
Protective fiber coatings had to be resistant to changes in material properties induced by such filling compounds, which often comprised fumed silica dispersed in mineral oil.
An alternative to the ‘‘loose-tube’’ cable design was the ‘‘direct-strand buffer’’ construction. An example consisted of a fiber that was coated with a silicone coating to 400-μm outer diameter and then over extruded with a nylon jacket to 900-μm outer diameter. Fibers of this type were wound about a strength member that had a lower expansion coefficient than that of the tight-buffer coating materials.
In tight-buffered or up-jacketed fiber, 250-μm fiber is overcoated with a tough thermoplastic extrusion, such as a Nylon 12, polyethylene or PVC. These thicker fibers provide improved handling and mitigate the use of a cabling gel.
Tight-buffered cables have been used in premise applications for more than 2 decades. Though widely used, thermoplastic extrusions are limited by slow line speeds, 100–200 m/min, and high scrap rates.
UV materials have successfully been employed as a cushioning layer between flame-retardant thermoplastic extrusion and the fiber. Construction of these types of fibers involves up-jacketing a standard 250-μm fiber to 400–500μm with a UV coating and then extruding with a colored thermoplastic to bring the final thickness to 900μm.
Another approach to tight buffering using UV-curable materials involves up-jacketing a colored 250-μm fiber with a clear UV resin directly to 500 or 900 μm. The development of flame-retardant UV cure coatings has been reported.
Montgomery et al. described the use of a tight-buffer coating containing a pigmented flame retardant, thus giving performance typically observed by thermoplastics. These UV-curable up-jacketed fibers can be processed on a modified ink-coloring machine, giving much faster processing speeds, up to 600 m/min, than thermoplastics.
During the early to mid-1980s, Japan began using UV-curable matrix materials to bond fibers in four-fiber ribbon arrays. AT&T had earlier pioneered 12-fiber Adhesive Sandwich Layer Ribbons, in which fibers were packaged between two plastic tapes coated with pressure-sensitive adhesive. UV-curable matrix materials for ribbon application became more popular globally, with the growing demand for fiber, in the 1990s.
Prior to the cabling of fiber, in either loose-tube or ribbon configuration, the vast majority is colored with a UV-curable ink, in an off-line operation. UV-curable inks are designed to provide high-quality color on fiber for good identification and are typically applied at 3- to 5-μm thickness.
The UV-cured ink layer has a hard and slick surface finish to allow suitability in both loose tubes and ribbons. It is also designed to have excellent adhesion to the secondary coating and to provide excellent resistance to cabling gel. Colors are designed to be bright and distinct, giving good color retention over time.
Specialty Coatings
Low RI coatings are used for cladding of plastic optical fiber or glass cores. They are by nature relatively low-modulus materials and are, therefore, often protected by overlaying a standard secondary coating.
The lower RI coating is preferred for fiber used in laser and amplifier applications. The low RI coating increases the numerical aperture (NA) value, allowing for higher power inputs and thus higher powered lasers. These coatings typically have an RI less than 1.41.
For application as a single coat, a modulus of approximately 120–200 MPa is desirable; however, when used as the primary layer in a dual-coating system, a modulus of 15–50 MPa may be used.
Basics of Optical Fiber Chemistry
In the late 1970s and early 1980s, UV-curable coatings designed for the protection of optical fiber transitioned from single coatings based on epoxy acrylate chemistry to dual-layer coatings based on urethane acrylate chemistry. These coatings are composed of one or more urethane acrylate oligomers, diluent monomer(s), photoinitiator(s), and various additives.
1. Oligomers
Urethane acrylate oligomers are based on stoichiometric combinations of diisocyanates (DICs), polyols, and some type of hydroxy-functional terminating species containing a UV-reactive terminus (A). Urethanes are known for their toughness and flexibility, a combination that adds value to the performance and protective nature of the coatings in which they are contained (Fig. 4.2).
Depending on the properties desired, different types of polyols are chosen. Typically, oligomers are made using the types of polyols shown in Figs. 4.3–4.5:
Urethane acrylate oligomers employed in UV-curable optical fiber coatings generally range between 1000 and 10,000 number average molecular weight. Higher molecular weights tend to be exceedingly viscous. The viscosity can be reduced by either heat and/or addition of diluent monomer.
Typical viscosities of coatings supplied to producers of optical fiber are less than 10,000 mPa•s. The viscosity of coatings, as they are applied to fiber, is typically 2000 mPa•s or less. In general, high-molecular-weight urethane acrylate oligomers are used in soft low-modulus coatings, while low-molecular-weight oligomers are employed to produce hard high-modulus coatings.
Acrylated epoxies are another type of oligomer commonly used in optical fiber coatings. Epoxy acrylates are tough fast-curing materials that have good chemical resistance. They tend to be used in secondary coatings (Fig. 4.6).
2. Monomers
Diluent monomers can be monofunctional or multifunctional with respect to UV-reactive terminal groups. Monofunctional diluent monomers are generally more efficient in their diluency; however, they are only capable of reacting linearly and will not add to the crosslinking of polymer networks.
Monofunctional monomers can be aliphatic (e.g., isodecyl acrylate [IDA]), aromatic (e.g., phenoxyethyl acrylate [PEA]), or alicyclic (e.g., isobornyl acrylate [IBOA]). In general, monofunctional monomers find their greatest utility in soft primary coatings, which are designed to have very low modulus. Monofunctional monomers generally give good flexibility and low shrinkage.
Multifunctional monomers are most often employed in secondary coatings, which have higher modulus and crosslink density relative to primary coatings. Multifunctional monomers are added for fast cure speed, increased crosslinking, and tensile strength.
Monomers possess a variety of useful properties beyond their acrylate functionality. These property differences provide skilled formulators the latitude to design coatings with optimized performance with regards to viscosity, cure speed, tensile properties, glass transition temperature profile, oleophobic/hydrophobic balance, adhesion, and long-term durability in various environments.
3. Photoinitiators
Photoinitiators, as their name implies, initiate the photopolymerization process by absorbing light. In UV-curable acrylate systems, they form radicals by cleavage (Norrish type I) or hydrogen abstraction (Norrish type II).
The effectiveness and efficiency of photoinitiators is largely governed by the their absorption spectra and extinction coefficients. It is important to appropriately match the absorption spectrum of a photoinitiator with the emission spectra of the lamps employed for coating cure.
4. Adhesion Promoters
Primary coatings based exclusively on oligomers, monomers, and photoinitiators generally do not provide adequate adhesion to the glass fiber. This is especially true in humid environments. It is, therefore, necessary to incorporate an adhesion promoter or glass-coupling agent.
Typically, adhesion promoters have an organic functional group, which bonds, or associates, with the coating. The adhesion promoter bonds to the glass surface through hydrolysis and condensation reactions.
Alkoxy silanes are commonly employed as adhesion promoters in fiber optic coatings.
The organofunctional end of the adhesion promoter covalently links with the oligomer or other radiation-curable moieties (Fig. 4.7). The glass-coupling moiety is generally inorganic in nature and bonds with the glass surface. Types of organic functionality include amino, epoxy, vinyl, methacryloxy, isocyanato, mercapto, polysulfide, and ureido.
The inorganic end of the coupling agent must have hydrolyzable groups, which leave silanol groups that can condense with the surface silanol groups of the glass fiber surface. Methoxy and ethoxy groups are typically employed as hydrolyzable groups of silane coupling agents.
Several types of non-silane adhesion promoters include chromium, orthosilicate, inorganic ester, titanium, and zirconium systems.
5. Other Additives
Other additives in fiber optic coatings include antioxidants and stabilizers and slip additives. Antioxidants and stabilizers are added to coatings to maintain the desired viscosity of the liquid composition and impart resistance to cured films against degradation by light and oxidation.
Hindered phenols are widely used antioxidants in fiber optic coatings, whereas hindered amines provide stability against light. Additives such as silicones are added to the coatings to modify the surface properties.
The standard components of a fiber optic coating are shown in Table 4.1, together with typical levels at which they are present, and their contribution to the coating.
As mentioned, acrylate systems cure by free radical photopolymerization. Photopolymerization occurs in several stages, as shown in the following subsections.
5.1 Initiation
Initiation involves the absorption of light, which, via several intramolecular energy transfers, yields primary radicals. This is followed by the addition of the resulting primary radicals in a Michael type of reaction to a suitable double bond, thereby resulting in propagating radicals (Fig. 4.8).
5.2. Propagation
As propagation reactions, all additions after the primary addition are counted and treated in a similar manner (Fig. 4.9).
5.3. Termination
Bimolecular termination: the standard that is employed in all classic descriptions of radical polymerization.
Primary radical termination: the propagating radicals are terminated by initiator radicals (Fig. 4.10).
In addition to acrylate systems, free radical polymerization may also employ thiol-ene chemistry. An alternative curing mechanism to free radical polymerization is cationic polymerization.
In thiol-ene chemistry, a thiol (mercaptan) radical reacts with an olefin in the presence of UV light. While thiol-ene systems have been evaluated for their potential use in optical fibers, to our knowledge, they have never been used commercially.
Cationic polymerization involves the acid polymerization of an epoxy or vinyl ether group. Typically diaryliodium or triarylsulfonium salts are used as photoinitiators. Epoxies are fairly slow curing; however, vinyl ethers cure extremely fast.
Advantages of cationic cure include an absence of oxygen inhibition and good through-cure. However, cationic cure suffers from moisture inhibition, limited raw material selection, and higher cost. With cationic polymerization, curing continues after irradiation.
Some hybrid systems such as acrylate free radical and cationic cure have also been employed in fiber optic coatings, but these are very rare.
Application of Coatings on the Draw Tower
Optical fibers are drawn from glass preforms at temperatures approximating 2000°C. The majority of tower systems employ the wet-on-dry application method, wherein fibers pass through a primary coating applicator, followed by one or more UV radiation–emitting lamps, and then through a secondary coating applicator, again followed by one or more UV radiation–emitting lamps.
In an alternative wet-on-wet application protocol, fibers are passed through both the primary and the secondary coating before UV exposure.
It is imperative to apply concentric coating layers, to prevent damage to the fiber during the drawing operation, and to maximize fiber strength and microbending resistance. Nonconcentric coating application puts differential stress on the fiber when cured.
An unevenly coated fiber will experience non-uniform forces during periods of coating expansion and contraction and is susceptible to greater attenuation of light signals passing though the fiber.
The earliest coating applicators used were of open cup geometry with either rigid or flexible tips. Some of these included a temperature-controlled cylinder, which contained fitted split conical dies that produced ‘‘constant coating thickness over a wide range of coating viscosities and drawing speeds.’’ Tapered dies aided concentric application of each of the coatings to the fiber.
At drawing speeds greater than 1.5 m/sec, open cup applicators tended to have problems with coating meniscus collapse and bubble entrainment. Paek and Schroeder studied a number of factors that were critical to increasing operating speed.
These factors included fiber cooling rate, applicator efficiency, and available UV energy. They noted that shear force in the coating die needed to be minimized to avoid beading on the fiber due to non-Newtonian flow effects of the coating. The conical dies, used in open-cup applicators, were calculated to produce a shear rate of 10,000 sec-1, at a speed of 1 m/sec.
Paek and Schroeder were among the first to publish results on the behavior of forced cooling of bare glass fibers, during draw, to prevent deterioration of the coating. This doubled the allowable drawing speed from 1 to 2 m/sec.
Paek increased the height and diameter of the coating cup to increase the ‘‘wetting time and minimize the air-trapping effect in the interface between the coating and the fiber.’’ These modifications facilitated a coating speed of approximately 5 m/sec.
Kimura et al. reported on the effect of fiber temperature, forced convection of liquid resin, and viscosity of coating. They found that the fiber temperature had to be maintained at less than 50°C when applying a viscous coating, with a standard open-cup die.
A fiber temperature of up to 120°C could be tolerated when a forced convection of liquid was brought about by the use of a rotating rotor within the die. Draw speeds could be increased when the viscosity of coating was decreased from 2500 to 1200 mPa•s.
Geyling et al. reported on their development of a pressurized applicator, wherein the coating was injected radially from a cylindrical plenum and filled a thin bore at the applicator.
The thinner coating channel stabilized the coating application and facilitated application speed of approximately 2.5–3.0 m/sec. Pressurized die construction evolved during the early 1980s and became the standard coating application tool, because of the improvement it offered in coating circulation and meniscus stability.
Kassahun and Viriyayuthakorn described the improvements offered by pressurized coating application, 40–80N/cm2, towards bubble-free coatings on fiber.
Paek and Schroeder reported substantial improvement in high-speed drawing up to 12 m/sec. Fibers were determined to have uniform 50-micron coating thickness and concentricity along a 50-km length. The fibers survived a proof test stress level of 0.7 GN/m2 (100,000 psi), and most survived stress levels of twice that level. Draw speed rates of 0.7 and 10 m/sec were found to produce single-mode fibers with identical optical properties.
Draw tower coating speeds ranged from 1 to 2 m/sec in the 1979–1981 timeframe. Improvements in optical glass technology began to yield larger preforms, which significantly improved production efficiency. As a result, fiber prices became more competitive, fiber consumption jumped, and the demand for faster drawing technology was created.
In 1985, Sakaguchi and Kimura published one of the first reports on pressurized coatings being applied to fiber at speeds up to 20 m/sec.
Paek and Schroeder found that for an open-cup applicator, the coating thickness remains constant as the draw speed increases. They found, however, that with a pressurized applicator, thickness depends heavily on the draw speed and decreases as the speed increases.
The two primary controls for fiber coating application consist of temperature and pressure. UV-curable coatings have a nonlinear decrease in viscosity with increasing temperature. Viscosities drop precipitously during a temperature increase from room temperature to 45°C.
Increasing temperature further to 55 and 65°C produces diminishing decreases in viscosity (Fig. 4.11).
It is important to have knowledge of the rheological properties of optical fiber coating resins at high shear rates. Steeman et al. described this relationship using the well-known time–temperature–superposition technique to avoid limitations of traditional direct measurement steady state methods, such as viscous heating in capillary viscometers and torque limitations in rotational viscometers.
A typical viscosity versus shear rate plot of a fiber optic coating is shown in Fig. 4.12. Fiber coatings exhibit shear thinning behavior, whereby the viscosity is Newtonian at low shear and decreases with increasing shear rate. The shear thinning behavior is generally controlled by the oligomer in the coating. Fiber optic coatings exhibit viscoelastic behavior.
1. Coating Cure Speed Measurement Techniques
During the mid- to late 1980s, coating cure speeds were increased through design improvements. Many of these improvements came from the availability of new coating raw materials. As cure speeds improved, so did the test methods employed for their measurement.
An early method monitored the development of modulus in free coating films as a function of UV dose, or more appropriately termed UV energy density. Typical curves showed a rapid rise in modulus at low UV energy density and then a slow rise after further exposure.
Some coatings did not reach their ultimate modulus until exposed to an energy density of 2 J/cm2 or greater; 3.5 J/cm2 was chosen as the standard energy density for determination of a cured film’s physical properties: tensile strength, elongation at break, and modulus at 2.5% elongation.
It is typical to measure the cure speed in terms of the dosage required to cure the coating to 95% of its ultimate modulus, as shown in Fig. 4.13.
As demand for faster curing coatings grew, and with the advent of faster curing monomers and more efficient photoinitiators, UV energy densities of 1 J/cm2 or less were required to achieve fully developed physical properties. Around this time, another analytical method became popular for the characterization of cure speed.
Fourier transform infrared (FTIR) spectroscopy had long been used to identify chemical species and to follow chemical reactions. In the 1970s and 1980s, a number of papers were published, which described how this technique could be used to follow the disappearance of absorbance peaks corresponding to acrylate unsaturation.
Julian and Millon reported on how FTIR could be used as a quality-control test for monitoring UV-curable coating cure speed. However, they cautioned that UV doses required for property development were different from those required to cause disappearance of acrylate unsaturation, suggesting that ‘‘loss of acrylate unsaturation is only one of a number of mechanisms which lead to the development of a properly cured coating film.’’
While the FTIR cure speed measurement method related more to the inherent chemical crosslinking taking place on UV exposure, the development of realtime DMA resulted in a more time-efficient means to evaluate the property development of a coating.
Coating cure is dependent on the peak intensity, emission spectrum, temperature, and geometry of the lamps to which it is exposed. It is also dependent on the coating’s absorption of incident UV radiation. It has not been possible to exactly measure the environment encountered by the coating as it is exposed to UV energy on the draw tower.
However, it is possible to measure both the reacted acrylate unsaturation (RAU) on cured fiber coatings and the in situ modulus of coatings cured on fiber. Therefore, some approximations can be made for the conditions that produced these types of properties.
Other cure measurement analytical tools include monitoring of glass transition temperature development, dielectric relaxation, and differential scanning photocalorimetry.
2. Cured Properties of Coatings on Fiber
Hussain employed a torsional pendulum technique comprising small coated optical fiber specimens to determine the suitability of coatings and their processing parameters. Using this technique, he was able to compare the shear modulus properties of different primary coatings as cured on fiber.
Frantz et al. evaluated a number of techniques for determining the extent of cure on optical fibers. These techniques included infrared spectroscopy (FTIR), differential photochemistry (DPC), solvent evaporation rate analysis (ERA), solvent extraction, water/solvent soak, dielectric measurement, hydrogen generation, weight loss, oxidation onset, abrasion resistance, coating strip force, and coating pullout. FTIR, DPC, ERA, and solvent extraction were found to have the greatest sensitivity and reproducibility for measuring the degree of cure of a given coating on fiber.
In situ modulus is a useful test for measuring the modulus of the coating on the fiber. It has been observed that the in situ modulus could be substantially lower than the flat film modulus even at high cure degree. Small variation of the in situ RAU can result in large differences in in situ modulus. Even with the same degree of cure, large variations of polymer networks may be formed, largely because of variations in the intensity of UV light and the resultant heat from the drawing process.
The shear modulus of primary coatings can be measured in situ by pullout test using TEM or DMA (Fig. 4.14).
Traditional techniques involved the use of an adhesive to immobilize the fiber on a substrate cylinder. A new method using mechanical clamping has the advantage of easy sample preparation and very small data scattering. Steeman et al. measured the primary modulus on a DMA by applying an oscillating shearing stress to the coating.
The in situ secondary modulus can be measured by applying an oscillating tensile stress to a stripped coating tube on a DMA. The coating tube is prepared by separating the glass fiber from the coating, in liquid N2. Because the modulus of the primary coating is several orders of magnitude lower than that of the secondary coating, it can be assumed that its effects are negligible, so the measured Young modulus of the coating tube can be assumed to come from the secondary coating.
Strip force provides an indication of the ease of stripping the coating from the glass. The coating must have adequate adhesion to the glass while being mechanically strippable for splicing and connectorization. There is little or no correlation between strip force and primary coating adhesion. The modulus, deformation, and failure mode of the secondary coating are the dominating factors that influence strip force.
Pullout force measures the force required to pull a glass fiber out from the coating layers. It gives a measure of the adhesion of the primary coating to the glass. The sample is glued to a support, prepared by gluing the fiber onto a support tab. The application area, the uniformity of the glue, and drying time can significantly affect the results. A mechanical clamping method has been tested, but the measured force inevitably is a complex result of adhesion force and friction force.
The FTIR/ATR technique can be used to measure cure of both the primary and the secondary coating on fiber. In terms of %RAU, the cure of the inner surface of primary coating is measured by cutting through the coating layers to the glass, removing the glass fiber, and laying the coating flat on a diamond attenuated total reflectance (ATR) crystal, thus exposing the primary. The degree of cure of the secondary coating can be measured by directly measuring the %RAU on the fiber surface.
3. Test Methods for UV-Curable Liquids and UV-Cured Films
Julian reviewed many techniques used to characterize the quality of radiation-curable coatings. For UV-curable coatings, it is also important to characterize the quality of the raw materials that are used to formulate the coating.
For coatings to be of commercial utility, they must have sufficient liquid stability in a variety of containers (plastic bottles and lined steel drums) and in heated application dies and coating reservoirs. Pasternack et al. mentioned that changes in properties could be ‘‘evaluated by accelerated aging of the liquid coating at temperatures high enough to cause changes within reasonable times but still below thermal degradation levels.’’ A typical temperature for such testing is 54°C (130°F).
Chawla et al. published a very thorough study on the factors that affect measurement of the mechanical properties (tensile strength, elongation, and modulus) used to characterize UV-cured films of optical fiber coatings. The study led to a number of improvements in equipment (more sensitive load cells and more accurate gauges for specimen measurement) and environmental controls (temperature, relative humidity, and cured film preparation parameters). These improvements were important to both characterizing the properties of new coatings and providing accurate and precise measurements of batch-to-batch variation in commercial coating production.
The modulus of primary coatings is typically calculated by taking the slope of the stress–strain curve from the origin to 2.5% strain (i.e., the segment modulus technique). For secondary coatings, secant modulus is typically measured and calculated from a tangent to the single point of the stress–strain curve at 2.5% strain. Chawla et al. found that there was no difference in which approach was used to measure the modulus of secondary coatings, but the segment modulus was found to give greater accuracy for primary coatings.
For optical fiber coatings, it is customary to characterize the mechanical properties of a coating by running a temperature sweep on a dynamic mechanical analyzer. A coating film is subjected to a controlled oscillating strain at a fixed amplitude and frequency, whereby the elastic modulus \(E'\), the viscous modulus, \(E''\), and tan\(\delta\) are measured. Information regarding crosslink density and glass transition can be obtained from the plot. There are a variety of methods for measuring the Tg of a coating. The peak tan\(\delta\) is often referred to as the ‘‘glass transition point,’’ but because the glass transition is a region rather than a single point, it is customary to quote the temperatures at which \(E'\) = 1000MPa and \(E'\) = 100 MPa. Typically, polymers exhibit glassy behavior at low temperatures and rubbery behavior at high temperatures. The equilibrium modulus, measured at the minimum of the \(E'\) curve, is proportional to a coating’s crosslink density. An example of DMA curves for a fiber optic primary coating is shown in Fig. 4.15.
As mentioned earlier in this chapter, DMA curves are useful to characterize the aging behavior of a film by inspecting the changes between spectra taken before and after environmental exposures.
Szum observed that coatings with glass transition temperatures above room temperature, notably secondary coatings, exhibited changes in physical properties over time. DMA analysis showed that these changes in properties were due to relaxation of the polymer network, rather than chemical changes. In general, it was observed that a high Tg crosslinked polymer’s properties were dependent on heat history. If a UV-cured film was quenched (i.e., quickly cooled after cure), it was found that the modulus of the film tended to increase over time, through densification, as the cured coating relaxed.
Julian reviewed a number of characterization methods for UV-curable liquids and cured films. These included viscosity measurement, cleanliness (freedom from particulate contamination), molecular weight determination, cure speed, mechanical property measurement (tensile properties and DMA properties), and thermal stability.
Julian cited three goals for equipment utilization, as mentioned in the paper: to provide a thorough characterization of materials, both raw materials and finished coating formulation; to perform analyses at reasonable cost; and to improve product consistency.
4. Coating Adhesion
Coating adhesion was thought to be a prerequisite for retention of fiber strength. This is why many optical fiber coatings in direct contact with glass have contained some type of adhesion-promoting agent since the time of Vazarani’s patent. Silane coupling agents have typically filled this role. Many of these silane molecules contain three alkoxy silane groups on one end of the molecule and an organofunctional terminus of some type at the opposite end. The organofunctional group is selected to react with the coating network and, thus, be tied to the glass through the alkoxy silane groups. Silane coupling agents improve the adhesion of coatings in wet and dry environments. In 1982, Plueddemann produced an excellent reference on the chemistry of silane coupling agents.
Overton and Taylor stated that reliable adhesion of the primary to glass is necessary to protect against coating delamination. Coating delamination decreases the fiber’s robustness and degrade its performance. They reported that adhesion is achieved though intermolecular forces, achieved by intimate contact between coating and glass, and chemical bonding afforded by silane coupling agents.
The adhesion to glass for some early fiber primary coatings was very high and made coating removal for splicing and terminations difficult. Chemical removal was often the preferred method to avoid damaging fibers in the field. As cable fiber counts began to increase, outside craft personnel spent more and more time making fiber connections. Amos et al. reported on a primary coating with improved lower stripping force and its suitability for mechanical removal without damage to the underlying fiber.
Overton and Taylor described a coating-removal test that resolved the problem of measuring the adhesive strength between the primary coating and the glass. The pullout test revealed that the temperature dependence of the pullout force is similar to that of the primary coating’s modulus. Continued decline in pullout strength at temperatures much greater than the coating Tg was attributed to the relaxation of hoop stress in the secondary coating.
Summary
Many aspects of the protective coatings have an impact on the final optical fiber performance, including the following:
| Primary coating feature | Performance impact |
| Composition | Fiber strength/fatigue, long-term durability |
| Reactivity | Drawing speed |
| Refractive index | Concentricity monitoring |
| Viscosity | Ease of processing |
| Tg | Low-temperature microbending loss |
| Modulus | Buffering (protection against microbending); strippability (residue left on fiber) |
| Adhesion | Delamination resistance, strippability |
| Secondary coating | Performance impact |
| Composition | Long-term durabilty |
| Modulus | Shielding (protection against lateral load), strippability, abrasion resistance, handling ability |
| Surface properties | Ink adhesion, fiber friction |
| Crosslink density | Chemical resistance |
The next tutorial discusses in detail about stimulated transitions in the classical oscillator model.