How the Glass Optical fibers are coated with UV-Curable Acrylate

The acrylate coating process is more important in a drawing process as coating materials and their application can influence fiber loss and strength. The coating should be applied within a clean environment immediately after the fiber comes out of the furnace to reduce the contamination of the surface of the silica fiber. It must be applied concentrically with sufficient thickness without damaging the pristine surface of the fiber and must be solidified very rapidly before the fiber reaches the take-up capstan.

Acrylate coating is applied to protect the glass fiber from external abrasion and contamination and thus preserve its intrinsic strength. Coating process plays an important role in the reduction of micro-bending losses which occur due to the geometry variation over distances of the order of wavelength of light. Colored coating is also applied by some of the basic optical fiber manufacturers to avoid coloring stage at the time of cable manufacturer.

Low Young’s modulus coating has been shown to be effective in reducing micro-bending by cushioning the fiber from non-uniform external stresses. Coating defects such as voids, lumps and uncoated sections also introduce micro bending losses. The first coating is with soft UV curable acrylate and compared to the Second layer which is of hard acrylate.

The applied acrylate layers are cured by passing through the ultra violet chambers. Inside the UV chambers the acrylate layers will get polymerized and form a long chain. The UV is used for generating the free radicals required for polymerization reaction. The presence of Oxygen must be avoided while the free radical reaction happens. Ultra violet rays breaks down the double bonds in acrylate molecule and thus produce free radicals. These free radicals will react more easily with Oxygen and thereby killing the whole purpose of polymerization. Therefore it is necessary to avoid the presence of Oxygen at the UV chambers.

Nitrogen is the cheapest gas which will not react with free radicals inside the UV chamber and thus allow the polymerization to happen. Polymerization means saturation of bonds. When the polymerization happens at a level of 90% to 95%, this is considered as a good level of reaction. The curing level is measured in terms of % RAU. This means the percentage of Reacted Acrylate. Curing level, or % RAU less than 90% may generate sticking of the layers of optical fiber on the spool. This is due to the uncured acrylate molecules. This will create process problems and may not be able to use further.

The optical fibers are taken for proof testing or screen testing. This is nothing but a tensile test for drawn optical fibers. Proof testing of optical fibers will ensure the strength and reliability of optical fibers in the subsequent process of cable making and laying. During proof testing the weak portions of the fiber will break at the machine. There is a bit controversy over the proof testing as the proof testing itself induce micro cracks on the fiber, which will break in the cabling process.