About UV-Curable Fiber Optic Materials

Got a question about UV-curable fiber optic materials?  Maybe we've answered it here.  If not, feel free to contact us with your question.

Yes.  All UV-curable inks incur pigment settling over time.  To ensure uniform color, it is recommended that Cablelite® inks be rolled at 8 rpms in their original bottles for a minimum of 4 hours prior to use.  Re-rolling is suggested if the bottle is not consumed within 24 hours.

The concentricity of a coating to glass fiber is controlled by the inking or coating dies.  These dies are self-centering:  forces inside the die naturally push the fiber to the center of the die (most energy-efficient).  The most common concentricity problems are caused by dies that are dirty or damaged.  These flaws interfere with the roundness of the die, pushing the fiber off center.  For wet-on-wet coating systems, the problem is even more complicated.  In these compound die sets, the primary (inner) coating die must be set concentric to the secondary (outer) coating die.  This takes considerable skill and experience and becomes even more difficult when dies are dirty or damaged.

There are three tests that can be used to determine if the Cablelite® or DeSolite® material is sufficiently cured:

Solvent Rub Test:  This test is only used for testing the cure of inks.  A strong solvent (MEK) is applied to a cotton cloth or pad, which is then wrapped around the fiber and rubbed back and forth to remove the ink.  Measuring the number of rubs it takes to remove the ink indicates the degree of cure.  Properly cured inks should pass 200 MEK Double Rubs.   This is a fast, easy test, but the reproducibility is not good.

Infrared (IR):  This test, referred to as "IR," "Percent RAU" or "FTIR," uses Fourier Transform Infrared Spectroscopy (FTIR) to determine the percentage of Residual Acrylate Unsaturation (%RAU).  A liquid IR spectra is determined before and after complete curing, indicating the degree of cure for a fiber sample.  The test measures the loss of acrylate unsaturation in response to various levels of UV radiation or to a control.  This test is used for UV-curable inks, coatings and matrix materials.

Percent Extractables:  For this test, a strong solvent is heated and used to extract soluble portions of the UV-curable coating.  The degree of cure (as a percentage) is determined by calculating the ratio of non-extractable content at a given exposure (below that required for a full cure) to the non-extractable content at full cure.  A coating is considered fully cured when repeated exposure to UV light does not result in significant decreases in extractable content.  This test is time-consuming (it can take 12 hours to run) and therefore is becoming less popular.  Further, it is only an accurate test if the amount of extractables at full cure is known to the tester.

Natural (coated) fiber can vary in color from completely colorless (water white) to slightly yellow.  This variation is primarily due to the color of the components used in the composition of the protective fiber coating.  Components (such as photoinitiators, stabilizers, monomers, etc.) which may possess a slightly yellowish color often correlate to better fiber performance and are chosen specifically for their unique performance benefits (i.e., stability, reactivity, adhesion, etc.).

The recommended optimum film thickness of fiber for Cablelite® 751Inks is 3-5 microns.  These inks are designed to provide glossy, vibrant colors at low, cost-effective film thicknesses.  The colors are easily distinguishable from one another and meet the EIA-359-A color specification.

Yes.  Oxygen inhibits the cure process.  In worst cases, the inked fiber surface remains very tacky and can cause significant fiber attenuation.  Nitrogen provides a relatively cheap and readily-available method of purging air from the cure chamber during processing.

To determine how much nitrogen is needed, refer to your line speed.  The faster the line speed, the more nitrogen required.  In the past, DSM Desotech recommended an average flow of 20 liters per minute for line speeds of 400-600 m/min.  Now that average line speeds are much faster, nitrogen flow must be increased proportionately.

When injecting nitrogen into the cure chamber, care should always be taken to provide adequate ventilation for both the extraction of the gas as well as for the equipment operators.

There are a number of variables that affect the curing of UV-curable materials.  The two most important are:

The amount of UV light that reaches the material.  For proper cure to occur, adequate exposure to UV light is necessary.  Keep in mind that the following factors can affect the amount of UV light reaching your material:

• The thickness of a coating (particularly important with pigmented systems)
• Both the age and UV output of the bulb
• The time of exposure
• The conditions of the quartz tubes
• The condition of the reflectors
• Positioning of the coating (must be in the focal point of the reflectors)

The atmosphere at the outer surface of the coating.  With all UV-curable materials, a successful cure depends upon the generation and propagation of free radicals when exposed to light.  Oxygen interrupts the cure reaction by trapping free radicals rather than allowing them to propagate.  The more oxygen present, the more inhibited the cure will be.  To prevent this from happening, oxygen must be displaced from the cure environment.  This is typically done with the injection of nitrogen into the cure chamber.

Quartz tubes can be cleaned either with heat or with a cleaning solution.  High temperature muffle furnaces set at 500ºC for four hours are a preferred method because tube handling (and therefore breakage) is minimized.  However, there are also cleaning solutions available for overnight soaking.  DSM Desotech has used Contrad 70, available from Fisher Scientific.

Quartz tubes should be changed before any discoloration is noticeable.  Tubes that are not "water white" should be cleaned or replaced.  On inking machines, the best way to ensure that the tubes are clean is to change them with every shift change.  On draw towers, quartz tubes should be changed whenever the draw is stopped.