Epoxy. Exploring core cracks during the curing process.

When I was asked to help with the 2023 FIS catalog this year, I was assigned the exciting category of epoxy. You’ve got me, that’s a smidge of sarcasm. Lol.

I figured this would be a straightforward and simple endeavor.  Come to find out there is a bit more to consider when choosing the correct epoxy than I originally gave credit. When I first jumped in, my knowledge led to the well-known basics, such as blue dyed epoxy is ideal for jobs that require polishing. UV curing epoxy is handy for mass production. Quick cure is exactly that, quick and saves time.

My first step was to jump on websites and check out the spec sheets for each individual epoxy that we offer. There began the turning of the gears in my head.  I began to wonder. Why exactly are there so many different curing schedules for each individual epoxy?  So, I did as any inquisitive mind would, I jumped on Google to see if I could grease those squeaky gears.  I came across individuals that claimed that they were having issues with transmission and signal loss after curing epoxy for assemblies. This seemed to happen with multimode fiber. Specifically, 62.5/125 um multimode, stating that the multimode fiber was victim to core cracking.  My brain immediately cued to C&C Music Factory’s 1991 hit. “Things that make you go hmmm.”

I wanted to explore more about epoxy and core cracking, and I am happy to share.

Core Cracking.

Here are a few images of a core crack.

Core cracking occurs infrequently. These core cracks are very hard to detect and require backlighting to be seen. The cracks are curved and extend thru the diameter of the core but not into the cladding of the fiber. The backlighting allows us to clearly see the defects. The cracks that occur can have a significant impact on the optical performance of the fiber, reducing the transmission of the light and leading to significant signal loss. Core cracking also will create higher attenuation if not caught and will require higher power requirements.

What is the Cause?

Fiber optic core cracking occurs during the curing process of the epoxy.  This cracking is caused by the tensile forces that are generated during the curing process, as the epoxy contracts it puts pressure on the outer diameter of the fiber. The core of the multimode fiber is more susceptible to cracking than the cladding of the fiber. The epoxy contracts putting pressure on the fiber and the core gives way. Single-mode fiber is rarely affected by this core cracking because the core of the fiber is more strongly condensed and smaller in diameter.  Therefore, core cracking of this specific kind during curing is almost solely found in multimode fiber.

NASA had an example of issues with core cracking in multimode fiber when terminating connectors using TRA-CON F253. The TRA-CON F253 did meet NASA requirements for outgassing but did have issues with termination at times. They found that the epoxy would shrink from 3% to 7% depending on the mass. Due to this shrinkage and the higher temperatures needed to cure this epoxy, (100 C for 15 minutes), the multimode fiber was much more susceptible to core cracks.  NASA eventually tested and moved to epoxy that cured at a lower temperature but still maintained the same outgassing properties needed.  The epoxy cured at 80 C for 2 hours to maintain those passing outgassing standards, and the lower temperature cure schedule overcame the core cracking. [1]

Identifying Fiber Optic Core Cracking.

Identifying fiber optic core cracking can be challenging, as the cracks are often small and can be difficult to see with the naked eye.  Some of the common methods for detecting fiber optic core cracking include microscopy, interferometry, and scatter light analysis.  These techniques use light to examine the surface of the fiber optic core and detect any cracks or defects.

Preventing Fiber Optic Core Cracking.

The prevention of core cracking is essential to ensure the reliability and performance of fiber optic systems.  Some of the ways to prevent core cracking include:

• Proper storage and handling of components and epoxy to minimize the risk of the damage during installation.
• Proper preparation of the fiber optic core prior to termination, including cleaning and degreasing to remove any contaminants that may interfere with the curing process.
• Careful mixing and application of epoxy, to minimize the risk of cracking during the curing process.
• Proper temperature control.

Ideal Temperatures for Curing Epoxy to Avoid Core Cracking.

When given the option to cure epoxies at different curing schedules lean towards using the schedule with the lowest temperature.  Curing at temperatures of 90 C or less is suggested to minimize core cracking.  It is important to monitor the curing process carefully and ensure the epoxy cures evenly. This can be achieved by using proper curing techniques, such as oven or heat lamp curing.

Check out our entire line of epoxy at https://www.fiberinstrumentsales.com/fiber-optic-consumables/epoxy-adhesives.html.

We have a large selection of epoxy in several cure forms. If you’re looking for quick cure, room cure, or blue dye epoxy we have offerings from our own FIS brand. We also offer many other options from popular brands such as TRA-CON, LOCTITE, and Epo-Tek. A friendly reminder, please don’t forget the importance of proper preparation and cleaning.  FIS can be your one stop shop for cleaning products, consumables, tools, epoxy, heat curing ovens and accessories.

If you have questions about cure schedules check the spec sheets that are attached for each product, as we provide all cure schedules from each manufacturer.

Need further assistance?  Don’t hesitate to reach out to any of the sales staff via live chat through our website, by phone or email.

Thanks for reading!

REFERENCE: [1] Melanie Ott and Patricia Friedberg, NASA Goddard Space Flight Center, and Barry Siroka, Fiber Optics Center, “Fiber Optic Epoxy to Alleviate Core Cracking During Termination,” https://photonics.gsfc.nasa.gov/tva/meldoc/foepoxy.pdf