Exotherm! Wait, What? Why Do I Need To Know This?

Exothermic Action In CIPP Liner

If you are in the lining business and don’t know what exotherm is, stay tuned and finish this post. All of you that have lined pipe with the CIPP process have witnessed exotherm. Exotherm describes the heat given off by the combining of compounds that are forming new materials. As resin forms rigid solid chains while it is transforming from a liquid to a solid, heat is given off by the process. Uncontrolled, the heat generated in forming these chains can reach temperatures in excess of 400F for some resins. For those of you that have lined pipe, you’ve probably seen the left over resin left in the bucket get hot and cure. Depending on the amount of left over resin, the greater the amount, the hotter the exothermic process becomes and some of you have seen the cracked resin hardened in the bottom of the bucket or the bucket catch fire. I’ve written in previous posts about letting resin get too hot during exotherm and the subsequent stress cracking that can occur if the temperature runs wild. This stress cracking leads to the possibility of future failure by allowing roots to grow through these cracks. So our challenge is to control exothermic temperature.

“But, you say, I’ve lined pipe for several years and let the liner cure without any regard to controlling the temperature and I’ve had no problems. In fact, all of my liners look perfect with no evidence of any stress cracking or other problems with the liner”.

So here’s why your liners look okay and most likely are perfect. With the catalysts, promoters, hardeners and other initiators that start the forming of chains turning the liquid resin to a hardened finished product. We are working with vary in thickness from 2mm to 6mm in the lateral lining business. These sizes are relatively thin and during an ambient curing process are regulated by the ground temperature, which dissipates heat and controls the temperature. That’s the reason that lining in a cold ground condition, ambient curing takes longer than when the ground is warmer. Many of you that line a lot of pipe, have opted to use heat to initiate the curing process faster. You’ve either chosen hot water or steam as the medium of choice for this process. You are using the heat source as a vehicle to overcome the heat lost to the ground temperature by heating the inside of the liner and letting the heat wash in turbulent flow throughout the liner. Here’s

Image of Exothermic Process

where things get tricky. We’ve set the hot water system up to cycle when the temperature reaches a set point, not allowing the temperature to “run wild”. The heater senses return temperature at or above set point and cycles on and off as needed to maintain that temperature. This is automatic. Steam on the other hand takes two separate components and ties them together. To control exotherm with steam, the operator must manually increase airflow and retard steam production as the temperature rises.

I’ve included a thermo GIF of exotherm inside a pipe above. This GIF shows a pipe curing at ambient temperature and the temperatures recorded at various points. Notable is the exothermic process. It doesn’t occur evenly throughout the liner but various locations are at different stages of the process, some curing faster and some curing slower. Also of note was the controlled temperature we used for the pipe being being cured as well as the resin temperature during the mixing and impregnating of the tube. All things were equal, yet the actual exotherm stages were variable during the process. This tells us that slight differences will speed up or retard exotherm and thus curing of liners. It also shows the exothermic process “moving” from hot spots toward colder areas.

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