Backing Material Properties of Covalence Shrink Sleeves (formerly Raychem shrink sleeves)
When attempting to determine physical properties of the backing of heat shrinkable sleeves, it is incredibly important that you are aware of the fact that different manufacturers products will exhibit dramatically different qualities.
This is incredibly overly simplified and any true in depth analysis would require a much more thoroughly educated communicator. But for the purpose of this specific discussion, I am going to break backing types down into two different categories: High Density and Co-Polymer heat shrinkable backings.
Aside from heat shrinkable backing construction / chemical make up; it is also important that any potential user or specifier of heat shrinkable sleeves understand that there are also two commonly used crosslinking methods out in the world today; radiation crosslinking and chemical crosslinking.
In recent months I've seen more and more field joint coating specifications coming across my computer that specifically say: products from India or China will not be accepted. One of the (many) reasons for this is that chemical crosslinking is very often the crosslinking method of choice in those countries. Why is that? Thanks for asking - that is because chemically crosslinking is dramatically less expensive than radiation crosslinking is. Less expensive cross linking method (plus lower quality backing, lower quality adhesives and lower cost labor) leads directly down the path to 'lower quality product.' And after a few field problems and coating failures -- the path generally leads to a "I will not accept _______" clause on coating specifications and RFQ's.
So, what advantages is a high density backing going to have over a co-polymer backing? What advantage is a radiation crosslinked backing going to have over a chemically crosslinked backing? Interestingly, the differences strengths and weaknesses are going to be pretty similar in both cases, so using a low cost co-polymer backing with a chemically crosslinked backing is going to act like a double whammy (a double whammy isn't good).
By being selective about the backing construction and cross linking method you are essentially getting a product with better: penetration resistance, cut through, abrasion resistance, tensile strength, Shore D hardness, shrink force, heat/flame resistnace and impact resistance. Let's look just a tiny bit closer at those characteristics below:
Penetration resistance: how well does the material withstand attempt to penetrate it? That pretty much sums it up. How does this come up in a practical sense? During backfill, there is a chance that this could come into play. It would also come into play in a case where the pipeline is resting on something (sharp-ish). The weight of the pipeline, plus the dirt, would be squeezing the coating between a rock and a hard place (so to speak) and penetration resistance can become an incredibly important characteristic.
Cut through: similar to penetration resistance, but picture a 'gash' instead of a hole being poked in the coating. This could happen when the line is being lowered into the ditch, or it could happen during a road bore or directional drill, or it could happen due to normal pipe movements through out the life the pipeline (picture the coating being forced to 'slide along' a sharp-ish edge due to one of the forces listed there).
Abrasion resistance: How does the backing hold up to normal back and forth pipe movements along the soil, rocks, roots, whatever? Over the course of months or years, is the backing thickness of the sleeve going to be worn away by this movement? Will you eventually end up with a backing that is only 10 mils thick? How about if your line will be involved in a 2000 foot road bore? Is that going to compromise your coating? Good abrasion resistance is important, don't let anyone fool you.
Tensile strength: how much force does the backing take before it breaks/splits? This is a little like cut through and penetration resistance, but instead picture a tremendous amount of weight putting only one small section of the coating into sheer. How much force will it take before the coating gives?
Shrink Force: how much force is involved as the backing shrinks down to the pipe? This is amazingly important. Did you know that when considering the true purpose of a shrink sleeve, you have two components: backing and adhesive. The backing has two functions: 1) as a vehicle to get the adhesive onto the pipe and 2) to physically protect that adhesive for the life of the line. As a shrink sleeve is installed, the adhesive flows and fills....thanks to the force of the shrinking backing exerting force on the adhesive and assuring that it fills every little area. Without the proper shrink force, your adhesive is likely to be inconsistently distributed. That isn't good. You do not want an inconsistently distributed adhesive on your pipeline underneath your backing.
I'll stop there for now. But let me just sum up by saying that choosing which shrink sleeve manufacturer you use is incredibly important. There are a ton of different methods for some folks out there to reduce their costs by modifying their products in a way that is likely not going to be obvious...until they've been installed and buried for a few months. Sure, you will likely be on to the next project; but ultimately, someone is going to have to answer for those decisions to compromise coating quality in order to save a few pennies.
Post a Comment