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Wednesday, December 2, 2015

Heat Shrink Sleeve Coating Thickness

Coating Thickness of Heat Shrink Sleeves

      When determining coating thickness of different heat shrinkable sleeve options, there are several things you will want to consider.  I will look at each individually below and then sum up at the end.  Those three components are:  Backing Thickness; Adhesive Thickness and (where applicable) Epoxy Thickness.  First up:  Backing Thickness.

     It is important to note that much literature for heat shrinkable sleeves will contain two different thickness for backing.  These two things are 'as supplied' and 'fully recovered.'  Some folks find these confusing so let me explain.  First of all - let's change gears and look at a really big rubber band like you might use for rehabilitation of a shoulder injury.  At rest (fully recovered) that rubber band is going to be at its thickest point.  Now stretch that rubber band out as far as it will go (or as far as it can go) and you will notice that it is thinner when stretched out (this can illustrate supplied thickness).  Relax the rubber band again and you will see the thickness grow back to its resting state.

     The supplied thickness of any particular shrink sleeve is that 'stretched' out version of the rubber band.  The fully recovered thickness would be that rubber band at rest.  Because of the nature of heat shrink sleeves:  acting as a vehicle for applying adhesive to a substrate and pressuring that adhesive into all cracks, voids, imperfections and step down areas, that heat shrink sleeve really needs to be able to shrink to something smaller than the substrate diameter.  So a shrink sleeve on a 6" pipe really needs to be manufactured in such a way that it will fully conform to that 6" pipe.  How is that done?  By making a shrink sleeve that would actually shrink down to something like 5.5"....IF IT WERE ABLE TO.  Because it is unable to on a 6" pipeline, the shrink sleeve applies its pressure to the adhesive; forcing a good bond to the substrate and the PE jacket; and holding that pressure throughout the life of the pipeline.  Based on all of that -- the 'fully recovered thickness of the backing' that you will see on a data sheet is very often going to be slightly thicker than what you would actually see during an install.  In the field; heat shrink sleeves simply do not 'fully recover' because the substrate does not allow it.  Hopefully that makes sense.

     When considering the adhesive thickness; generally only one data point is recorded: adhesive thickness as supplied.  This is an excellent and accurate number when purchasing a proven, high quality product like Covalence (Seal for Life / formerly Raychem) manufactures.  What is there to discuss beyond that?  Plenty.  The adhesive actually gets quite a bit thicker during installation.  How does that happen?  Let me give a slightly oversimplified example:  say you have a 10 foot section of WPCT material.  Now say you shrink that down fully.  When you are finished; that WPCT section is going to be something closer to 7.5 feet long.  This is because WPCT is designed to shrink 25% to 31% (so in reality, that 10 foot section could shrink to just under 7 feet long).  So the surface area of the PE crosslinked backing has diminished by 25+%; and now the adhesive thickness that was covering 10 feet of material will not be covering only 7.5 feet of material.  In theory; an increase in thickness of 25%.  But it doesn't end there.  When the shrink sleeve is installed, one of the purposes of the shrink sleeve is to force adhesive out at the edges of the sleeve; creating an impenetrable barrier that water and oxygen will be unable to permeate.  So some of that 25% added thickness will be lost; but certainly not all of it.  How much is left?  That is a complicated question as it will be hinged on how snugly the sleeve was wrapped around the pipe; how properly the pipe was preheated; how much the shrink sleeve was heated and shrunk; how much adhesive might have been pushed around when rolling the overlap area of the sleeve; etc; etc.

     Finally, we can look at epoxy thickness where applicable.  Products that utilize an epoxy are generally:  HTLP60; HTLP80 and DIRAX.  With some products; epoxy thickness is critical and difficult to manage.  With stand alone epoxies it is not unusual that you need to work up multiple layers of epoxy in order to achieve 30 mils; 45 mils or even 60 mils of thickness in order to meet whatever end user specification you are working from.  That isn't the case with our product.  With our products we are generally looking for a thickness of epoxy that would be measured in microns.  Certainly visible to the eye.  Certainly covering every single square millimeter that the heat shrink sleeve will touch (and beyond in order to act as an inspection tool for construction foremen).  But there is no difficult thickness requirement and really not even a thickness gauge required. 

     So there you have it.  More than you ever wanted to know about heat shrink sleeve coating thicknesses.  Tomorrow I will follow up with a post outlining data from some of our actual products. 

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