Shrink Sleeve Mastic Thickness and the Girth WeldThough not a question that comes up too often these days, during the time when heat shrink sleeves were exploding in the pipeline coatings market (and other markets as well) there was often a contentious debate about mastic adhesive thickness....and what the minimums should be in order to assure proper mastic flow and filling around the weld bead...and at the step down areas from the factory applied coating to the bare steel. Below is one such analysis completed in 1979.
Scope: This report presents data and observation on the ability of various corrosion preventive products used in joint protection to fill and seal over and around a weld bead. The results of this analysis will also cross over (based on factory applied pipeline coating thickness) to shed light on the required thickness of adhesive on different pipeline coatings to properly fill at the step down area from factory applied coating to bare steel.
Test Procedure: Samples were applied to a 4.5" diameter steel pipe with a circumferential weld bead. Test samples were applied in accordance with the manufacturer's installation instructions. After 24 hours, pipeline coating test specimens were examined for voids or leak paths around the are of the weld bead. During this test, weld bead dimensions were .100" high and .350" wide.
#1: Heat Shrink Tubular Sleeve with total adhesive thickness of 25 mils
#2: Heat Shrinkable Tube Sleeve with total adhesive thickness of 30 mils (<- Covalence)
#3: Heat Shrink Tube Sleeve with total adhesive thickness of 10 mils
#4: Heat Shrinkable Wrap Around sleeve with total adhesive thickness of 70 mils (<- Covalence)
#5 Heat Shrink Wrap Sleeve with total adhesive thickness of 25 mils
#6 Cold Applied Tape with total adhesive thickness of 30 mils
#7 Cold Applied Tape with total adhesive thickness of 40 mils
Observations After Testing
#1: With a 25 mil supplied adhesive thickness -this shrink sleeve lacked enough flow and volume to fill areas along weld beads leaving a continuous leak path under the shrink sleeve.
#2: (30 mil) The sealant thickness and flow properties were sufficient to obtain complete filling of the weld bead area.
#3: (10 mil) The mastic showed good flow properties but lacked enough sealant thickness to adequately fill around the weld bead area.
#4: (70 mil) There was sufficient amount of sealant present which flowed well upon heating to give a void-free seal in the weld bead area.
#5: (25 mil) Insufficient sealant thickness plus low flow properties contributed to a void line along the weld bead.
#6 & #7: (30 mil and 40 mil) Both of these samples exhibited continuous void lines along the sides of the weld bead. It has been found that during the application of cold applied tapes, there is a bridging effect which traps air to cause voids along the weld bead. Most sealants have limited flow when applied at room temperature and do not fill well around the weld bead.
Conclusions: The table above lists the various samples tested and gives a comparison of the sealant thicknesses. Sample 1, 2 and 3 are tubular heat shrinkable sleeves (supplied in the shape of a tube) which, due to their recovery ratio, will have variable sealant thicknesses depending on the degree of recovery.
Results indicated that an adhesive thickness of 30 to 35 mils is needed to adequately fill around the weld bead on the test fixture.
As indicated in several of the observations, a second factor which is equally important is the ability of the sealant to flow during installation. Samples 1 and 5 both showed low flow characteristics when heated; this contributed to the lack of filling. Samples 6 and 7 also showed very low flow because of being cold applied. As noted in the observations, this is a common and known shortcoming of cold applied tapes.
In general, a product must have a balance of adequate coating thickness (approximately 30 mils or above) and good flow characteristics during installation to deliver a void free corrosion protection coating to the weld bead.