Mastic
Mastic tape, wrapped into what a sleeve cannot bridge
The sleeve recovered down clean, and the pocket at the connector shoulder is still full of air.
A shrink part recovers onto a step and bridges it. Under that bridge there is air.
The pocket sits where the geometry changes: at a connector shoulder, at a lug palm, at a screen cut, at the shoulder of a gland. On a medium voltage joint the air in it partially discharges and erodes insulation from the inside, at a rate nobody can see. On a low voltage one it fills with water and stays wet. Sealed assemblies do not dry out.
So the void has to be gone before the shrink part goes over it. Filling it is the whole job of mastic.
Nothing on this page shrinks. Mastic is a cold applied butyl sealant on a release liner, worked into the void by hand pressure and nothing else. Gala Thermo Shrink Pvt. Ltd. manufactures it in three graded compounds, coded HBMT, HRMT and HSCM. They are supplied inside the same cable joint and termination kits as the shrink parts.
Two properties decide whether a wrap becomes one body or stays a stack of layers. Adhesion holds the first turn onto the jacket, the metalwork or the screen. Self amalgamation makes the second turn forget it was ever a separate turn. A wrap that has amalgamated has no interface left inside it to leak along. A wrap that has not is a spiral leak path at the pitch of your hand.
The manufacturer rates both as good on all three grades and publishes no number for either. That is a qualitative claim and it should be read as one.
Mastic tape — HBMT black, HRMT red and HSCM yellow
A cold applied butyl sealant supplied on a release liner, in cut strips and in rolls. It fills and water seals the voids inside heat shrink cable joint and termination kits. The manufacturer is Gala Thermo Shrink Pvt. Ltd.
- Black HBMT seals and weatherproofs LV and MV joints, terminations and telecom installations
- Red HRMT makes the watertight seal on MV termination breakout boots, non-tracking tube and lugs
- Yellow HSCM is the stress relief and void filling grade, for connectors and screen cuts on MV work
- Adhesion and self amalgamation both rated good across the three grades
- UV resistant, weather resistant and resistant to chemicals
- Two codes run to 25000 mm on a roll, HBMT-14 in black and HRMT-05 in red
| Type | Cold applied butyl sealant, non shrinking |
|---|---|
| Grades | Black HBMT, red HRMT, yellow stress control HSCM |
| Service temperature | -20 °C to +90 °C, identical on all three grades |
| Width | 20, 25 or 35 mm minimum, fixed by code |
| Thickness | 1.7, 2.0 or 2.5 mm, toleranced at ±5% |
| Supplied length | 100 mm to 25000 mm, fixed by code |
| Form | Strips or roll, decided by the code and not by the order |
| Dielectric strength | 6 kV/mm black, 10 kV/mm red, 15 kV/mm yellow, all minimum |
| Volume resistivity | 1 × 10^14 Ω·cm minimum on red, the highest of the three |
| Dielectric constant | 10 minimum on yellow, which is the point of that grade |
| Adhesion and self amalgamation | Rated good on all three grades, with no figure given |
| Environmental | UV and weather resistant, resistant to chemicals |
Selection chart, all sixteen codes
| Gala code | Grade | W min (mm) | L min (mm) | T ±5% (mm) | Form |
|---|---|---|---|---|---|
| HBMT-01 | Black | 35 | 100 | 2.5 | Strips |
| HBMT-02 | Black | 35 | 200 | 2.5 | Strips |
| HBMT-03 | Black | 35 | 300 | 2.5 | Strips |
| HBMT-04 | Black | 35 | 450 | 2.5 | Strips |
| HBMT-05 | Black | 35 | 1000 | 2.5 | Roll |
| HBMT-06 | Black | 35 | 2000 | 2.5 | Roll |
| HBMT-08 | Black | 35 | 150 | 2.5 | Strips |
| HBMT-14 | Black | 35 | 25000 | 2.5 | Roll |
| HBMT-15 | Black | 35 | 400 | 2.5 | Roll |
| HRMT-01 | Red | 35 | 300 | 2.0 | Strips |
| HRMT-02 | Red | 35 | 450 | 2.0 | Strips |
| HRMT-05 | Red | 35 | 25000 | 1.7 | Roll |
| HRMT-08 | Red | 35 | 450 | 1.7 | Strips |
| HRMT-19 | Red | 35 | 300 | 1.7 | Strips |
| HSCM-06 | Yellow | 20 | 100 | 1.7 | Strips |
| HSCM-09 | Yellow | 25 | 450 | 1.7 | Strips |
W is width, L is length, T is thickness. W and L are minimums; T carries a ±5% tolerance. The grade column is read off the code prefix, which the manufacturer's datasheet defines: HBMT black, HRMT red, HSCM yellow stress control. Form is a property of the code. You do not choose between strips and a roll at one length.
Published properties, grade by grade
| Property | Black HBMT | Red HRMT | Yellow HSCM | Method |
|---|---|---|---|---|
| Tensile strength, MPa | 0.1 min | 0.5 min | 0.1 min | ASTM D638 |
| Ultimate elongation, % | 300 min | 300 min | 300 min | ASTM D638 |
| Density, g/cm³ | 1.30 ± 0.2 | 1.20 ± 0.2 | 1.30 ± 0.2 | ASTM D792 |
| Service temperature, °C | -20 to +90 | -20 to +90 | -20 to +90 | IEC 216 |
| Dielectric strength, kV/mm | 6 min | 10 min | 15 min | ASTM D149 |
| Volume resistivity, Ω·cm | 1 × 10^10 min | 1 × 10^14 min | 1 × 10^9 min | ASTM D257 |
| Dielectric constant | 2 to 3 | 3 max | 10 min | ASTM D150 |
| 50 Hz loss factor | 0.035 | 0.035 | 0.035 | Not stated |
| Adhesion and self amalgamation | Good | Good | Good | Not stated |
Thirty-five millimetres is the width on every black and red code, so the choice inside a grade is length, form and thickness. Where the datasheet prints no test method the column says so, rather than carrying down the method from the row above.
Not published — get these four in writing
The mastic sheet is dense on electrical properties and silent on four things a specification usually needs. There is no shelf life and no storage temperature. The same manufacturer does publish both on its non-shrinkable insulation tape, at five years minimum between +10 °C and +30 °C. There is no minimum application temperature: butyl tack drops away on a cold morning long before the -20 °C service floor is anywhere near. There is no peel or adhesion figure in newtons per centimetre, only the word good. And there is no substrate list. Adhesion onto a lead sheath, a semi-conducting screen or a painted gland body is neither promised nor ruled out. Ask the manufacturer for those four before a specification leans on them.
Where this is the wrong part
Mastic fills and seals. Every job below is one it will lose.
- An insulation build-up. Black and yellow are 0.1 MPa minimum tensile. A layer built up out of that carries no mechanical load, and it creeps under the pressure of whatever is shrunk over it. The build-up grade is EPR tape at 3 MPa: see building insulation by hand.
- An unrestrained gap. Butyl keeps flowing under pressure for the life of the joint. Confine it under a sleeve, a boot or a tube. Left open at one end, it migrates out of the void it was meant to fill.
- A screen cut filled with black. Dielectric constant is 2 to 3 on black and 10 minimum on yellow. Black in a stress relief position leaves the field ungraded, and that joint fails later and out of sight. Which grade takes which joint sets the three against each other.
- Anything that has to recover with heat. A bend or a bolted joint no tube will thread onto takes heat shrink tape. Mastic will not shrink and does not pretend to.
- A duty outside -20 °C to +90 °C. That band is the same on all three grades and there is no high temperature variant in the chart.
- An exposed outer surface. There is no abrasion resistance and no mechanical wall here. Something always goes over mastic.
- A torn cable jacket, on its own. Mastic under nothing is not a repair. The part that restores the jacket is a wraparound cable repair sleeve, and mastic goes in only where that sleeve has a step to cross.
Read next
- What mastic tape is, and what it actually doesThe definition, what cold flow means on site, and how much of it a joint needs
- Black, red or yellow masticThe three grades compared, with the consequence of each substitution
- Building insulation by handGNT 1 and GNT 2 EPR tape at 42 kV/mm, for the job mastic cannot take
- Heat shrink tapeThe tape that does recover, for bends and bolted joints a tube will not fit
- Wraparound cable repair sleeveThe part the mastic is sealing underneath on a damaged jacket
- Reading a heat shrink datasheet without taking it on trustWhat ASTM D149, D150, D257 and D792 measure, and what a minimum figure means
Say what the mastic has to sit under
Name the grade, the form, the length and the part it goes inside. Black at 450 mm in strips and black at 25000 mm on a roll are two different lines. An enquiry that names neither costs three extra emails before anyone can price it.