Flame-retardant protective clothing is a system, and the zipper is part of the system. A coverall made from an inherently flame-retardant fabric can still fail a thermal exposure test because the zipper tape melted, the slider conducted heat to the skin, or the tape shrank and opened a gap. The component is small, but it sits at the centre front, directly over the torso.
This article describes a typical project: an industrial workwear coverall for workers exposed to heat, flame or electric arc, made from an aramid or modacrylic blend, with a front zipper, a storm flap and two chest pockets. The buyer references established protective clothing standards and expects the garment to be certified as a whole, including the zipper.
1. Requirement: no melt drip, no gap, no hidden heat path
Requirements for this category should be written with a test house in mind:
- Material: no thermoplastic component that melts and drips when exposed to flame.
- Heat transfer: the zipper assembly must not create a local hot spot that transfers heat faster than the surrounding fabric.
- Shrinkage: the tape must not shrink enough to open the front or distort the flap under heat.
- After-wash performance: the FR properties and the mechanical function must survive the specified number of industrial washes.
- Coverage: the flap must cover the chain completely, so no metal or coil is directly exposed.
The first point eliminates most standard nylon and polyester zippers. That single constraint reshapes the whole specification.
2. Selection: inherent FR tape, metal or FR coil
The tape is the critical component. A standard polyester tape can melt and drip, and a treated FR finish may wash out. Specify a tape made from an inherently flame-retardant fibre, such as an aramid blend, or a tape that has been qualified as part of the garment's certification. Ask for the tape's own test data, not just the fabric's.
For the chain, metal is often the safer choice because it does not melt, but a metal chain conducts heat and can feel cold or hot against the body. A large moulded coil made from an FR polymer is an alternative where the standard permits it, but the polymer must be qualified. A common compromise is a metal chain with a covered slider and a deep flap, so the metal is not in direct contact with the skin.
The slider and puller should be metal with a finish that survives industrial washing, or a qualified FR polymer. Avoid decorative lacquers and plastic-coated pullers, which can melt or burn. A large puller is easier to use with gloves, but it must be able to be tucked under the flap so it does not present an exposed hot surface.
3. Key processes: flap, thread and assembly
The flap is the primary thermal protection for the zipper. It should overlap the chain by at least 30 mm, be made from the same FR fabric as the shell, and be wide enough that the chain is not exposed when the wearer bends. A double flap, with an inner gutter, is common on garments for high heat exposure.
Thread must also be FR or inherently non-melting. A polyester thread that melts at a seam can open the flap at the moment it is needed. Use the thread specified for the garment certification, and keep the stitch density consistent; a very dense stitch perforates the FR fabric and can reduce its protective performance.
Assembly should avoid placing the zipper directly against the skin. Add an inner facing strip behind the chain, made from the FR shell fabric, so the wearer's undergarment does not contact metal. At the collar, cover the top stop with a soft FR tab. Check that no sharp chain end is exposed when the garment is worn open.
4. Testing and acceptance
Test the garment, not the component in isolation, and use an accredited laboratory:
- Vertical flame: test the zipper area and the flap, and record after-flame and after-glow time, plus any melting or dripping.
- Heat transfer: radiant and convective heat tests at the placket, compared with the shell fabric.
- Arc flash: where the garment is rated for arc protection, test the placket area as part of the garment.
- Shrinkage: measure the front opening before and after the specified wash cycle, and after heat exposure.
- Wash durability: the full industrial wash cycle, then repeat the flame and mechanical tests on a reduced basis.
- Function: 2,000 cycles after washing, with a check that the flap still covers the chain.
Keep the test reports linked to the zipper lot. Certification is granted to a combination of materials and construction, and a later substitution invalidates it even if the new part looks identical.
5. Common risks and how to close them
The most serious risk is a tape or thread that melts, which turns a protective garment into a hazard. The second is a flap that is too narrow, exposing the chain when the wearer moves. The third is shrinkage after industrial washing, which shortens the front and opens a gap at the chest. The fourth is a metal slider that conducts heat or causes a cold-weather contact burn.
There is also a compliance risk that is entirely organisational: buying a "FR zipper" from a catalogue without confirming that the specific tape, thread and slider combination is covered by the garment's certification. Any change of supplier, finish or tape composition should trigger a re-test, not a paperwork update. For protective clothing, the cost of re-testing is part of the cost of the product, and the cost of skipping it is borne by the wearer.