Halogen-Free PBT: GWIT, GWFI and the Difference Between the Two Glow-Wire Tests

Polybutylene terephthalate (PBT) compounds rated for connectors, terminal blocks, relay bases, and power distribution components are frequently specified with halogen-free flame-retardant packages when end-product glow-wire testing is part of appliance safety certification. The material is semicrystalline, with a melting point near 223 °C, and halogen-free grades based on organic phosphinates, melamine polyphosphate, and nitrogen synergists have become a standard route to meet IEC 60695-2-11 end-product glow-wire requirements without brominated flame retardants. Two separate material screening methods, IEC 60695-2-12 for the glow-wire flammability index (GWFI) and IEC 60695-2-13 for the glow-wire ignition temperature (GWIT), are closely associated with halogen-free PBT because appliance standards such as IEC 60335-1:2020 clause 30.2.3.2 permit material pre-selection based on these values. The relationship between GWFI and GWIT is not interchangeable: a 30% glass-fiber-reinforced halogen-free PBT plaque at 1.5 mm may be listed with a GWFI of 960 °C and a GWIT of 775 °C. The reason is that GWFI permits a post-removal self-extinguishing interval, whereas GWIT records sustained ignition during the 30 s contact period.

What Distinguishes GWIT from GWFI in Halogen-Free PBT Grades?

Under IEC 60695-2-12, GWFI is the highest temperature, typically evaluated in 25 K steps, at which a horizontally mounted specimen does not ignite, or if it ignites, the afterflame and afterglow extinguish within 30 s after glow-wire removal, and no flaming or glowing dripping ignites a tissue paper layer placed 200 mm below the specimen. Under IEC 60695-2-13, GWIT is the lowest temperature at which the specimen ignites for 5 s or more during the 30 s glow-wire application. Both tests apply a 4.0 mm diameter 80/20 nickel-chromium wire loop heated electrically to a specified tip temperature, with a contact force of 1.0 N ± 0.2 N. The specimen thickness must always be reported; typical values are 0.75 mm, 1.5 mm, and 3.0 mm. A GWFI value obtained at 1.5 mm is not valid for a 0.4 mm connector wall because thermal mass and glass-fiber orientation differ.

Published data for this specific configuration is limited for very thin molded sections below 0.75 mm. Because the standardized plaque geometry in IEC 60695-2-12 and IEC 60695-2-13 is 60 mm × 60 mm, the results cannot be directly transferred to injection-molded parts with knit lines, ribs, or variable wall thickness. This limitation is significant for halogen-free PBT connectors, where the ignition pathway may follow a glass-fiber-rich knit line rather than the homogeneous plaque surface.

Glow-Wire Apparatus Geometry and Thermal Boundary Conditions

The glow-wire source in all three methods is specified in IEC 60695-2-10. The loop is formed from 80/20 nickel-chromium wire with a diameter of 4.0 mm. A sheathed thermocouple inserted into the tip controls the wire temperature to ±5 K. The specimen is clamped to a carriage, and the glow wire is applied for 30 s ± 1 s with a force of 1.0 N ± 0.2 N. In the GWFI method, a white tissue paper wrapped around a wooden block is located under the specimen and is part of the pass/fail criterion; in GWIT, the tissue paper is not required because the endpoint is defined by specimen ignition during wire contact. The standard specimen geometry is 60 mm × 60 mm square with a minimum thickness of 0.75 mm ± 0.1 mm. End-product testing under IEC 60695-2-11 applies the same glow-wire apparatus directly to the molded part at the specified test temperature, which may be 750 °C, 850 °C, or 960 °C depending on the application and the clause invoked by the appliance standard.

Halogen-free PBT compounds that meet GWFI 960 °C and GWIT 775 °C thresholds usually rely on phosphorus-based flame retardants rather than nitrogen-only systems. A typical 30% glass-fiber-reinforced formulation contains 15 wt% to 20 wt% of an organic phosphinate, with melamine polyphosphate or a nitrogen synergist added at 5 wt% to 8 wt%, plus 0.2 wt% to 0.5 wt% of hindered phenol and phosphite stabilizers. The flame-retardant mechanism operates in the condensed phase, through intumescent char formation catalyzed by phosphorus acids, and in the gas phase, through radical scavenging. Under the 30 s glow-wire exposure, the surface temperature exceeds the oxidative decomposition onset of PBT, which is often reported in thermogravimetric analysis at 380 °C to 400 °C. The practical result is that ignition resistance depends less on the initial polymer melting point and more on the thermal conductivity of the glass network and the ability of the char layer to form before the 5 s GWIT ignition criterion is reached.

Production compounding of halogen-free PBT requires a tight thermal window. On a 40:1 L/D twin-screw extruder with atmospheric venting and glass-fiber side-feeding, melt temperature measured at the die is normally kept between 250 °C and 265 °C. Above 270 °C, the phosphinate package can release acidic decomposition species that hydrolyze the PBT ester bonds, causing molecular weight loss and reducing the GWIT by approximately 25 K in molded plaques. The same thermal excursion produces visible yellowing in 30% glass-filled grades after 30 s to 60 s residence time. Pre-drying in a desiccant dryer at 120 °C for 4 h to a residual moisture level below 0.02% is mandatory; at relative humidity above 60%, dried pellets can regain sufficient surface moisture to increase hydrolysis during molding. Injection molding of these compounds typically uses a barrel profile of 240 °C at the feed throat, 255 °C in the compression zone, and 260 °C at the nozzle, with a mold temperature of 80 °C to 100 °C and a back pressure of 3 bar to 5 bar.

Halogen-free status for PBT compounds is frequently evaluated using IEC 61249-2-21, which defines halogen-free as total chlorine below 900 ppm, total bromine below 900 ppm, and total halogen below 1500 ppm. This definition was developed for printed circuit board base materials but is applied by many connector and relay manufacturers to PBT molding compounds. Compliance with IEC 61249-2-21 does not itself confer glow-wire performance; a compound may be halogen-free and still fail GWFI because the flame-retardant package is inadequate for the 960 °C wire temperature.

When Halogen-Free PBT Encounters the 30-Second Ignition Window

At a glow-wire tip temperature of 960 °C, the surface of a halogen-free PBT specimen is heated far beyond the temperature range used in standard UL 94 small-flame testing. The 30 s application period creates a steep thermal gradient: the material immediately under the wire loop carbonizes and forms char, while the underlying polymer remains below its melting point. The char layer can act as an insulating barrier, which is why many halogen-free PBT grades pass the GWFI test at 960 °C even though their GWIT is 775 °C. During GWFI testing, if ignition occurs, the specimen is allowed 30 s after removal for the afterflame to extinguish; the phosphorus-based char layer is often sufficient to prevent continued combustion. During GWIT testing, the criterion is stricter because ignition for 5 s during contact means that the char layer has already failed under the applied thermal load. The difference between the two values therefore indicates whether ignition is delayed long enough to meet a post-removal self-extinguishing target, not whether the material is non-ignitable.

This distinction has direct consequences for material substitution. A halogen-free PBT grade with a UL 94 V-0 rating at 0.8 mm may not automatically satisfy appliance glow-wire requirements if its GWIT is below the threshold specified by IEC 60335-1:2020 clause 30.2.3.2. Under that clause, material pre-selection for unattended appliance current-carrying parts commonly requires GWFI of at least 850 °C and GWIT of at least 775 °C. If a manufacturer replaces a qualified 30% glass-filled halogen-free PBT with a lower-cost 15% glass-filled grade having the same UL 94 rating, the thinner char network and higher polymer volume may reduce GWIT below the acceptable limit. Published data for this specific configuration is limited for weld-line areas and for part walls below 0.75 mm.

When interpreting GWFI and GWIT values, specimen thickness is not a secondary detail. A 0.75 mm plaque usually yields lower GWFI and GWIT values than a 1.5 mm plaque because the smaller polymer volume stores less heat and the char layer is thinner. For halogen-free PBT, published data commonly show a 25 K to 50 K reduction in GWFI when moving from 1.5 mm to 0.75 mm, with a similar or smaller reduction in GWIT. Published data for this specific configuration is limited for very high-flow grades used in thin-wall connectors, because the high melt flow rate requires lower molecular weight PBT, which can reduce thermal stability and char integrity under glow-wire contact.

In an injection-molded terminal block produced from a halogen-free PBT grade with 30% glass fiber and a nominal wall thickness of 1.2 mm, the end-product glow-wire test under IEC 60695-2-11 is frequently conducted at 960 °C. The location of the glow-wire tip relative to the gate and weld line changes the result because glass-fiber orientation in the weld-line plane is predominantly parallel to the surface and provides a continuous thermal conduction path. Plaque data from IEC 60695-2-12 and IEC 60695-2-13 may indicate GWFI 960 °C and GWIT 775 °C at 1.5 mm, while the molded part fails at the weld line. For this reason, appliance manufacturers may require both standardized plaque data and an end-product glow-wire test on actual production geometry before approving a halogen-free PBT compound.