The Dryer Broke on a Friday: What to Do Before Moulding PBT Wet

Polybutylene terephthalate (PBT) is a semicrystalline thermoplastic polyester with a melt temperature near 223 °C, a dry density of approximately 1.31 g/cm³, and a normal injection-moulding melt window between 240 °C and 270 °C. The ester bond in the polymer backbone is hydrolytically sensitive; virgin pellets equilibrate at roughly 0.15–0.25 wt% water at 23 °C and 50 % RH, depending on crystallinity, pellet shape, and storage time. When those pellets enter the melt stream above 250 °C, residual water attacks the ester linkages, releasing terephthalic acid end groups and 1,4-butanediol end groups. The acid end groups catalyse further cleavage, so a small amount of moisture can produce a disproportionate molar-mass loss. A Friday dryer outage therefore forces a sequence of tests and drying actions before a PBT lot can be released to the press.

Storage conditions before drying determine the severity of the Friday incident. PBT bags exposed to humid plant air for 8 h after the dryer stops can shift from 0.02 wt% to 0.05 wt% moisture, particularly if the pellets are warm from a previous drying cycle. A half-filled hopper left open to 60 % RH air can re-absorb surface moisture within 30–60 min. For this reason, the failed dryer should be sealed at the hopper throat, fill port, and return-air line immediately, not after shift change. Unopened original packaging prevents most re-moisture during a short outage, but an open gaylord or bulk bin cannot be considered protective once the dry-air purge is lost.

What Residual Moisture Value Justifies Starting a PBT Moulding Run After Dryer Failure?

Most PBT suppliers specify 0.02 wt% maximum residual moisture before injection moulding. The range from 0.03 wt% to 0.05 wt% is borderline and is associated with sporadic splay, elevated melt flow rate, and lower tensile elongation retention. Moisture determination is performed by Karl Fischer titration according to ISO 15512:2019; if a moisture balance is used for rapid screening, it should be referenced against titration for the specific compound because additives, fillers, and regrind can distort the loss-on-drying result. After a dryer failure, samples must be drawn from the hopper throat rather than the top surface, because interrupted drying creates vertical moisture gradients in the hopper.

When a Desiccant Wheel Dryer Loses Dew Point on a Friday

A desiccant wheel dryer for PBT normally supplies hot air at 120–140 °C with a dew point of −40 °C or lower. If the wheel fails and the dew point climbs above −20 °C, the drying air may re-load the pellets with moisture. The immediate requirement is to isolate the wet lot and purge the hopper with dry air at a positive pressure of 0.1–0.2 bar. The material must not be run wet to purge the barrel. Vented barrels, back-pressure changes, or higher melt temperatures merely accelerate hydrolysis; they do not remove the water before the compression zone.

1. Close the hopper inlet and isolate the wet lot to prevent cross-contamination of dry material.
2. Draw a hopper-throat sample and measure moisture using Karl Fischer titration or a calibrated moisture balance.
3. Switch the hopper to a backup desiccant dryer, vacuum oven, or twin-tower desiccant dryer with supply air at −40 °C dew point.
4. Do not restart moulding until the measured moisture is ≤0.02 wt%.

If no second desiccant dryer is available, a stainless-steel vacuum oven is the most controllable batch alternative. Pellets are spread to a bed depth of 50 mm or less, heated to 110–120 °C, and held at ≤50 mbar absolute pressure for 4–6 h. The vacuum pump and seals must be rated for continuous operation at that temperature, and the oven should be purged with dry nitrogen before breaking vacuum. A tray dryer using uncontrolled plant air is generally not sufficient for wet PBT: compressed air may have a dew point of +2 °C to +5 °C, and an unheated hopper cannot desorb water to 0.02 wt% within a practical Friday-to-Monday window.

Plant compressed air is not automatically equivalent to desiccant-dried air. A refrigerated compressed-air dryer typically leaves a pressure dew point near +3 °C; after expansion at the hopper, the supplied air is far too wet for PBT. A membrane dryer can deliver −30 °C to −40 °C pressure dew point at limited flow, but the dew point must be measured at the hopper inlet, not in the compressor room. Published membrane dryer performance data show a loss of drying capacity when inlet air temperature exceeds 30 °C; therefore a temporary compressed-air membrane dryer installed outdoors on a hot Friday may not meet the required dew point unless the air is cooled before the membrane module.

Production-scale injection moulding of wet PBT typically appears as silver streaks, splay, or nozzle drool. On a 100-tonne hydraulic-clamp machine with a 30 mm general-purpose screw, a lot at 0.06 wt% moisture can show visible surface defects within the first 5–10 shots at a melt temperature of 260 °C. The cavity may still fill, but the part can carry a shifted melt flow rate and reduced notched impact strength. The mechanical loss may not be obvious at the press; it is detected only by testing moulded specimens according to ASTM D638-14 and ASTM D256-10. Operators should not interpret a complete fill as evidence that the material was dry.

Hydrolytic Chain Scission in PBT at Melt Temperatures Above 250 °C

The hydrolysis rate in PBT follows an Arrhenius relationship with melt temperature. At 240 °C, dry pellets with 0.02 wt% residual moisture can tolerate normal residence times of 8–10 min without measurable intrinsic viscosity loss. At 260 °C, the same moisture content can produce detectable molar-mass loss in 4–5 min. The reaction produces acid end groups that autocatalyse further ester cleavage, so degradation accelerates once the first chain scission occurs. A general-purpose screw with an L/D ratio of 20:1 and a compression ratio of 2.5:1 to 3.0:1 is common for PBT; a barrier screw improves melting consistency but does not remove water. Long residence caused by short shots, hot-runner hold, or oversized barrels therefore multiplies the risk when feed moisture is above the 0.02 wt% limit.

After emergency drying, verification uses melt flow rate measurement at 250 °C with a 2.16 kg load according to ISO 1133-1:2022. A virgin PBT with a baseline MFR of 15–20 g/10 min may shift upward by 20–40 % if the material was processed or held at 0.06 wt% moisture. Melt flow rate is a screening indicator, not a substitute for intrinsic viscosity; dilute-solution viscometry in a phenol/tetrachloroethane solvent at 30 °C remains the direct measure of chain length. A drop from 1.0 dL/g to 0.85 dL/g confirms hydrolytic damage even if the pellet moisture has been brought below 0.02 wt% afterward. Quarantine decisions should therefore rely on both residual moisture and melt viscosity data.

Drying Equipment Specifications for PBT Pellets

Desiccant drying systems for PBT should deliver air at 120–140 °C, a dew point of −40 °C or lower, and an airflow of roughly 3.7 m³/h per kg/h of throughput. The hopper residence time should be 4–6 h for virgin material and 6–8 h for regrind. In a 200 kg/h production line, a hopper volume of 1,000–1,200 L is typical to maintain that residence. Hoppers should be insulated and fitted with internal air distribution cones; PBT pellets can bridge, and stagnant zones can hold moisture even when the measured inlet air is dry. A twin-tower desiccant dryer with closed-loop regeneration is preferred over a single-tower hopper dryer because the regeneration cycle maintains the required dew point during continuous operation.

Regrind complicates the Friday dryer failure because ground PBT carries higher surface moisture, lower bulk density, and fines that retain water. A blend of 30 wt% regrind with 70 wt% virgin pellets can require 2 h additional drying relative to virgin-only material, depending on the fines content. Fines below 500 µm should be screened with a 2 mm top screen and a 0.5 mm bottom screen before drying; they clump, restrict airflow, and can raise the blended feed moisture above the 0.02 wt% limit even when the virgin component is dry. Wet regrind should be quarantined until it has been dried and tested separately. Published lot-to-lot kinetic data for wet PBT regrind under production-scale screw conditions are limited, so the conservative quarantine approach is used.

Even when pellets are dried correctly, mould temperature and barrel residence time define the remaining hydrolytic risk. PBT moulds should be held at 80–120 °C to develop proper crystallinity and release; a mould below 60 °C may produce a glossy surface that temporarily hides splay but does not restore molar mass or impact strength. Barrel set points should be profiled from 240 °C at the feed zone to 270 °C at the nozzle. The shot size should occupy 40–60 % of the barrel capacity; oversized barrels increase residence time and accelerate hydrolysis when trace moisture is present. The screw cushion is normally held at 2–4 mm, and back pressure is set at 5–10 bar hydraulic to maintain shot consistency. If the melt becomes gassy or the screw recovery becomes irregular, the lot is not acceptable for continued moulding.

Lot release after a Friday dryer failure is based on a written record of the failed dryer dew point, the alternate drying method used, the lot number, the post-drying moisture value, and the melt flow or intrinsic viscosity result. If any of those records is incomplete, the wet PBT remains quarantined. A temporary downstream adjustment in moulding conditions does not substitute for the measured ≤0.02 wt% moisture requirement.