Within PBT logistics—container loading, weight limits and moisture protection in transit—each control is determined by the material’s semi-crystalline polyester structure and its melt-processing sensitivity to residual water. Unfilled PBT grades for injection moulding and extrusion are shipped as pellets in 25 kg multiwall paper bags with low-density polyethylene liners, 1,000 kg flexible intermediate bulk containers, or 1,250 kg octabins. Solid-state density measured according to ISO 1183-1 is 1.30–1.32 g/cm³ for unfilled grades, while apparent pellet bulk density under ISO 60 is 0.78–0.85 g/cm³; a 25,000 kg bag load therefore occupies approximately 29–32 m³ of a 67–76 m³ container interior, making the cargo mass-limited rather than volume-limited. The processing threshold is narrow: at melt temperatures above 250 °C, moisture levels above 0.02 wt% hydrolyse the terephthalate ester bond, reducing intrinsic viscosity and tensile elongation at break as measured by ASTM D638-14. This logistics chain therefore has two operating boundaries—the mass limit on container and road payload, and the moisture limit at the extruder throat—that are managed through loading geometry, floor-load distribution, and liner-sealed desiccation.

Which residual moisture in a 25 kg PBT bag forces re-drying before melt processing?

Residual moisture in PBT is determined by ISO 15512 Karl Fischer titration on pellets drawn from the centre of a sealed liner, not from surface scrapings or paper bag fibre. Undried unfilled PBT injection grades typically contain 0.05–0.08 wt% water after bagging; the release limit for direct melt processing is usually ≤0.02 wt%. The 24 h water absorption of unfilled PBT is 0.07–0.09 wt% under ISO 62 at 23 °C, and equilibrium immersion at 23 °C is roughly 0.2–0.4 wt%; a monolayer paper sack exposed to high ambient humidity can therefore exceed the 0.02 wt% limit without visible dampness. Some high-speed extrusion lines with vented 32:1 L/D twin-screw extruders tolerate up to 0.03–0.04 wt% if vacuum venting is maintained at −0.08 MPa gauge, but unvented injection moulding barrels do not have this margin. If a container lot arrives above 0.02 wt%, re-drying in a desiccant-hopper dryer with air dew point −40 °C and inlet temperature 120 °C for 4 h is the standard correction; above 0.08 wt%, drying time is extended to 6–8 h to avoid residual moisture pockets in the hopper. The moisture limit is a processing quarantine threshold, not merely a transport specification: it determines whether the lot is released directly to the moulding cell or diverted to drying at the receiving plant.

Container floor-load distribution and 25 kg bag stacking in a 40 ft high-cube

Standard sea-freight containers for PBT are series 1 containers conforming to ISO 668:2013 and tested to ISO 1496-1:2013. A typical 40 ft high-cube container has internal dimensions 12.032 m × 2.352 m × 2.695 m and a maximum gross mass printed on the CSC plate of 30,480 kg; tare mass is commonly 3,900–4,200 kg, leaving a net payload of approximately 26,300–26,580 kg. PBT bag loads are not a uniformly flat bulk solid. A palletized stack of 40 × 25 kg bags creates a unit load of 1,000 kg on a 1.2 m × 1.0 m footprint, yielding a static bearing pressure of approximately 8.2 kPa. The static pressure is below the container floor rating, but forklift axle loads during loading and uneven stacks over floor cross members can produce local wheel loads of 4,000–5,000 kg, which is why loading plans use plywood or steel spreader plates under forklift paths. For a 40 ft high-cube, 20 palletized unit loads of 1,000 kg fit in a single-tier pattern, producing a net cargo mass of 20,000 kg; this is intentionally below the container payload limit to accommodate pallet mass, dunnage, and destination-road axle restrictions. Floor-stowed 25 kg bags can increase the net cargo to 25,000 kg, but that configuration is used only when the receiving extruder has bulk bag emptying and the risk of torn liners from double-tier handling is accepted.

When ocean-freight dew point exceeds the container skin temperature

Moisture protection in transit is a dew-point problem rather than a simple relative-humidity problem. A container loaded at 30 °C and 70% RH in a Southeast Asian port contains air with a dew point of approximately 24 °C; as the vessel moves into cooler air, the container roof and sidewall skins can fall below 24 °C, causing condensation on the inner steel surfaces. This condensate drips onto top cartons and pallet covers, and if the pallet wrap is torn, it enters the bag liner through needle holes. PBT pellets are not visibly wetted by container sweat, but local moisture uptake at the top layer can rise above 0.06 wt% within 72 h if the liner is open. Two condensation modes govern PBT transit. Ship sweat forms on the container shell when the vessel crosses into colder water; cargo sweat forms on the cooler surface of the pallet stack when a cold container is opened in warm humid air at destination. The latter is often the larger source of visible water on bag surfaces because the sealed container is opened before the pallet temperature has equilibrated to ambient. For sea-freight PBT, 25 kg bags are specified with a 0.10 mm low-density polyethylene liner and a moisture-tight tie or heat seal; palletized loads are covered with a 150 µm polyethylene pallet cover from the top down to the pallet fork entry, then secured with polyester strapping. The container is inspected for holes of 6 mm or larger, broken door gaskets, and residual cargo moisture before loading. Ventilation is generally closed for PBT because fresh marine air introduces more water vapour than it removes; the cargo space is instead maintained with desiccated headspace.

When a container loaded with PBT in Singapore arrives at a European rail terminal at −20 °C, the absolute humidity trapped in the headspace is held by the cold air as ice or subcooled water films on the container interior. Opening the doors immediately can create a convective front that deposits cargo sweat on the outer bag surfaces even though the pellets remain frozen. The standard receiving procedure is to allow 12–24 h of door-closed equilibration inside the warehouse after the container skin has approached indoor temperature, then open and log headspace relative humidity. If the headspace exceeds 75% RH, the lot is segregated for Karl Fischer sampling before liners are cut.

Desiccant mapping for tropical routes cannot rely on fixed bag counts alone

Desiccant placement in PBT containers is governed by internal dew-point mapping. A single desiccant bag placed near the door is ineffective because moisture enters through door seals and roof corner castings; the condensation front moves from the container skin inward through the pallet stack. On tropical routes, container desiccant suppliers commonly specify 500–1,000 g of calcium chloride or molecular-sieve desiccant per cubic metre of interior volume, which for a 40 ft high-cube translates to 38–76 kg total desiccant, distributed horizontally behind the door headers, along the sidewall corrugations, and under the top cartons. Published data for PBT-specific desiccant demand under all route profiles is limited; therefore the allocation should be based on data-logger readings of dew point and absolute humidity rather than a fixed bag count. ISO 9001:2015 shipment-release records should retain the logger traces and desiccant certificates. PBT load plans that use 150 µm pallet covers can reduce desiccant demand, but the covers must not be perforated for ventilation because the objective is isolation from the container atmosphere. Destination inspection includes measuring headspace relative humidity; if it exceeds 75% RH, the lot is placed in quarantine and sampled for Karl Fischer moisture before liners are cut. This procedure prevents wet bags from entering the dryer hopper where a −40 °C dew-point air supply would be spent removing surface water rather than preserving intrinsic viscosity.