When a container of PBT arrives wet, the immediate processing risk is not the visible free water on the outer packaging; it is the moisture already absorbed by the semicrystalline polyester pellets. Polybutylene terephthalate takes up water reversibly at ambient temperature, but the same water becomes a hydrolytic chain-scission agent when the resin is heated above its melting range. A shipment found with soaked slip sheets, saturated desiccant pouches, pooling water inside the outer wrap, or punctured liner film must therefore be quarantined before melt processing. Incoming resin specifications for injection moulding grades typically require moisture content below 0.02 % by Karl Fischer titration, because residual moisture above that threshold has been associated with reductions in intrinsic viscosity, increases in melt volume-flow rate, and loss of tensile elongation. The first line of documentation for a potential claim includes the carrier delivery receipt with a wet notation, unopened photographic evidence of the exterior, and the batch or lot number from the packaging label. Hydrolysis of polybutylene terephthalate proceeds by ester linkage scission, generating carboxyl end groups and lowering molecular weight. The reaction is autocatalytic because carboxyl end groups can catalyse further hydrolysis; a wet lot that is dried too late may already possess latent damage even if the post-drying moisture is low. A retained sample should be collected from at least three separate locations in the shipment, sealed in aluminium-laminated barrier bags, and stored at 23 °C under 50 % relative humidity pending analysis.

Why Does Water Ingress into PBT Pellets Create a Melt-Processing Failure Risk?

PBT is processed in the melt state at barrel set temperatures commonly between 245 °C and 260 °C; at these temperatures, dissolved water attacks ester groups at a rate that becomes significant within the normal residence time of an injection screw. The equilibrium moisture content of PBT at 23 °C and 50 % RH is approximately 0.2 %, an order of magnitude above the melt-processing limit. Wet packaging can therefore elevate pellet moisture to levels that require active desiccant drying rather than surface wiping or warehouse air exposure. Residual moisture levels as low as 0.03 % have been reported in polymer processing literature to cause silver streaking and lower elongation at break because the evolved steam disrupts the melt stream and the molecular weight drops. The acceptable moisture content for unfilled injection moulding PBT is generally ≤ 0.02 %; for glass-fibre-reinforced grades, the limit is similar, but the visible result of excess moisture is often splay and fibre exposure at the part surface. Water content is determined by Karl Fischer coulometric or volumetric titration after heating the specimen in a sealed vial; ISO 15512-1:2019 and ASTM D6869-17 describe suitable methods. The specimen mass is typically 0.5 g to 1.0 g, and the result is reported in percent by mass. The packaging barrier has failed when free water is present inside the liner. This is not equivalent to normal moisture regain during storage under uncontrolled humidity; it is a containment integrity failure. A desiccant dryer delivering air at a dew point of ≤ −40 °C is the minimum acceptable equipment for wet-recovered PBT. Hopper inlet air temperature should be 120 °C for unfilled PBT and 120 °C to 140 °C for glass-fibre-reinforced grades, with a residence time of 4 h to 6 h. For flame-retardant PBT grades containing brominated or phosphorus-based additive packages, the drying temperature is often limited to 80 °C to 100 °C to avoid additive migration or degradation. The hopper must be insulated, and the return-air side checked for bypass leakage. A production-scale desiccant dryer with an airflow of 3.7 m³/h per 1 kg/h of polymer throughput is typical, but the hopper capacity must match the required residence time; a 100 kg/h injection moulding machine consuming 2 kg per shot therefore requires a hopper capacity sufficient for at least 400 kg when the drying time is 4 h.

Desiccant drying parameters for wet-recovered PBT

Grade type	Drying air temperature	Minimum drying time	Dew point	Residual moisture limit
Unfilled injection moulding PBT	120 °C	4 h	≤ −40 °C	≤ 0.02 %
Glass-fibre-reinforced PBT	120 °C to 140 °C	4 h to 6 h	≤ −40 °C	≤ 0.02 %
Flame-retardant PBT	80 °C to 100 °C	4 h to 8 h	≤ −40 °C	≤ 0.02 %

If the container has been wet for more than 72 h, drying alone cannot restore molecular weight; the hydrolytic damage may have already occurred during storage. In that situation, intrinsic viscosity and melt volume-flow rate are measured before processing. A shift in MVR greater than 20 % from the supplier certificate of analysis is a rejection indicator. Drying is a moisture-removal operation, not a corrective action for chain scission. On a production-scale twin-screw compounding line with a 40:1 L/D ratio and a vacuum vent set to −0.08 MPa gauge, wet PBT can present as fluctuating melt pressure and rapid screen-pack loading. The vent vacuum is intended to extract volatiles, but if the incoming pellet moisture exceeds 0.05 %, the vent port may flood with steam and deposit low-molecular-weight condensate. Extruder operators have observed torque instability and die-face deposits that are not corrected by raising barrel temperature; instead, the material should be returned to quarantine and the dryer checked for air leakage, saturated desiccant beds, and incorrect hopper loading. A melt flow check per ISO 1133-1:2022 at 250 °C with 2.16 kg load will frequently show an MVR increase compared with the certificate of analysis if hydrolysis has begun. The sampling protocol for a wet container must account for water distribution. Top pellets may be wet while bottom pellets may be saturated, or vice versa. Grab samples from the surface are not representative. A tiered sampling of at least nine points, three from the top third, three from the middle third, and three from the bottom third, reduces the risk of accepting a partially hydrolysed lot. Samples should not be pre-dried before Karl Fischer analysis, and the vial headspace must be minimised because atmospheric moisture exchange affects the result. The analytical balance used for specimen mass must have a readability of 0.1 mg.

When the Outer Packaging Is Intact but Condensation Is Visible on the Liner

Visible condensation on the inside of a sealed PE liner does not automatically mean the resin is out of specification. The inner liner may have been sealed in a high-humidity environment, trapping moisture vapour inside the package. The weight of the container, the condition of the desiccant pouches, and the presence of free water should be recorded before opening. A sample should be collected from the top, middle, and bottom of the container using a clean metal or glass tube, placed immediately into a pre-weighed Karl Fischer vial, and sealed. If the liner is punctured, the container is not acceptable for direct production use; a claim is likely, and the entire lot must be segregated. If the outer wrap is wet but the liner is intact and desiccant pouches are still friable, the pallet may be dried in a dry room for 24 h at 15 % RH before sampling, but the outer wrap should not be removed in an uncontrolled wet environment. Desiccant pouches, whether silica gel or molecular sieve, are not a substitute for hopper drying.

Claim Documentation, Retention Triggers, and Analytical Records

The claim file is initiated when the incoming lot fails the agreed moisture specification or shows visible free water, punctured liners, or saturated desiccant that no longer controls the package environment. The minimum evidentiary set includes the bill of lading marked “wet on arrival,” dated photographs of the exterior before opening, and the supplier’s certificate of analysis. A nonconformity report under ISO 9001:2015 clause 8.7 should reference the lot number, the total quantity impacted, and the quarantine location. If the material is to be used after drying, a deviation or concession must be recorded with the drying curve and the post-drying Karl Fischer result.

Evidentiary set for moisture-related PBT claims

Document	Standard or legal basis	Key data to record
Carrier delivery receipt / CMR consignment note	CMR Convention / Incoterms 2020	Date, time, seal number, wet notation
Photographic evidence	ISO 9001:2015 clause 7.5.3	Exterior wrap, pallet, liner, desiccant condition
Karl Fischer moisture report	ISO 15512-1:2019 / ASTM D6869-17	Sample mass, heating temperature, result in %
Intrinsic viscosity record	ISO 1628-1:2021	Solvent, temperature, result in dL/g
Melt volume-flow rate	ISO 1133-1:2022	250 °C, 2.16 kg, result in cm³/10 min
Quarantine and disposition log	ISO 9001:2015 clause 8.7	Lot number, quantity, storage location, decision

If drying recovery is attempted, the documented record must include the initial moisture result, drying equipment type, dew point, air temperature, residence time, post-drying moisture result, and any MVR or intrinsic viscosity measurement. Without this sequence, a later product failure cannot be traced to a temporary dryer fault or an incomplete drying process. The retained sample should be at least 2 kg, sealed in an aluminium barrier bag, labelled with the dispute reference, and kept in a dry area at 23 °C until the claim is settled. The carrier should be invited to inspect jointly if the damage is external; unilateral disposal destroys evidence. Post-drying release criteria should include residual moisture ≤ 0.02 % and intrinsic viscosity within the supplier specification. If the viscosity has dropped by more than 5 % from the certificate of analysis, the lot should be restricted from thin-wall connectors, electrical insulation parts, and safety-related automotive components because embrittlement risk is elevated. The carboxyl end group concentration is a sensitive indicator of hydrolytic damage, but the acceptance limit is grade-dependent and must be taken from the supplier specification rather than a generic value. Where quantitative data for a particular resin configuration are not available, the conservative disposition is to reject the lot for critical production and use it only in non-structural, non-electrical applications after documented drying and testing.