Polybutylene terephthalate (PBT) is a semicrystalline polyester whose storage stability is governed by hydrolytic attack on the ester repeat unit rather than by simple additive separation. An unopened foil-laminated package with desiccant typically preserves the moisture content of an unfilled injection-moulding grade below 0.04% by weight as determined by ISO 15512. This as-shipped value is below the normal 0.02%–0.04% processing threshold for standard melt conversion. Once the barrier is broken, the material begins a cumulative moisture-exposure interval described on production floors as the sealed-bag clock. The interval is not a fixed expiry date; it is the elapsed time over which ambient temperature, relative humidity, pellet geometry, and initial carboxyl end-group concentration combine to raise the moisture content toward a critical threshold before drying or melt-phase hydrolysis becomes decisive.

Weight-loss methods alone are insufficient for borderline PBT shipments because oligomer and additive volatility can be mistaken for water. The reference method for water content is Karl Fischer titration according to ISO 15512, with coulometric techniques specified by ASTM D4019 used in plant laboratories for rapid checking. Transfer of pellets to the analyser must occur in a low-dew-point environment; a delay of 30 s in 25°C/60% RH air can increase the reported moisture by 0.005 percentage points or more. For this reason, the sealed-bag clock cannot be assessed by visual appearance, and it must be anchored to a measurement method.

Does Hydrolysis Begin Before Moisture Exceeds the PBT Drying Specification?

Hydrolysis in PBT does not require saturated pellets. The ester bond adjacent to the tetramethylene segment is susceptible to chain scission at melt temperatures as low as 240°C when moisture is present at 0.02%. The reaction is pseudo-first order in moisture concentration over the narrow range relevant to injection moulding, and the liberated fragments increase the concentration of carboxyl end groups. Because carboxylic acid groups catalyse further scission, the process is autocatalytic; a resin with an initial carboxyl end-group content of 25 meq/kg can degrade measurably faster than a stabilised grade with 10 meq/kg under the same moisture load. For unfilled PBT processed at a barrel set point of 250°C, vendor drying guides specify a maximum residual moisture of 0.02% when the intended residence time exceeds 3 min or when the tool contains hot-runner manifolds above 260°C. Hydrolytic degradation during moulding is detected as a shift in melt volume-flow rate under ISO 1133-1. A change greater than 20–30% relative to the lot’s original value is routinely used in incoming QC to flag molecular-weight loss, and the same samples often show tensile strength retention below 90% when tested according to ISO 527-1.

At the moulded-part level, hydrolytic degradation is not always visible immediately. The property most sensitive to earlier chain scission is frequently not tensile strength but notched impact strength under ISO 180/1A. Impact toughness may drop by more than 20% before the melt flow rate shift exceeds 30%. For parts that must meet a minimum Charpy or Izod value, this failure mode is particularly dangerous because it can escape process monitoring that relies only on fill time and cushion position. The visible warning signs—silver streaks, brown specks, or mould deposits—appear when the moisture load is already high enough to produce gaseous degradation products in the melt.

Moisture uptake in a cylindrical PBT pellet is not homogeneous during the first hours. The surface layer saturates quickly, but the core remains dry. If a dryer removes only the surface water rapidly, core moisture persists and can migrate to the surface after the pellets enter the barrel. This is why moisture specifications are written at the pellet composite level, not at the surface of a grab sample. The sealed-bag clock therefore has at least two distinct phases: a fast surface-adsorption phase immediately after opening, and a slower diffusion-limited phase that depends on temperature and relative humidity. The following table summarises representative moisture-exposure windows for unfilled PBT pellets after breaking a foil-lined package.

The Sealed-Bag Clock as a Cumulative Humidity Exposure Metric

The sealed-bag clock is not reset by closing the top of an open bag. A foil laminate with an internal moisture vapour transmission rate below 0.01 g/m²/day acts as a barrier, but a folded open bag allows humid plant air to circulate. If a bag is split for 10 min and then sealed with tape, the trapped headspace at 55% RH can elevate the moisture content of the top layer by 0.01–0.02 percentage points within 12 h. The only reliable reset is active drying in a closed-loop desiccant system with a dew point of -40°C or lower, followed by storage in a conditioned hopper or airtight container. Warehouses that use first-in-first-out rotation often assign a maximum open-bag exposure of 8 h at 23°C/50% RH or 4 h at 23°C/65% RH before re-drying is mandatory. These limits are operationally derived, not standardised, and should be verified against the specific resin lot because initial carboxyl end-group content and pellet crystallinity shift the uptake curve.

Post-industrial PBT regrind is not equivalent to virgin pellets for storage purposes. The irregular surface area of ground sprues and runners increases the air-facing area, and the carboxyl end-group concentration is often higher after a first heat history. A regrind fraction above 25% in a blend can reduce the allowable open-bag exposure by roughly 30%, and the material should be dried to 0.015% if the tool contains hot-runner passages. These values must be adjusted for each recovery stream because the particle size distribution and residual stabilizer content are not identical between processors.

When Storage Temperature Exceeds 30°C or Relative Humidity Fluctuates

When a warehouse cycles from 22°C at night to 38°C in the day, condensation risk on cold pellet surfaces appears when the surface temperature drops below the dew point of the surrounding air. In a non-barrier bag, water film at the pellet surface is absorbed quickly and can produce localised moisture contents above 0.1% even when the average room humidity is only 60% RH. Temperature also affects solid-state hydrolysis and antioxidant consumption. A sustained warehouse condition above 30°C combined with 70% RH can reduce the allowable open-bag shelf life by roughly half compared with 23°C/50% RH. In such environments, the incoming material should not be stored against exterior walls, beneath roof vents, or in direct sunlight. Pallets should be conditioned to production hall temperature before opening; a temperature differential greater than 5°C between the pellets and the ambient hall is a common handling threshold adopted to avoid condensation.

The drying step must be matched to the accumulated moisture history. A desiccant dryer with a -40°C dew point and inlet air at 120°C for 4–6 h is the standard method for moving PBT from 0.05%–0.10% moisture down to 0.02%. Hot-air hopper dryers that draw plant air are generally inadequate in humid conditions because the dew point of the drying air remains above -10°C, and residual moisture after 4 h may not fall below 0.04%. On a production line, the true drying endpoint is not the dryer residence time alone; it is the moisture content at the feed throat. Portable coulometric Karl Fischer instruments operating according to ASTM D4019 or ISO 15512 can be used to sample pellets immediately before the feed throat and to identify moisture regain caused by long conveying lines or open machine hoppers. If the feed-throat moisture exceeds 0.02%, the operator should reduce the hopper inventory, inspect return-air filters, or blank off the hopper to plant air.

Production-scale experience with 40:1 L/D reciprocating injection units indicates that the visible signs of moisture-induced hydrolysis appear most frequently when the material has been exposed to plant air above 65% RH for more than 4 h without pre-drying. However, the absence of silver streaks does not confirm that the dried pellet moisture content is acceptable. Intrinsic viscosity measurements in phenol/1,1,2,2-tetrachloroethane according to ISO 1628-5 provide a more reliable record of chain scission. A drop in intrinsic viscosity of 0.04 dL/g or more relative to virgin material is a clear indication that hydrolysis has occurred, even if the melt flow rate shift measured by ISO 1133-1 remains within the supplier specification. For glass-fibre-reinforced PBT, moisture uptake is diluted by the 30% glass content, but the polyester matrix remains the reactive phase; therefore the same dryness limit of 0.02% is applied to the matrix rather than the total compound. Flame-retardant PBT grades with brominated or phosphorus-based additives may have broader moisture tolerances, but published data for specific formulations is limited, and those grades require supplier-specific conditioning data rather than generic PBT drying curves.