Shipping PET Resin: Moisture Barriers, Container Liners and Claims

Polyethylene terephthalate (PET) resin moving through ocean freight is not an inert dry cargo; the pellet is a hygroscopic solid that exchanges moisture with the headspace air until the water activity of the polymer surface and the surrounding atmosphere approach equilibrium. Bottle-grade PET at 23 °C and 50 % RH equilibrates to approximately 0.20–0.30 wt% moisture, while saturation in water approaches 0.60 wt%. Unventilated containers operating at 35–45 °C and 80–95 % RH produce surface moisture levels above 0.40 wt% on outer pellet layers during voyages longer than 14 days. Injection stretch blow molding and sheet extrusion require pellet moisture no greater than 0.005 wt% (50 ppm) for intrinsic viscosity retention, making the shipping barrier a direct process-control variable rather than a packaging accessory. Moisture entering the melt hydrolyzes ester linkages during processing at 270–290 °C, reducing intrinsic viscosity measured by ASTM D4603-18 and generating acetaldehyde measured by headspace gas chromatography under ASTM F2013-10; once hydrolytic chain scission has occurred, drying cannot restore molecular weight.

The sorption isotherm of PET at 23 °C is nonlinear. At 50 % RH the equilibrium moisture lies near 0.25 wt%; at 80 % RH it can exceed 0.40 wt%, and the slope steepens further above 90 % RH. Because bottle-grade pellets are partially crystalline, moisture diffusion through the pellet is slow relative to the headspace humidity changes. The outer shell of the pellet can exceed the processing limit while the core remains below it, producing a radial moisture gradient that a single Karl Fischer measurement does not resolve. This gradient is the reason discharge sampling under ISO 15512:2019 Method B must be performed on multiple locations across the top, sidewall, and bottom zones of the container, not from one composite sample taken at the discharge chute. The center of the bag produces the driest result and is the least useful sample for claim adjustments.

On production-scale injection molding lines with throughputs between 150 kg/h and 250 kg/h, wet resin is detected as screw torque instability, sporadic unmelted pellets, and higher preform haze before the dryer alarms respond. A desiccant wheel dryer sized for 0.10 wt% incoming moisture may not return pellets to 50 ppm when the silo inlet moisture reaches 0.40 wt%. Desiccant wheel dryers with supply-air dew points below −40 °C and air temperatures of 160–180 °C are the production benchmark; residence times below 4 h with wet feedstock leave the pellet core above the processing limit even if the surface is dry. Moisture analysis by ISO 15512:2019 Method B or ASTM D6869-17 is therefore the first acceptance parameter at the silo inlet, not a routine quality formality.

What Moisture Barrier Specification Applies to Resin Shipments?

The moisture barrier specification derives from the allowable moisture gain between pack-out and dryer inlet, not from film thickness alone. A liner film is characterized by water vapor transmission rate under ASTM F1249-20 at 37.8 °C and 90 % RH, oxygen transmission rate under ASTM D3985-17 at 23 °C and 0 % RH, and flex-crack resistance under ASTM F392/F392M-23. The purchase specification should state a WVTR ceiling at the voyage upper temperature, because WVTR increases with temperature and humidity gradient. A 125 µm LLDPE/LDPE single-layer liner may exhibit a WVTR between 3.0 g/(m²·day) and 8.0 g/(m²·day) at 37.8 °C and 90 % RH; a three-layer PE-EVOH-PE coextrusion typically reduces the value to 0.5–1.5 g/(m²·day), and foil-containing laminates fall below 0.05 g/(m²·day). The stated values are supplier-technical-bulletin ranges, not universal constants; film grade, tie-layer thickness, and sealant thickness shift the results.

Because the liner is installed inside a corrugated steel container, puncture and abrasion resistance are as important as WVTR. The film must survive bulkhead edges, wooden dunnage, and the repeated load shifts of ocean transit. Slow-rate puncture resistance is evaluated under ASTM F1306-19; seal strength is measured under ASTM F88/F88M-21. A liner with superior barrier but low puncture resistance fails through pinholes at the floor and sidewall corners, producing localized moisture ingress that the center-bag moisture sensor does not detect. The technical audit of a container liner therefore combines barrier, seal, puncture, and flex performance, with the failure mode weighted by the actual loading pattern.

Desiccant packs placed inside the liner reduce headspace humidity but cannot remove moisture already absorbed by the pellets. Calcium chloride desiccant lowers the headspace dew point, yet the resin mass is a much larger moisture sink than the air volume. The desiccant quantity is calculated from the free air volume, the liner WVTR, the voyage duration, and the expected temperature envelope; without voyage-specific temperature and WVTR data, the calculation is not transferable. Desiccant placement at the nose and above the top surface of the pellet pile is required because those zones experience the most severe condensation cycles.

Container Liner Film Architecture and Heat-Seal Integrity

Film architecture is not a single property but a layered response to moisture vapor, liquid water, flexing, and heat-seal stress. Three-layer polyolefin liners place a low-seal-initiation-temperature resin on the inner surface, a high-strength LLDPE or HDPE core, and an outer layer resistant to tear propagation. When EVOH is incorporated as a barrier core, the structure suppresses WVTR but requires continuous polyolefin encapsulation because EVOH loses oxygen barrier function when moisture swells the polymer; the WVTR of the total structure remains governed by the polyolefin skins. Aluminum foil laminates provide the lowest WVTR but concentrate stress at the bottom gusset welds and at the top skirt folds. The tie layer between foil and PE must maintain adhesion through the expected voyage temperature range, and corrosion of the aluminum layer occurs where salt-laden water penetrates a seal defect.

EVOH is a moisture-sensitive oxygen barrier; when it absorbs moisture, its oxygen barrier declines, but the WVTR of the total structure is dominated by the polyolefin layers. Aluminum foil laminates provide near-zero WVTR but require stress dissipation at the corner welds to avoid flex cracking. The tie layer between PE and foil must maintain adhesion through the expected voyage temperature range. Where the same liner is reused or where the container is washed with salt water, residual chloride on the container floor accelerates aluminum corrosion at the bottom weld. Published data for this specific configuration is limited, but the corrosion mechanism is consistent with galvanic attack at exposed foil edges.

Heat-seal integrity at the bottom gusset is the most frequent liner-origin moisture path because the seal is formed during liner fabrication and then flexed during installation. Seal strength under ASTM F88/F88M-21 should exceed the film tear strength so that failure occurs in the film, not at the seal. A seal that separates at less than the film yield stress indicates a welding defect or contaminated seal area. During loading, the bottom seal bears the dynamic pressure of the pellet column and the abrasion of the discharge chute. Installation procedures that fold the liner at the corners without strain relief create the same failure signature as a seam defect, and the distinction is only visible after the liner is removed and laid flat.

Inside a 40 ft high-cube container, the liner is specified by bulkhead dimensions and loading method, not by resin grade alone. The gap between the liner skirt and the top rail is a recurrent moisture path; if the liner is not tensioned and taped at the top, warm humid air enters during the day and condenses on the cooler resin surface at night. The resulting dew point condensation migrates through the pellet bed, creating wet zones near the sidewalls that are difficult to sample. Loading operations that discharge resin from a silo through a telescopic chute generate electrostatic surface charge on the pellets; the charged pellets repel one another and produce an open bed, increasing the air contact area at the top of the pile. The top surface of the pile is therefore the first region to exceed moisture specification and the region most commonly omitted from sampling because discharge sampling draws from the center of the bag.

If the Container Liner Fails at the Bulkhead Corners

When a liner puncture occurs at the lower bulkhead corner, the defect path behaves as a capillary inlet under the stack pressure of 25–30 t of resin in a 40 ft container. Water entering the puncture spreads laterally along the floor before wicking upward through the pellet bed. The visible wetting front understates the affected volume because PET pellets at 0.40 wt% moisture may remain free-flowing and visually dry while already above the 0.005 wt% processing limit. The affected resin must be segregated by distance from the puncture, and the claim calculation must include not only the wet pellets but the geometric volume of the moisture diffusion zone. Bulk samplers that take a single composite from the discharge chute cannot resolve the boundary of this zone.

Thermal cycling of the container drives condensation even without liner puncture. A container moving from 40 °C daytime to 10 °C nighttime can create an internal dew point that exceeds the surface temperature of the liner and the pellets. Moisture condenses on the top sheet and drips onto the cargo. Container roof condensation becomes a problem when the resin is loaded at high humidity and the container is sealed before the headspace is conditioned. Headspace loggers with ±0.5 °C and ±3 % RH accuracy should be placed at the door end and the nose end, not only at the center of the bag. The nose-end record is the most diagnostic for condensation because the cargo at the nose undergoes the largest thermal lag.

Compiling a Defensible Claim Data Package

A moisture-related quality claim is adjudicated against the bill of lading values, not against the buyer’s dryer performance. The claimant must demonstrate that the pellets at discharge deviated from the contracted specification and that the deviation originated from the transport barrier or stowage, not from pre-existing moisture at pack-out. A defensible claim package includes the pack-out moisture report under ISO 15512:2019 or ASTM D6869-17, the liner installation log, the headspace temperature and humidity records, the discharge moisture report, the intrinsic viscosity results under ASTM D4603-18, and the retained samples from both the rail car and the container. Missing link data is the most common claim failure because the same moisture value can be produced by a breached liner, a condensation cycle, or a wet silo at load port.

Intrinsic viscosity loss in wet PET is not reversible, but the magnitude of loss must be separated from the normal thermal history of the resin. Bottle-grade PET with an initial IV of 0.82 dL/g will show a different hydrolysis signature when processed wet than when processed dry. The claim surveyor should request IV from the incoming pellet, not only from the finished preform. A drop of ≤0.03 dL/g from the bill of lading value is sometimes accepted as normal drying and processing variation; larger drops accompanied by discharge moisture above 0.10 wt% indicate a transport-related moisture excursion, not ordinary process variation. This threshold is contractual, and the parties should fix the exact ΔIV and moisture values in the sale contract.

Container liner claims often turn on whether the moisture entered as liquid water through a breach or as water vapor through an intact barrier. Liquid water ingress at a puncture creates high localized moisture and often leaves a visible contamination line, while vapor ingress through an underspecified film produces a diffuse moisture rise across the pellet bed. The two mechanisms require different claim treatments and different corrective actions. A puncture requires repair of the liner installation procedure and may shift responsibility to the loading crew; a high-WVTR liner requires re-specification of the barrier film. The distinction is made by comparing moisture mapping from multiple sampling ports against the headspace humidity record. If the highest moisture values align with the puncture coordinates, the breach is the primary cause. If the moisture rise is uniform and the WVTR was near the contract limit, the barrier film itself is the source.

Moisture barriers for shipping PET resin are defined not only by the film data sheet but by the installed geometry, the heat-seal integrity, the voyage thermal cycle, and the moisture sampling plan. A specification that stops at the phrase “moisture barrier liner” without a WVTR ceiling, flex-crack method, seal-strength minimum, and puncture test leaves the quality claim dependent on after-the-fact sampling rather than controllable transport parameters.