Key takeaways
- Hygroscopic resins — PA, PET, PC, PBT, ABS, PMMA — absorb water into the polymer matrix, not just onto the pellet surface, so they cannot be re-dried by wiping or warming the bag; they need a hot-air or desiccant dryer to the producer's spec before processing.
- The two failure modes are different: surface moisture on any resin causes splay and bubbles, while absorbed water hydrolyses the condensation polymers (PET, PA, PC, PBT) at melt temperature — permanently cutting molecular weight and intrinsic viscosity, which no amount of later drying can restore.
- Typical pre-processing moisture targets are demanding: PET below roughly 50 ppm (0.005%), polycarbonate below roughly 200 ppm (0.02%), and ABS/PA in the 0.02–0.20% band — verified by Karl Fischer titration, not a damp-to-touch judgement.
- Most in-transit damage is condensation (container or cargo sweat), not a leaking roof — controlled by foil/PE inner liners, container desiccant bags sized to cargo weight and voyage length, and by reading moisture-as-shipped on the Certificate of Analysis.
There are two completely different moisture problems in a polymer shipment, and conflating them costs money. The first is surface moisture — condensation that lands on the pellet skin and flashes off in the barrel as splay, silver streaks or bubbles; it is a cosmetic and processing nuisance that drying removes cleanly. The second is absorbed moisture: water that has migrated into the polymer matrix of a hygroscopic resin, where at melt temperature it hydrolyses the chains and permanently destroys molecular weight. The first is annoying. The second is irreversible, and it is the reason PA, PET, PC and PBT carry drying specs at all.
Knowing which resins are hygroscopic — and treating their inner liners, desiccants and pre-drying as part of the spec rather than packaging detail — is what separates a clean engineering-plastics order from a dispute over short shots and brittle parts. It belongs on the RFQ and the Certificate of Analysis, not in a phone call after the container lands.
Hygroscopic grades pull water into the polymer through hydrogen bonding and free-volume diffusion; the moisture is held internally and cannot be wiped or briefly warmed away. Non-hygroscopic grades — the commodity polyolefins and PVC — only carry surface condensation. Nylon is the extreme case: PA6 will absorb several percent water at equilibrium in humid air, which is why it ships sealed and is dried hard before moulding. For the resin families themselves, see our engineering plastics comparison and the PA6 vs PA66 guide.
| Resin | Hygroscopic? | Typical dry-to target | Damage if wet |
|---|---|---|---|
| PET | Yes (strongly) | ≤ ~50 ppm (0.005%) | Hydrolysis — IV / molecular-weight loss, brittleness |
| PA6 / PA66 (nylon) | Yes (strongly) | ~0.10–0.20% (grade-dependent) | Splay, hydrolysis, strength loss |
| Polycarbonate (PC) | Yes | ≤ ~200 ppm (0.02%) | Hydrolysis, silver streaks, haze |
| PBT | Yes | ≤ ~0.04% | Hydrolysis — molecular-weight loss |
| ABS / SAN | Yes (mild) | ~0.02–0.10% | Splay, surface defects (rarely permanent) |
| PMMA | Yes | ~0.02–0.08% | Bubbles, splay |
| PE / PP / PS / PVC | No (surface only) | Generally none | Surface splay if visibly wet |
The mental image of a leaking container roof is mostly wrong. The dominant in-transit moisture risk is condensation — 'container sweat' (water forming on the steel ceiling and walls and dripping onto cargo) and 'cargo sweat' (water condensing on the cooler pellets themselves). Both are driven by the dew point: as a box transits climate zones, day-night and sea-air temperature swings drop surfaces below the dew point of the air trapped inside, and water comes out of that air. A container loaded warm and humid in a tropical port, then chilled crossing higher latitudes, is a textbook sweat event — no rain required.
Surface moisture is a drying problem. Absorbed moisture in a condensation polymer is a molecular-weight problem — and you cannot dry molecular weight back in.
Protection is layered, and no single measure is sufficient on its own. The producer's job is to pack dry and seal well; your job is to specify it and to re-dry on intake regardless. The controls that matter, in rough order of leverage:
- Sealed inner liner — a foil-laminate or co-extruded PE liner inside the 25 kg bag or FIBC, heat-sealed, is the primary moisture barrier for hygroscopic grades. Confirm liner type the same way you confirm bag weight on packaging and container loading.
- Container desiccant bags — hanging calcium-chloride poles or pouches absorb the airborne moisture that drives sweat. Size them to cargo weight and voyage length (suppliers rate them in grams of moisture capacity); a single token bag in a 21 MT box is theatre, not protection.
- Dry, cool packing — sealing the liner in low-humidity conditions traps less water inside to begin with; packing hot resin into a sealed bag bakes moisture in as it cools.
- A dry container — the box interior should be inspected dry and undamaged before loading, ideally as part of a pre-shipment inspection and its photo record.
- Desiccant indicator cards or a humidity logger in high-value loads give you evidence of what the cargo actually experienced if a dispute arises.
Even a perfectly packed, perfectly dry container of nylon or PET must be dried before it is moulded, because a hygroscopic resin re-absorbs moisture from ambient air the moment a bag is opened. The producer's processing datasheet sets the conditions; the principle is hot air or, for the demanding grades, a desiccant dryer that delivers air at a low dew point (typically around -40°C) so the drying air itself is drier than the resin. Indicative regimes: PET at ~150–180°C for 4–6 hours in a desiccant dryer; PA at ~70–80°C; PC at ~120°C for 3–4 hours; ABS at ~80°C for 2–4 hours. A simple hot-air oven will dry ABS but will not get PET or PC dry enough, because the surrounding air is not dry enough to pull the last of the water out.
| Resin | Dryer type | Indicative temp / time | Why |
|---|---|---|---|
| PET | Desiccant (low dew point) | ~150–180°C, 4–6 h | Crystallisation + very low residual moisture needed |
| PC / PBT | Desiccant | ~120°C, 3–4 h | Tight target below hot-air capability |
| PA6 / PA66 | Desiccant or hot air | ~70–80°C, 4–8 h | High uptake; some grades moulded to a controlled level |
| ABS / PMMA | Hot air often sufficient | ~80°C, 2–4 h | Mild uptake, surface-driven defects |
Moisture is one of the cleanest places to prevent a dispute on paper. Specify the inner liner, the desiccant practice and the moisture-as-shipped acceptance limit on the contract, and require the CoA to report the as-shipped figure and its test method — Karl Fischer titration for the engineering grades, loss-on-drying for less critical ones. That gives your processing team a baseline to dry against and gives you a documented position if hydrolysis ever shows up in a failed part. For the resins where this matters most, route the specification through our engineering plastics desk.
The practical takeaway: separate the two moisture problems and price them differently. Surface condensation is a packing-and-drying problem you can manage with liners, desiccants and an intake dryer. Absorbed water in PET, PA, PC or PBT is a molecular-weight problem that only the producer's pre-drying spec — followed on both sides of the voyage — prevents. Treat the liner, the desiccant and the dry-to target as part of the grade, and you ship parts; treat them as packaging trivia, and you ship a dispute. When in doubt, talk to the desk before the container is sealed.
Frequently asked
Questions on the desk
Which polymers are hygroscopic and which are not?
Hygroscopic resins absorb water into the polymer itself: polyamide (PA6, PA66), PET, polycarbonate (PC), PBT, ABS, PMMA, SAN, PLA and TPU. Non-hygroscopic resins — PE, PP, PS, PVC — pick up only surface moisture that is easy to drive off. The distinction matters because hygroscopic grades need a proper dryer to a specified residual moisture level before processing, while non-hygroscopic grades usually do not.
Why does drying matter so much for PET and nylon?
PET, PA, PC and PBT are condensation polymers. At melt temperature, any water present drives the polymerisation reaction backwards — hydrolysis — which cuts the molecular chains. The result is a permanent drop in molecular weight (for PET, a fall in intrinsic viscosity) that weakens the part and cannot be recovered by re-drying. This is why the pre-drying spec is a quality requirement, not a cosmetic one.
What residual moisture level do hygroscopic resins need before moulding?
It varies by polymer and must come from the producer's processing datasheet, but typical targets are: PET below about 50 ppm (0.005%), polycarbonate below about 200 ppm (0.02%), nylon in roughly the 0.10–0.20% band for many grades (lower for some), and ABS around 0.02–0.10%. Confirm moisture by Karl Fischer titration or a calibrated moisture analyser rather than by feel.
How do you stop resin getting wet in the container?
Most in-transit moisture damage is condensation — 'container sweat' or 'cargo sweat' — caused by temperature swings across a voyage, not a leaking box. Defences are layered: a sealed foil or PE inner liner on bags and FIBCs, container desiccant bags (typically calcium-chloride hanging poles) sized to the cargo weight and transit time, dry packing in low-humidity conditions, and inspecting that the container interior is dry before loading. Even so, hygroscopic resin must be re-dried on arrival before use.
Should moisture be specified on the contract and CoA?
Yes. Put moisture-as-shipped, the required inner liner and desiccant practice, and the acceptance limit on the RFQ and contract, and require the Certificate of Analysis to report the as-shipped moisture figure and the test method. This gives you a documented baseline if a hydrolysis or splay dispute arises and tells your processing team how aggressively to dry on intake.
General market commentary from the OmniaStrata desk, provided for information only. It is not legal, financial, tax, or trading advice, and it is not an offer or a commitment to any terms. Figures such as price ranges, spreads, financing costs, and credit periods are illustrative market context, not OmniaStrata's rates or terms. Actual contract terms — including price, payment instrument, credit, insurance, and Incoterms — are agreed in writing on a per-transaction basis and at OmniaStrata's discretion. Market conditions change; figures reflect the publication date.