Polybutylene Terephthalate Moisture Vs Water Absorption Two Numbers Buyers Confuse
Polybutylene Terephthalate Moisture vs Water Absorption: Two Numbers Buyers Confuse
In a PBT datasheet, a maximum pellet moisture content of 0.02% after drying can appear next to a water absorption value of 0.2% at 23°C and 50% RH. The first number is a process control limit. The second is a standardized material property measured on a molded specimen. Polybutylene terephthalate is polymerized from 1,4-butanediol and dimethyl terephthalate or terephthalic acid. The resulting polyester backbone contains ester groups that hydrolyze in the presence of water at melt temperatures, so moisture content is specified to protect molecular weight during extrusion or injection molding. Water absorption, defined by ISO 62:2008 or ASTM D570-98, measures how much water a conditioned specimen takes up from immersion or humid air over a defined period. Confusing the two does not alter the chemistry but regularly leads to incorrect drying, inaccurate incoming inspection limits, and disputes between molding operations and raw-material suppliers.
What Does a PBT Datasheet Actually Separate When It Lists 0.05% Maximum Moisture and 0.2% Water Absorption?
Residual moisture in PBT pellets, also called water content, is the mass fraction of water present in the polymer at a given time. It is measured by Karl Fischer titration according to ISO 15512:2016 or by loss-on-drying methods such as ASTM D6980-17. In the injection-molding supply chain, the value is typically 0.05–0.15% by mass for material stored at 23°C and 50% RH in unsealed containers. A drying specification of ≤0.02% is a delivery gate before melt processing. It is not a saturation limit, and it does not describe the amount of water a molded PBT part will absorb in service.
Water absorption under ISO 62:2008 is different. A specimen is immersed in distilled water at 23.0 ± 0.5°C for 24 h and then weighed. Unfilled PBT typically reports 0.08–0.15% water absorption under this condition. Equilibrium uptake in air at 23°C, 50% RH is commonly 0.2–0.3%. Saturation in liquid water over extended periods may reach 0.4–0.5% depending on crystallinity and filler content. These are material properties that inform dimensional stability, electrical behavior, and long-term hydrolysis resistance of the molded article. They do not instruct a drying operator how to set a desiccant dryer.
| Measured parameter | Test method | Reported range for unfilled PBT | Relevance |
|---|---|---|---|
| Residual moisture in as-received pellets | ISO 15512:2016 (Karl Fischer), ASTM D6980-17 | 0.05–0.15% by mass after storage at 23°C, 50% RH | Determines dryer setpoint and required residence time before melt processing |
| Maximum residual moisture before melt processing | Supplier specification with Karl Fischer verification | ≤0.02% by mass for unfilled and glass-fiber reinforced PBT | Prevents hydrolytic chain scission and intrinsic-viscosity loss |
| Water absorption after 24 h immersion in distilled water | ISO 62:2008 Method 1, ASTM D570-98 | 0.08–0.15% by mass at 23°C | Material property for service moisture uptake, not a drying specification |
| Water absorption at equilibrium in air | ISO 62:2008 | 0.2–0.3% by mass at 23°C, 50% RH | Long-term dimensional and electrical performance in humid air |
In a desiccant wheel hopper dryer processing a 30% glass-fiber reinforced PBT, the drying condition is normally set to 120–130°C for 4–6 h, with a drying-air dew point of ≤ −30°C. These values are derived from the residual moisture specification, not from the water absorption value. A dryer operator who attempts to calculate the required drying time from the 0.2% water absorption figure will under-dry the material. Loss-on-drying at 160°C may report 0.04–0.08% total volatiles for a material that contains only 0.015% water by Karl Fischer titration. The discrepancy arises because low-molecular-weight terephthalate oligomers and other volatiles are released under heat. ISO 15512:2016 Method B is therefore preferred when a dispute exists regarding moisture content. For PBT pellets, validation of residual moisture in a molding facility typically uses a coulometric Karl Fischer instrument with a sample size of 0.5–1.0 g and a titration temperature of 160–180°C. A moisture value above 0.02% at the feed throat of an injection molding machine is not offset by the fact that the material is subsequently melted; hydrolysis occurs within seconds to minutes at melt temperatures.
| PBT grade type | Dryer temperature range | Typical drying time | Maximum residual moisture | Water absorption after 24 h immersion, 23°C | Equilibrium uptake at 23°C, 50% RH |
|---|---|---|---|---|---|
| Unfilled injection-molding grade | 120–130°C | 4–6 h | ≤0.02% | 0.08–0.15% | 0.2–0.3% |
| 30% glass-fiber-reinforced grade | 120–130°C | 4–6 h | ≤0.02% | 0.07–0.12% | 0.15–0.25% |
| Flame-retardant unfilled grade | 100–120°C | 4–6 h | ≤0.02% | 0.10–0.18% | 0.2–0.35% |
Compounding of PBT is typically performed on a co-rotating twin-screw extruder with an L/D ratio of 32:1 to 44:1, followed by vacuum venting at −0.08 MPa to −0.095 MPa to remove volatiles before pelletization. Residual moisture in the finished pellet is not fixed by the water absorption property of the resin; it depends on pellet-cooling water carryover, dryer residence time, and ambient humidity during packaging. This production-scale behavior explains why as-received moisture can vary from lot to lot even when the PBT water absorption value is identical across lots. A certificate of analysis that reports water content by ISO 15512:2016 is therefore more useful to a molder than a datasheet water absorption value for setting the pre-drying protocol.
Equilibrium Uptake and Melt-Phase Hydrolysis Kinetics
PBT is not as hygroscopic as polyamide, but it is water-sensitive in the melt. Equilibrium moisture absorption in air at 23°C, 50% RH is approximately 0.2%, while a glass-filled grade may report 0.07–0.10% after 24 h in water. The lower filler volume fraction explains part of this difference, but the polyester matrix remains the hydrolyzable phase. The glass transition temperature of PBT is about 45–60°C, and the crystalline melting point is approximately 223–225°C. Above the melting point, water reacts with ester groups in a pseudo-first-order hydrolysis process. Chain scission reduces molecular weight and intrinsic viscosity. Intrinsic viscosity of PBT, measured according to ISO 1628-5 in a phenol/tetrachloroethane mixture at 25°C, is typically 0.80–1.00 dL/g for injection-molding grades. A drop below 0.70 dL/g after processing is generally associated with reduced tensile elongation and impact strength.
Published kinetic constants for commercial PBT grades vary with catalyst residues, end-group concentration, and chain extenders, so supplier-specific processing bulletins specify a maximum residual moisture rather than a universal hydrolysis rate constant. The practical threshold is consistent: moisture above 0.02% by mass before melt processing can produce intrinsic-viscosity loss that is measurable after a single injection-molding cycle. This is why the 0.02% number appears on drying instructions while the 0.2% water absorption number appears on the property table. One is a kinetic safeguard. The other is an equilibrium or quasi-equilibrium material response.
On an injection molding machine with a screw diameter of 40 mm and a barrel profile of 240–270°C, undried PBT at 0.08% residual moisture can generate visible splay on a connector housing and reduce the melt viscosity by 10–20% relative to material dried to 0.02%. The measured water absorption value of the same grade may remain 0.1% after 24 h immersion. That value does not predict the splay defect because splay is a function of liberated steam during mold filling, not of the water-uptake property of the solid specimen. Nozzle temperature, back pressure, and mold venting influence the appearance of splay, but the root cause in moisture-sensitive polyester processing is usually residual water. A moisture level above 0.02% is also more damaging at higher melt temperatures because the hydrolysis reaction rate increases with temperature. For glass-filled PBT, the same drying limit applies because the polyester matrix still hydrolyzes even though the filler reduces the bulk water-absorption percentage.
Storage conditions shift the moisture content of the pellets without changing the datasheet water absorption value. At warehouse relative humidity above 60%, PBT pellets take up surface moisture quickly and should be dried before processing even if the original certificate of analysis reported 0.02%. Sealed containers, dry-air purge, or desiccant bed storage are used to minimize moisture regain. The residual moisture specification is therefore a time-dependent process variable. The water absorption value remains a material property measured on a test specimen under defined conditions. Both numbers are valid, but they cannot be directly substituted.
When a Moisture Analyzer Over-Reports Water and the Water Absorption Value Cannot Correct the Dryer
Quality-control laboratories sometimes use a halogen moisture analyzer for speed. In PBT, the method can over-report water because additives, sizing agents on glass fiber, and low-molecular-weight oligomers volatilize at the test temperature. A halogen analyzer set to 160°C may report 0.06% total volatiles while a Karl Fischer titration according to ISO 15512:2016 reports 0.018% water. If the incoming-inspection specification is written only as ≤0.02% moisture and the method is not defined, a buyer may reject material that is actually process-ready. Conversely, accepted material with a low loss-on-drying reading may still contain too much water if the sample was not protected from ambient humidity during handling. The water absorption value of 0.2% does not correct this. It is a separate test under ISO 62:2008 or ASTM D570-98 that requires a molded specimen or a conditioned test piece, not pellets. For drying control, the relevant method is water-specific Karl Fischer titration, not water absorption. For service performance, the relevant method is water absorption under the intended exposure. Combining them into one specification creates a false contradiction between 0.02% and 0.2%.
Specimen preparation for water absorption is also influential. An injection-molded PBT disk with a thickness of 2 mm reaches 24 h immersion uptake faster than a 4 mm specimen. Crystallinity differences caused by mold temperature affect the amorphous phase, where water diffuses preferentially. A mold temperature of 80–120°C for PBT produces higher crystallinity than a cold mold at 20–30°C. The higher crystallinity reduces equilibrium water absorption by a few tenths of a percentage point, but the residual moisture specification before processing remains unchanged. This is another reason why a water absorption measurement cannot be used as an incoming moisture check: the test specimen history is not the same as the pellet lot history.
