TPU Drying and Moisture Control: How to Prevent Hydrolysis, Bubbles, and Process Drift
A drying recipe is only one part of moisture control. TPU can absorb water during storage, after a bag is opened, inside an underperforming dryer, through leaky conveying, from wet masterbatch or regrind, and again while dried material waits at the machine. The practical goal is a verified dry-material path from sealed package to feed throat—not simply a heater setpoint.
Quick answer: TPU is hygroscopic, and excess moisture can create bubbles, splay, unstable extrusion and hydrolytic molecular-weight loss during melt processing. Use the current supplier guidance for the exact grade, but verify the whole system: incoming package condition, dryer temperature, dry-air dew point, airflow, bed loading, residence time, throughput, dried-pellet temperature, closed transfer and moisture at or near the feed throat. Do not apply one universal temperature, time or moisture limit to every TPU.
Why moisture control matters before and during melt processing
Water on or within a TPU pellet becomes more than steam when the polymer is heated. It can also participate in hydrolysis, splitting susceptible polymer chains and reducing molecular weight. A visibly foamy melt is an obvious warning, but the more difficult case is a part that looks acceptable while melt strength, viscosity or retained mechanical properties have changed.
| Stage | Moisture-related risk | Evidence to collect |
|---|---|---|
| Storage and bag opening | Condensation on cold material; humid-air exposure; damaged or repeatedly opened packaging | Package condition, storage temperature/humidity, bag-open time and resealing practice |
| Drying | Insufficient temperature, dry-air quality, airflow or residence time; overloaded hopper | Actual inlet/return temperature, dew point, airflow indication, bed level, throughput and residence-time calculation |
| Transfer and blending | Ambient pickup; leaks; wet receiver, hose, masterbatch, additive or regrind | Closed-path inspection, leak/receiver checks, component moisture and waiting time |
| Melt processing | Hydrolysis, gas, unstable viscosity or added degradation from heat/residence/shear | Actual melt temperature, residence time, pressure/load, appearance, flow/viscosity and retained properties |
| Post-process diagnosis | Symptoms attributed to moisture when venting, contamination or overheating is responsible | Known-dry control, instrument check, marked samples and one-variable trials |
BASF’s Elastollan processing guide states that its TPU granulate rapidly absorbs atmospheric moisture and gives a portfolio-level moisture target and drying table. Those values apply to BASF’s products, not automatically to GOAFLEX grades. The transferable lesson is to protect dry pellets, dry compatible secondary streams and refer to the selected grade’s current process sheet.
The five variables in an effective drying system
Drying happens when moisture can diffuse out of heated pellets and dry air can carry it away. A timer alone cannot prove this happened. Record the variables as a connected system.
| Variable | What it controls | Common trap |
|---|---|---|
| Temperature | Raises moisture-diffusion rate within the pellet | Displayed setpoint differs from pellet or return-air temperature; an excessive setting risks sticking or thermal damage |
| Dry-air dew point | Indicates how little water vapor the drying air contains | A heater-only unit makes pellets hot without providing sufficiently dry air |
| Airflow | Removes released moisture and distributes heat through the bed | Blocked filters, leaking hoses, wrong valves or channeling leave zones under-dried |
| Residence time | Gives pellets time at effective conditions | Hopper size is quoted without accounting for bulk density, throughput, refill pattern or short-circuiting |
| Bed loading and throughput | Determine whether every pellet sees the intended cycle | Production rate changes while the drying plan remains fixed |
Dew point is a dryer-health indicator, not a resin-moisture result. A good dew-point reading at the machine does not prove that every pellet reached the correct temperature for long enough. Conversely, an acceptable pellet-moisture result is stronger evidence than setpoints alone, provided the sampling and analytical method are controlled.
Where dried TPU can become wet again
- Opened bags or gaylords left in humid production areas, especially when partially used packaging is not resealed.
- Cold material opened before warming to room conditions, allowing condensation on pellets or packaging surfaces.
- Dried pellets discharged to open bins, carts or bags while waiting for a machine, changeover or quality release.
- Long or leaky conveying routes that draw humid room air, or receivers and filters that are not sealed and maintained.
- An uncovered machine hopper or a low-throughput period that leaves hot material exposed longer than planned.
- Wet color concentrate, additive, carrier, reclaimed material or regrind blended into an otherwise dry base resin.
- Regrind with high surface area stored openly after grinding; it still requires controlled identification, storage and drying.
Map the real path during a normal shift, startup, meal break, material change and overnight stop. Intermittent defects often follow handling events that a steady-state drying record does not capture.
Moisture symptoms—and defects that can look similar
| Observation | Moisture-related possibility | Other mechanisms to separate |
|---|---|---|
| Bubbles, foam or splay | Water flashes and/or hydrolysis occurs in the melt | Trapped air, volatiles, incompatible additives, overheating, long residence or poor venting |
| Lower viscosity or melt strength | Hydrolytic molecular-weight loss | Excess melt temperature/shear, wrong grade, ratio error or repeated reprocessing |
| Unstable extrusion output or die drool | Variable wet feed or degraded rheology | Feeding/surging, screen restriction, temperature drift, screw/die mismatch |
| Brittle or weak parts | Hidden chain scission from wet processing | Contamination, poor fusion, knit-line design, thermal degradation or wrong conditioning |
| Silver streaks or surface haze | Gas/moisture in the melt | Mold air, surface contamination, poor dispersion, cooling or downstream damage |
| Defect after adding color/regrind | Secondary stream introduces moisture | Incompatibility, contamination, different melt behavior or thermal history |
P.A.T. Products describes dew-point verification as an important diagnostic when material has been heated but moisture symptoms persist. Its published temperature, time and ratio ranges remain supplier and grade context, not universal GOAFLEX instructions. A known-dry control and direct moisture result are safer than repeatedly increasing dryer settings.
Build a factory troubleshooting worksheet
- Grade, lot, package type and condition; storage temperature/humidity; date and time opened; reseal history.
- Dryer make/type, hopper capacity and bed level; set and actual supply/return temperatures; dew point; airflow or pressure indication; regeneration/alarm status.
- Material bulk density, production throughput, refill pattern, calculated minimum residence time and interruptions.
- Conveying layout, hose/receiver/filter condition, distance, ambient pickup points and machine-hopper cover or dry-air blanket.
- All blend components: color/masterbatch, additives and regrind, including carrier identity, ratio, storage and drying.
- Moisture sample location, sampling time, sealed-container handling, method, calibration/verification and result.
- Actual melt temperature, barrel/die settings, screw speed, back pressure, residence time, pressure/load and process trend.
- Defect photographs and retained samples marked by machine direction, cavity/web position, time, roll/shot and operator side.
- Finished-product comparison: appearance, dimensions, flow/viscosity indicator, tensile/tear or application-critical retained properties.
Verification: measure the resin and confirm the product
A moisture method must be appropriate for the material, expected range and plant decision. Karl Fischer titration is widely used for direct water determination, while loss-on-drying or moisture-analyzer methods infer mass loss and may also respond to volatiles. Whatever method is chosen, define sample mass, preparation, temperature/program, blank or calibration checks, sampling point, sealed transfer and acceptance rule. Do not compare values produced by unlike methods as if they were identical.
- Confirm the instrument and sampling procedure with a dry reference or other plant control.
- Sample incoming or stored material when investigating package history, then sample dried material as close as practical to the machine throat.
- Run a known-dry control under stable settings. If the defect remains, investigate venting, contamination, thermal history, equipment and grade suitability.
- Compare process evidence such as pressure, screw load, output, fill and surface—not moisture alone.
- Condition and test parts consistently. When risk is meaningful, compare flow/viscosity and critical mechanical or functional properties, not just appearance.
- Confirm the corrective action across normal lots, humid days, startups, rate changes and production duration before revising the control plan.
What to send GOAFLEX for a moisture-control review
- Exact TPU grade, lot, current TDS/process sheet, package and storage/open-bag history.
- Process route, equipment, screw/shot size, throughput, actual melt conditions, residence time and defect timing.
- Dryer type and capacity, hopper level, set/actual temperatures, dew point, airflow information, residence-time calculation and maintenance/alarm history.
- Conveying path and machine-hopper arrangement; all color, additive and regrind streams with ratios and drying history.
- Moisture method, sample location/handling, instrument checks and results for good versus problem runs.
- Marked good/failed samples, photos, flow/viscosity evidence and the finished-product properties that must be retained.
Troubleshooting intermittent bubbles, splay or process drift? Send the grade, dryer/transfer record, measured moisture, actual process data and retained samples to hello@goaflex.com. Also see GOAFLEX’s TPU film extrusion troubleshooting guide and TPU injection molding defects guide.
Sources and further reading
- BASF — Elastollan Processing Recommendations — supplier-specific guidance on hygroscopic storage, drying, secondary streams, regrind and processing; numeric conditions must not be transferred to another grade.
- P.A.T. Products — TPU processing and regrind: drying, defects, and fixes — practical secondary context on desiccant-dryer performance, dew point, moisture symptoms and regrind handling; verify all settings against the selected grade.
- Chen et al. — Moisture and injection-molded parts, Applied Sciences 2022 — open-access research showing moisture-related bubble and appearance effects and interactions with plasticization parameters for the studied TPU and PC systems.
- GOAFLEX — TPU Film Extrusion Troubleshooting — related diagnostic guidance from material storage and drying through the extrusion line.
- GOAFLEX — TPU Injection Molding Problems — related molding-defect context, including the need to separate moisture from tooling and process causes.
Image credits and licensing
Image 1: A Karl Fischer titrator illustrates direct moisture measurement; the suitable method and acceptance limit remain grade-specific. Creator/source: Datamax. Original source page. License: Public domain. Optimized draft copy: 146 KB; resized/compressed with no substantive alteration. ALT guidance: “Karl Fischer titrator used for laboratory moisture measurement”
Image 2: Generic molding pellets illustrate material handling; the pictured polymer is not verified as TPU. Creator/source: Teemeah. Original source page. License: CC BY-SA 3.0. Optimized draft copy: 147 KB; resized/compressed with no substantive alteration. ALT guidance: “Close-up of colored plastic pellets used for injection molding”
Image 3: An existing twin-tower desiccant compressed-air dryer schematic provides contextual dry-air vocabulary; it is not a TPU hopper-dryer specification. Creator/source: Brian S. Elliott. Original source page. License: CC BY-SA 4.0. Optimized draft copy: 91 KB; resized/compressed with no substantive alteration. ALT guidance: “Twin-tower desiccant compressed-air dryer schematic”
Image 4: A generic plastic extruder illustrates the feed-to-die process path. Creator/source: Cjp24. Original source page. License: CC BY-SA 3.0. Optimized draft copy: 176 KB; resized/compressed with no substantive alteration. ALT guidance: “Industrial plastic extruder with hopper, drive, barrel and downstream dies”
Context note: These search-sourced, commercially reusable visuals do not prove GOAFLEX ownership of the equipment or materials, creator endorsement, exact TPU chemistry, grade identity, process settings, test validity, or product performance. Confirm final CMS attribution and license display before publication.
Intellectual property notice: If any information or material in this article is believed to violate intellectual property rights, please contact GOAFLEX so it can be reviewed and addressed.
