A TPU part that feels flexible at room temperature can become much stiffer as temperature falls. Whether that change becomes a field failure depends on more than a datasheet temperature: grade chemistry, specimen geometry, conditioning time, strain rate, impact, prior aging, and the function the part must still perform.
Quick answer
Do not approve TPU for cold service from Shore hardness or one “low-temperature” value alone. Define the real duty cycle: minimum and typical temperature, exposure time, bending or impact mode, deformation rate, number of cycles, preload, fluid exposure, and the function that must remain intact. Use a grade-specific laboratory screen, then test the processed and pre-aged finished construction at the relevant temperature. ASTM D746 brittleness data can be useful, but the standard itself cautions that the result does not necessarily equal the lowest suitable service temperature.
What changes in TPU as temperature falls
TPU is a segmented polymer. Its soft and hard segments, morphology, additives, pigments, and processing history together determine how molecular motion changes with temperature. Cooling generally raises stiffness and can change damping, rebound, impact response, tear behavior, and crack-growth resistance. The transition is not a single universal switch shared by every TPU grade.
Glass-transition temperature is useful background, but its reported value depends on the measurement technique and conditions. BASF’s TPU handbook notes that dynamic and static methods can produce different transition values and that thermal history matters. Dynamic mechanical or torsional testing can therefore help compare the modulus and damping trend across temperature, but it does not replace a finished-part test.
Soft-segment chemistry can guide initial screening. Polyether-based TPU is often considered when hydrolysis resistance and low-temperature flexibility matter, while polyester-based systems may be attractive for other mechanical or oil-exposure requirements. That is a starting direction—not proof that every polyether grade will outperform every polyester grade in a specific cold application.
Why room-temperature Shore hardness is not enough
Shore hardness measures resistance to indentation under defined conditions. It is not a direct measurement of cold bending, impact energy, rebound, fatigue, sealing force, or cable handling. Two grades carrying the same room-temperature Shore rating may have different soft-segment chemistry, crystallization behavior, stabilizers, pigments, and modulus-versus-temperature curves.
Room-temperature input | What it can help screen | What it does not prove |
|---|---|---|
Shore hardness | Initial feel and indentation resistance under the stated method | Cold modulus, impact survival, rebound, crack growth, or minimum service temperature |
Tensile / elongation | Short-term uniaxial behavior on the stated specimen | Behavior at a notch, weld, tight bend, fast impact, or after cold cycling |
Tg or DMA data | Where stiffness and damping change for that method and specimen | Automatic pass/fail temperature for the final product |
Brittleness temperature | Failure probability under the defined impact test | Safe temperature for every geometry, load rate, and service function |
Choose the test by the cold failure mechanism
Test or evidence | Question it answers | Important limits to report |
|---|---|---|
ASTM D746 impact brittleness | At what temperature does a defined share of specimens fail under the specified impact condition? | Procedure, specimen, thickness, conditioning, impact setup, failure definition; not a universal service limit. |
Cold bend / mandrel test | Can a film, coated fabric, cable, or hose bend around a defined radius without visible cracking? | Temperature, dwell, radius, bend rate, orientation, reinforcement, inspection and leak/electrical checks. |
Low-temperature impact | Can the material or part tolerate a drop, strike, puncture, or collision while cold? | Impactor, energy, speed, notch, support, geometry, temperature and pass criterion. |
DMA or torsional stiffness | How do modulus and damping change across a temperature sweep? | Frequency, strain, heating/cooling rate, specimen history and selected transition definition. |
Rebound and cyclic flex | Does energy return or repeated bending remain acceptable after cold conditioning? | Cycle count, amplitude, frequency, rest time, heat buildup, crack criterion and recovery. |
Finished-part functional test | Does the cold product still seal, carry fluid, protect a conductor, bond, fit, or cushion? | Complete construction, assembly, environment, load sequence and acceptance limits. |
Testing laboratories list different low-temperature methods because they answer different questions. ARDL, for example, lists brittleness, bend, retraction, durometer, torsional stiffness, and low-temperature compression-set methods. E Squared similarly distinguishes bending, impact, rolling, and folding for coated fabrics. Select the method that reproduces the dominant deformation—not simply the test most often printed on a datasheet.
Application validation should reflect the construction
Application | Likely cold concern | Useful validation sequence |
|---|---|---|
Cable jacket | Stiff handling, jacket cracking at a bend, impact damage, conductor exposure | Condition cable reels/assemblies; bend at defined radius; impact and flex-cycle; inspect and run electrical/abrasion checks. |
Hose or tubing | Kink, wall cracking, leakage, fitting stress, pressure shock | Condition filled and empty assemblies; bend and pressure-cycle; inspect, leak-test, and measure recovery. |
Footwear / winter sports | Loss of flex or rebound, crack at grooves or bonds, impact discomfort | Condition complete component; flex and impact at temperature; examine sole geometry, bonds, traction and recovery. |
Film / coated fabric | Crease cracking, coating fracture, seam leakage or delamination | Cold-fold or mandrel-bend the complete laminate; cycle seams; inspect and retest hydrostatic/air retention as applicable. |
Molded protective part | Brittle failure at ribs, corners, gates, knit lines or fasteners | Test molded parts with actual wall transitions and assembly preload; add drop/impact and thermal cycles. |
Refrigerated equipment | Long dwell followed by movement, sealing, cleaning or impact | Age at operating temperature and fluids; actuate while cold; test dimensions, force and function before/after warm recovery. |
Pre-aging and geometry can change the result
A pristine plaque may pass while the product fails because cold is only one part of the exposure history. Include pre-aging when the application also sees heat, humidity, water, oils, cleaners, fuels, sweat, UV, or repeated sterilization. Hydrolysis or chemical swelling may alter molecular weight, dimensions, and crack resistance before the cold event occurs.
- Use the intended wall thickness; thin films, thick molded sections, and foams cool and deform differently.
- Include notches, gate scars, knit lines, perforations, sharp radii, cut edges, welds, adhesive layers, textile reinforcement, and fittings.
- Record drying, melt temperature, residence time, regrind, annealing, orientation, and conditioning. Processing damage or residual stress can become visible only after cooling.
- Control strain rate. A part may bend slowly but crack during a fast impact at the same temperature.
- Measure warm recovery as well as the cold result. Some parts regain shape after warming; others retain set, damage, or lost bond strength.
Common cold-service failures and first checks
Observed failure | Possible contributors to separate | Evidence to collect |
|---|---|---|
Crack at a bend or edge | Cold stiffening, radius too tight, notch, poor cut, orientation, prior aging | Crack-origin photos, section thickness, bend radius/rate, temperature history and control samples. |
Impact fracture | Brittle response, stress concentration, void, knit line, assembly preload | Impactor/energy, support condition, part orientation, molded quality and failure location. |
Hose kink or leak | Wall stiffening, reinforcement mismatch, fitting strain, insufficient radius | Pressure and bend sequence, fitting geometry, wall construction, leak location and warm recovery. |
Cable jacket split | Cold bend plus abrasion, installation damage, conductor geometry or notch | Mandrel radius, reel/installation temperature, jacket thickness, conductor bundle and abrasion marks. |
Loss of rebound or cushioning | Modulus/damping shift, foam cell behavior, set, slow recovery | Conditioning time, density/cell structure, cyclic load, recovery interval and retained dimensions. |
Bond or seam failure | Differential contraction, adhesive transition, weak surface preparation, frozen moisture | Full layer stack, overlap/edge geometry, bond process, peel/leak test at temperature and after cycling. |
A practical cold-service approval plan
- Write the duty cycle before selecting the test: minimum/typical temperature, dwell, ramp, motion, load, impact, cycles, fluids, and recovery.
- Screen exact candidate grades with comparable specimens and conditioning. Keep a room-temperature control and document all method details.
- Measure a temperature trend—not only one endpoint—when stiffness or damping transition is important.
- Process representative specimens and parts using intended drying, molding/extrusion, color, regrind, annealing, bonding, and assembly conditions.
- Run cold bend, impact, cyclic, or pressure/leak/electrical tests matched to the actual failure mode.
- Repeat after relevant heat/humidity, water, chemical, or UV pre-aging; test the complete construction and weakest interfaces.
- Confirm across normal lots and production conditions, then set incoming, process, and finished-part controls.
What to send GOAFLEX for grade and sample review
- Final application and drawing, including wall thickness, radii, notches, reinforcement, welds/bonds, fittings, and assembly preload.
- Minimum, typical, and peak temperatures; conditioning time; cooling rate; indoor/outdoor or refrigerated exposure; warm-recovery expectations.
- Deformation mode and rate: bend radius, flex amplitude/frequency, impact energy, pressure, load, drop height, or installation method.
- Exact fluids, cleaners, oils, fuels, humidity, water, salt, sweat, and UV/heat aging that occur before or during cold service.
- Current grade, hardness, chemistry if known, color/additives, drying and process history, regrind level, and known-good/failed samples.
- Required standard, specimen, cycle count, failure definition, and functional acceptance limits such as leakage, electrical integrity, retained force, rebound, appearance, or dimensional recovery.
Planning a cold-service TPU trial? Send the application, cold duty cycle, part drawing, aging conditions, and pass criteria to hello@goaflex.com. For adjacent material-selection context, see Polyether vs. Polyester TPU for Tubing and Hose and Industrial Hose Materials Explained.
Sources and further reading
- ASTM International — ASTM D746-24: https://www.astm.org/d0746-24.html — Scope, significance, and the explicit caution that impact brittleness does not necessarily define the lowest suitable service temperature.
- BASF — Elastollan TPU Material Properties: https://download.basf.com/p1/8a8082587fd4b608017ff4e900830157/en/Elastollan — Supplier-level background on chemistry, modulus-versus-temperature, glass transition, torsional response, impact, and the need to test finished parts.
- Akron Rubber Development Laboratory — Low-Temperature Testing: https://ardl.com/testing/low-temperature-testing/ — Overview of available brittleness, bend, torsional-stiffness, retraction, hardness, and compression-set methods.
- Smithers — Brittleness Testing: https://www.smithers.com/industries/materials/polymer/physical-testing/material-properties-testing/brittleness-testing — Testing-laboratory context for impact/flex brittleness methods and specimen preparation.
- E Squared — Cold Resistance: https://www.e2techtextiles.com/tg-cold-resistance/ — Application-oriented distinction among bend, impact, rolling, and folding tests for flexible polymers and coated fabrics.
Image credits and licensing
- Image 1: A boot in snow provides application context for cold-weather footwear. The pictured boot materials are not identified as TPU. Creator/source: Oto Zapletal. https://commons.wikimedia.org/wiki/File:Boot_on_snow.jpg. License: CC BY 4.0. Alt-text guidance: “Winter boot standing in snow, illustrating cold-service conditions for flexible polymer components”
- Image 2: A laboratory freezer used for low-temperature flexibility testing illustrates why conditioning equipment and temperature control are part of a valid method. Creator/source: Cjp24. https://commons.wikimedia.org/wiki/File:Environmental_chamber-Freezer.jpg. License: CC BY-SA 3.0. Alt-text guidance: “Laboratory low-temperature environmental chamber used to condition material specimens”
- Image 3: Cross-country equipment combines rigid and flexible components that experience impact, bending, and repeated loading in the cold. Material chemistry is not identified. Creator/source: HopsonRoad. https://commons.wikimedia.org/wiki/File:Cross-country_equipment--Skate_and_Classic.jpg. License: CC BY-SA 4.0. Alt-text guidance: “Cross-country skis, boots, bindings, and poles used in cold-weather service”
- Image 4: A refrigerated warehouse illustrates sustained cold exposure. The historical building does not identify any specific polymer system or GOAFLEX application. Creator/source: Unknown authorUnknown author or not provided. https://commons.wikimedia.org/wiki/File:Arkansas-_Jerome-Refrigerated_Warehouse_-_NARA_-_27813782.jpg. License: Public domain. Alt-text guidance: “Historical refrigerated warehouse building representing sustained cold-service environments”
Context note: These search-sourced photographs are used to illustrate application, equipment, or testing context. They do not prove GOAFLEX ownership of the pictured products, creator endorsement, exact TPU chemistry or grade identity, or the performance of any GOAFLEX material.
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.
