TPU Outsole Grip Is Not One Number: How to Validate Slip Resistance Across Surface, Contaminant, and Tread
Quick answer
A TPU outsole does not have one universal “grip value.” Slip resistance is the measured result of a specific footwear–surface–contaminant system under defined test conditions.
The same TPU compound can produce different friction results when you change outsole geometry, floor material, water or oil, heel versus forepart contact, sliding direction, temperature, tread wear, molding quality, or the construction of the finished shoe.
Use material specimens to screen candidate TPU grades, but validate grip on production-representative outsoles or completed footwear. Build a test matrix around the surfaces and contaminants in the intended environment, test new and worn conditions, and report every claim with its method and conditions. Avoid “non-slip.” A more defensible statement is “tested for slip resistance under the stated laboratory conditions.”
Why outsole grip is a system property
Coefficient of friction is sometimes presented like hardness or density: one number attached to a material. That interpretation is unsafe for footwear. Friction is generated at an interface that includes the outsole compound, tread, floor, and anything separating them.
Under dry contact, relevant mechanisms can include adhesion, deformation, and hysteretic losses as the elastomer moves over surface texture. Under contaminated contact, the tread must also manage fluid or debris. A compound that performs well when dry may not preserve the same contact mechanics on water-covered tile or oil-contaminated steel.
ASTM F2913-24 defines a laboratory method for measuring dynamic coefficient of friction between footwear and flooring under reproducible conditions. The result supports relative comparison under those conditions; it is not a universal material constant.
The useful development question is not “What is the coefficient of friction of this TPU?” It is “How does this compound-and-tread system perform in the contact modes, environments, and wear states that matter to the intended user?”
The six variables that should define a grip program
Outsole material
TPU formulation affects hardness, modulus, damping, elastic recovery, wear behavior, and chemical response. Processing matters too: moisture control, melt history, mold temperature, packing, cooling, surface replication, pigment, and conditioning can all affect the finished outsole.
Hardness alone is not a grip specification. Two compounds at similar nominal hardness can have different viscoelastic and surface behavior. Changing hardness also does not guarantee a predictable change in contaminated-floor friction. A 2021 Footwear Science study of artificially worn shoes found that tread design affected fluid force, friction, and response to wear in its glycerol-contaminated vinyl-tile system, while no hardness effect was observed within that study design. That does not prove hardness never matters; it shows why direct testing is necessary.
Tread geometry and drainage
Under liquid contamination, the outsole must establish contact through an intervening film. Channels, blocks, and edges can support fluid evacuation, but their behavior depends on dimensions, orientation, escape paths, local pressure, and deformation.
Important variables include:
- channel width, depth, continuity, and available void volume;
- open versus closed drainage paths;
- lug size, spacing, orientation, and leading-edge density;
- local contact pressure and articulated zones;
- heel and forepart geometry;
- directional versus multidirectional tread;
- how the pattern changes after wear or clogging.
“Deep tread” is not automatically “high grip.” A deep pattern with trapped pockets or poorly oriented channels may retain contaminant. A fine pattern may work differently after features wear away.
Floor surface
Ceramic or quarry tile, stainless steel, vinyl composite tile, painted concrete, unfinished concrete, and polymer flooring are not interchangeable counterfaces. Texture, porosity, waviness, coatings, maintenance history, and directionality can change the interaction.
Even floors with the same commercial description may differ by supplier, finish, wear, and cleaning practice. A useful report identifies the counterface precisely and documents preparation and replacement controls. “Tested on tile” is not enough information for engineering comparison.
Contaminant
Water is not a universal proxy for every wet workplace. Oil differs from water in viscosity and chemistry. Detergent can change wetting. Food fats, syrups, processing residues, mud, and particulates create different challenges, including tread blockage.
Define contaminant identity, concentration, temperature, application method, quantity or film-control procedure, and renewal frequency. If a laboratory substitutes one fluid for another, treat it as a model system, not automatic proof for the field contaminant.
ASTM F2913-24 tightened its suggested “oily wet” condition. SATRA’s summary of the 2024 revision notes the focus on corn oil. The broader lesson is that the contaminant must be specified closely enough for reproduction.
Temperature
Temperature can change the outsole’s mechanical response and the contaminant’s flow behavior. It can also affect ice, floor coatings, and complete-shoe stiffness.
Do not evaluate cold-room, winter, or hot-floor service only at unspecified ambient conditions. Condition footwear, floor, and contaminant as required; record temperatures and transfer time. If a standard method does not cover the intended extreme, agree on a supplementary protocol with the laboratory rather than silently modifying a standard test.
Wear and contamination history
Wear can round tread edges, reduce groove depth, expose new material, polish or roughen contact regions, create local flat areas, collect debris, crack lugs, or alter the real contact pattern. Wear does not always move friction in one direction; the result depends on the geometry and surface state it creates.
“After abrasion” should not mean an arbitrarily damaged specimen. Define a reproducible wear procedure, inspect the resulting outsole, and compare its morphology with field-returned footwear.
Use standards correctly: method, specification, and application matrix
ASTM F2913-24 is a measurement method
ASTM F2913-24 is a whole-shoe-tester method for dynamic coefficient of friction under reproducible laboratory conditions. Use it to establish a controlled baseline and compare iterations. Record the standard edition, contact mode, orientation, floor, contaminant, conditioning, specimen count, and individual results—not only a selected average.
ASTM F3445-24 addresses completed-footwear performance
ASTM F3445-24 is a separate performance specification used with ASTM F2913. Its scope cautions that controlled laboratory compliance does not establish performance for every situation and recommends considering alternative floors or contaminants where workplace hazards require them.
A development coupon can help select a compound, but it cannot establish finished-footwear conformity to a specification intended for completed shoes.
SATRA TM144 and ISO 13287
SATRA TM144:2021 determines dynamic coefficient of friction under conditions intended to simulate phases of walking in which slipping is likely. It covers footwear, outsole units, and soling materials on multiple surfaces, with or without contaminants. Finished whole shoes are preferred; component specimens can support pre-production development.
For personal protective footwear programs outside the US, or products intended for multiple markets, also review ISO 13287:2019 and the applicable finished-footwear requirements. Do not assume results or markings from different standards are interchangeable. Confirm current editions and regional requirements with an accredited laboratory or certification body.
A practical TPU outsole validation matrix
This development framework does not replace prescribed standard conditions.
| Factor | Screening level | Finished-design validation | Field-risk extension |
|---|---|---|---|
| Specimen | Molded plaque, slider, or outsole unit | Production-representative completed footwear | Production footwear after controlled wear or field-equivalent conditioning |
| Surface | One controlled reference surface | Required standard surfaces | Representative tile, steel, vinyl, concrete, or coated floor |
| Contaminant | Dry and water, if relevant | Conditions required by the chosen method | Workplace oil, cleaner, food residue, slurry, mud, or another defined hazard |
| Contact mode | Defined material contact | Heel and forepart modes required by protocol | Additional direction or mode justified by use |
| Temperature | Controlled laboratory condition | Standard conditioning | Intended low/high service condition with components conditioned |
| Wear state | New | New production shoe | Run-in plus defined intermediate and end-of-life states |
| Lots and sizes | Candidate batches | Multiple production-representative pairs | Multiple lots, sizes, cavities, and sites as risk requires |
| Output | Comparative ranking | Method-specific result and conformity decision | Condition-by-condition risk map plus failure-mode inspection |
Turn the matrix into an approval plan
- Define the use environment: floor types, cleaning chemistry, credible spills, temperatures, and likely tread blockage.
- Select the governing standard based on market, product category, customer specification, and intended claim.
- Set an internal development margin that accounts for uncertainty, process variation, and aging risk.
- Screen materials with identical specimen preparation and conditions.
- Lock tread and process, then test parts from representative tooling, cavities, and settings.
- Validate completed footwear across the sizes and feet required by the protocol.
- Add application-specific challenges with documented procedures, clearly separated from standardized results.
- Repeat after justified wear or aging states.
- Retain individual results and investigate low-performing specimens instead of reporting only an average.
- Reassess grip after changes to compound, pigment, additives, mold texture, tread, process, factory, or critical supplier.
Common mistakes in TPU grip development
Publishing a bare coefficient of friction
A number without method, surface, contaminant, contact mode, temperature, and specimen state is not decision-ready. Do not compare supplier values unless their protocols and reporting bases match.
Approving the resin instead of the shoe
A plaque test removes tread drainage, local deformation, heel geometry, assembly, and fit effects. Use it for screening—not as the final claim basis.
Testing only dry and new
Dry testing can miss fluid-management failure. New-shoe testing can miss tread loss, polishing, damage, or clogging. Build foreseeable use into a reproducible matrix.
Changing several variables at once
If compound, tread, molding settings, and floor all change, the result does not reveal the cause. Use controlled experiments during development, then validate the integrated design.
Treating abrasion resistance as retained grip
Low volume loss in a drum-abrasion test does not establish slip resistance. Abrasion and friction methods answer different questions. Track dimensional wear and test grip after a defined wear procedure.
Calling compliant footwear “non-slip”
No laboratory method covers every floor, contaminant, gait, maintenance condition, and wear state. Prefer condition-specific language and preserve the supporting laboratory report.
What to send GOAFLEX for a TPU outsole grip project
Send:
- footwear category, target market, and intended claim;
- applicable standard, customer protocol, or certification route;
- outsole drawing, 2D/3D tread files, and target thickness;
- target hardness and current reference compound;
- color, transparency, and appearance requirements;
- molding process, machine, gate concept, and tooling status;
- service floors and their finish or product designation;
- expected contaminants, concentrations, and temperatures;
- heel, forepart, and directional test conditions;
- target new and worn acceptance criteria;
- abrasion, flex-fatigue, hydrolysis, and chemical-exposure requirements;
- reference footwear, failed parts, and existing test reports;
- projected volume, timing, and production location.
GOAFLEX can use this information to discuss candidate material directions and a validation plan. Final slip-resistance claims should be based on the agreed finished-footwear program, not TPU chemistry alone.
For related guidance, see TPU for footwear, TPU abrasion testing, TPU chemical resistance validation, and TPU hydrolysis testing.
Developing a TPU outsole or troubleshooting inconsistent grip? Send the application, tread, process, and test conditions to hello@goaflex.com.
References
- ASTM F2913-24: Standard Test Method for Measuring the Coefficient of Friction for Evaluation of Slip Performance of Footwear and Test Surfaces/Flooring Using a Whole Shoe Tester.
- ASTM F3445-24: Standard Specification for Performance Requirements when Evaluating Slip Resistance of Footwear Using ASTM F2913 Whole Shoe Tester.
- SATRA: Updates to US Slip Resistance Standards, October 2024.
- SATRA TM144: Friction (Slip Resistance) of Footwear and Floorings, March 2021.
- ISO 13287:2019—Personal Protective Equipment—Footwear—Test Method for Slip Resistance.
- Hemler et al., Effect of Tread Design and Hardness on Interfacial Fluid Force and Friction in Artificially Worn Shoes, Footwear Science, 2021.
- Hemler et al., Identifying Shoe Wear Mechanisms and Associated Tribological Characteristics, Wear, 2016.
- Clarke and Carré, Tribology, Friction and Traction: Understanding Shoe-Surface Interaction, Footwear Science, 2013.
- Gauthier et al., Grip Mechanisms and Mechanical Characterization of Outsole Materials, Footwear Science, 2023.
- NIOSH, Preventing Slips, Trips, and Falls in Wholesale and Retail Trade Establishments.
Image credits
Image 1: Hotshot977, Shimano MT31 shoe with SH56 cleat—sole, public domain.
Image 2: Gail Williams, Liquid crosswalk, CC BY 2.0.
Image 3: Bernard Spragg. NZ, Sole parting company, CC0 1.0.
Image 4: Matt Howard / Argonne National Laboratory, Hydrogen Tribometer, CC BY-SA 2.0.
Image-use caution: These are search-sourced, commercially reusable contextual visuals. They do not prove GOAFLEX ownership of the photographed products, creator endorsement, exact TPU chemistry, grade identity, regulatory status, or application performance.
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.
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