Using DMA to Find Glass Transition Temperature (Tg): Method & Examples
Introduction: Friction and Slip Resistance in Footwear
Slip-and-fall accidents are among the leading causes of workplace injury and personal injury litigation worldwide. The friction performance of shoe outsoles against floor surfaces determines slip resistance — the ability to maintain traction and prevent falls across different floor types, contaminants (water, oil, cleaning agents), and walking speeds. Dynamic Mechanical Analysis (DMA) is a powerful tool for understanding the viscoelastic properties of outsole rubber and polymer materials, which fundamentally govern friction behaviour.
The Science of Shoe-Floor Friction
Friction between a rubber outsole and a floor surface involves two primary mechanisms:
Adhesion Component
Adhesion friction arises from molecular-scale bonding between the rubber surface and the floor asperities — determined by the contact area and the surface energies of the rubber and the floor. Softer, more compliant rubbers achieve a greater real contact area by conforming to the floor surface microtopography, thereby increasing adhesion and friction.
Hysteresis (Viscoelastic) Component
Hysteresis friction arises from energy dissipation in the rubber as it deforms over and behind the floor surface asperities during sliding. The rubber deforms as each asperity sweeps beneath it — storing energy on approach and dissipating some as heat on recovery. The ratio of dissipated to stored energy — the loss tangent (tan δ) — directly governs the hysteresis friction contribution. Higher tan δ at the relevant deformation frequency = higher hysteresis friction = better wet and dry traction.
How DMA Characterises Friction-Relevant Viscoelastic Properties
DMA Storage and Loss Modulus
DMA measures storage modulus (E’) and loss modulus (E”) as functions of temperature and frequency. At room temperature, the relevant frequency for walking-pace slip corresponds to the characteristic frequency at which an outsole rubber contacts floor asperities — typically 10 Hz to 10 kHz depending on walking speed and asperity size.
Tan δ as a Predictor of Friction
The hysteresis friction coefficient is proportional to tan δ at the relevant deformation frequency. Rubber compounds with high tan δ at room temperature and typical contact frequencies provide superior grip — particularly on wet or contaminated floors where adhesion is reduced.
Temperature-Frequency Superposition
The DMA master curve — constructed from measurements at multiple frequencies and temperatures using time-temperature superposition (WLF or Arrhenius shifting) — enables prediction of viscoelastic properties at the specific temperature-frequency combinations representative of real walking conditions.
DMA Test Methods for Footwear Outsoles
ASTM E1640 / ISO 6721 DMA Protocol
Rectangular specimens (approximately 35 × 10 × 2–3 mm) cut from the outsole are tested in dual-cantilever or tension mode over a frequency sweep (0.1–100 Hz) within the target service temperature range (−20°C to +40°C). The frequency and temperature at which tan δ peaks coincide with maximum energy dissipation — the optimum friction condition.
Correlation with Standardised Friction Tests
DMA tan δ values correlate well with the friction coefficient measured by standardised slip resistance tests:
- ASTM F2913 (Slip resistance by mechanical tribometer)
- EN ISO 13287 (Footwear slip resistance — pendulum method)
- SATRA TM144 (Slip resistance — ramp test)
High tan δ at relevant frequencies consistently predicts high pendulum coefficient of friction (PCoF) on smooth, wet floors — the most slip-hazardous condition.
Material Design for Optimal Friction Performance
Understanding the DMA viscoelastic profile enables rubber compound formulators to design outsole materials with optimised friction performance:
- Selecting elastomers with Tg close to room temperature maximises tan δ at use temperature — providing the highest energy dissipation per unit deformation
- Fillers (silica, carbon black) adjust stiffness and damping balance
- Oil and plasticiser content shifts the tan δ peak to lower temperatures — controlling the friction-temperature profile
Industrial Applications
Athletic footwear manufacturers use DMA to qualify new outsole rubber compounds before expensive moulding and wear trial programmes. Occupational footwear producers — safety shoes, and work boots — use DMA and standardised slip-resistance testing for EN ISO 20345 certification. Floor surface manufacturers correlate DMA outsole data with their floor-slip-resistance ratings to define compatibility guidelines.
Conclusion
Dynamic Mechanical Analysis (DMA) plays a crucial role in understanding and optimising footwear slip resistance by characterising the viscoelastic properties that govern friction behaviour. By analysing parameters such as storage modulus, loss modulus, and tan δ, manufacturers can design outsole materials that maximise energy dissipation and surface contact — key factors for improving grip on dry and contaminated surfaces. This scientific approach enables the development of safer, high-performance footwear while ensuring compliance with industry standards and reducing the risk of slip-and-fall incidents.
Why Choose Infinita Lab for DMA and Slip Resistance Testing?
Infinita Lab provides DMA viscoelastic characterisation and standardised slip-resistance testing for footwear materials through our nationwide, accredited polymer and product testing laboratory network.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
Why is DMA more sensitive than DSC for detecting the Tg of a golf ball core rubber? The Tg of polybutadiene rubber (at ~−90°C) produces a very small heat capacity change in DSC — difficult to detect above baseline noise. DMA detects the dramatically larger modulus drop and tan δ peak at Tg with far greater sensitivity. DMA Tg values for rubbers are typically 10–20°C higher than DSC values due to the frequency dependence of viscoelastic relaxations.
What frequency should be used for DMA Tg measurement of golf ball materials? 1 Hz is the standard reference frequency for most DMA Tg measurements — providing a balance between test speed and viscoelastic response at representative deformation rates. Golf ball impact occurs over ~0.5 millisecond — equivalent to ~2 kHz — so DMA data at multiple frequencies (1–100 Hz) enables prediction of ball performance at impact-rate deformation using time-temperature superposition.
How does the cover material Tg affect the feel of a golf ball? Stiffer covers (higher storage modulus at playing temperature, further from Tg) produce a firmer, clicking feel at impact. Covers with Tg closer to room temperature are softer and more compliant at impact — producing the "soft feel" preferred by tour players with cast polyurethane covers. DMA storage modulus at playing temperature correlates directly with cover stiffness and perceived feel.
Can DMA be used to compare original and counterfeit golf balls? Yes. DMA viscoelastic profiles — including Tg values, storage modulus magnitude, and tan δ peak height — are characteristic fingerprints of each material layer's formulation. Significant differences between an authentic ball's DMA profile and a suspect ball indicate different material composition — an effective tool for quality and authenticity verification.
What other thermal analysis techniques complement DMA for golf ball material characterisation? DSC (specific heat, melting point, crystallinity), TGA (crosslink density indirectly via swelling, rubber content), and TMA (CTE, dimensional stability) complement DMA. Together, this thermal analysis battery provides a comprehensive material characterisation package for golf ball polymer qualification and formulation development.
