What is Tg Temperature?
Tg (Glass Transition Temperature) is the temperature at which rubber changes from a flexible, rubbery state to a hard, glass-like state. Below Tg temperature, molecular chains lose movement freedom, making rubber stiff and brittle. This makes Tg a critical indicator for low-temperature performance – the lower the Tg, the better the rubber remains flexible in cold environments.
Key Factors Affecting Cold Performance
1. Molecular Traffic Jams: Space Hindrance Effect
Imagine trying to dance in a crowded room. Big side groups on rubber molecules (like methyl or benzene rings) act like “molecular elbows” that block chain movements.
Key Impacts:
- Side groups >3 carbon atoms change molecular motion from “slithering” to “jumping”
- Every 0.1 nm³ increase in side group size raises energy needed for movement by 8%
- Real-world example: Silicone rubber (PDMS) stays flexible at -100°C because its small side groups (-CH3) don’t block movement
Engineering Solutions:
- Winter tires use 98%+ natural rubber (avoiding stiff trans-structures)
- Rocket seals use silicone with 0.5% phenyl groups to balance flexibility and oil resistance
2. Molecular Magnets: Dipole Interactions
Simple Explanation:
Polar groups (-Cl, -CN) act like tiny magnets between chains. In cold conditions, these “magnets” lock molecules in place like a molecular zipper.
Cold Weather Challenges:
- At -30°C, nitrile rubber’s -CN groups become 40% more ordered (like soldiers freezing at attention).
- Each chlorine atom in neoprene adds 1.5D dipole strength – enough to triple stiffness below -40°C.
Smart Fixes:
- Hydrogenated nitrile rubber (HNBR) reduces Tg from -25°C to -45°C while keeping oil resistance.
- Conductive rubber for Arctic cables uses controlled dipoles to prevent hardening.
3. Molecular Lego: Crystallinity & Structure Order
Simple Analogy:
Well-organized molecules pack like Lego bricks. Natural rubber’s 99% cis-structure allows instant “crystal armor” formation when stretched at -25°C.
Temperature Effects:
- Every 1% increase in molecular order raises melting heat by 2 J/g
- Adding just 1.2% trans-structures lowers crystallization temperature by 15°C
Cold-Proof Designs:
- LNG seals use EPDM rubber with controlled ethylene chains to prevent crystallization below -160°C.
- Shape-memory medical devices use precisely ordered structures that unfold perfectly at body temperature.
How to Measure Tg Temperature？
Tg (glass transition temperature) is measured using three primary methods:
- DSC (Differential Scanning Calorimetry): Detects heat flow changes as the material transitions between glassy and rubbery states.
- DMA (Dynamic Mechanical Analysis): Measures changes in storage/loss modulus with temperature, identifying the peak in tan δ (damping factor).
- TMA (Thermomechanical Analysis): Tracks dimensional changes caused by thermal expansion/contraction during the transition.
Tg Values of Common Rubbers
| Material | Tg Range (°C) | Key Producers | Example Products |
|---|---|---|---|
| Natural Rubber (NR) | -60 to -70 | Michelin, Bridgestone | Hevea NR (Michelin) |
| Silicone Rubber | -123 to -80 | Dow Corning, Momentive | Silastic® (Dow) |
| Nitrile Rubber (NBR) | -35 to -15 | Lanxess, Zeon Chemicals | Perbunan® (Lanxess) |
| Fluorocarbon Rubber | -25 to -15 | Chemours (DuPont) | Viton® (Chemours) |
| EPDM Rubber | -60 to -45 | ExxonMobil, Arlanxeo | Vistalon™ (ExxonMobil) |
| Butyl Rubber (IIR) | -70 to -60 | ExxonMobil | Exxon™ Butyl Rubber |
| SBR (Styrene-Butadiene) | -60 to -45 | Synthos, Goodyear | Duradene® (Synthos) |
