rittleness and cracking have long been major concerns in cement‑based mortars. Adding redispersible polymer powder (RDP) greatly improves flexibility, yet the actual performance can vary widely. The root cause is often the glass transition temperature (Tg) of the RDP.

Tg is not just a technical number. It directly determines whether the polymer film stays soft or hard at ambient temperature, which in turn governs crack resistance, adhesion, and durability. Choose the right Tg, and you are already halfway to a successful formulation.

What Is Tg? The “Soft‑Hard Switch” of Polymers

Tg is the critical temperature at which a polymer changes from a brittle, glassy state to a soft, elastic, rubbery state.

– When the ambient temperature is below Tg, the polymer film remains stiff and unable to deform.
– When the temperature is above Tg, the film becomes flexible and can absorb stress.

In simple terms, lower Tg means a softer film, while higher Tg gives a harder film. This switch directly impacts the mechanical behaviour of the mortar.

Low Tg vs. High Tg: A World of Difference

The Tg of commercial RDP typically ranges from ‑10°C to +30°C. Different Tg values lead to vastly different mortar properties:

– Low Tg (e.g., ‑10°C ~ +5°C):
The polymer film is soft and highly flexible. It effectively absorbs substrate deformation and temperature‑induced stress, offering excellent crack resistance. Moreover, a soft film conforms better to the substrate, often resulting in higher adhesive strength. Typical examples are VAE‑based RDP grades.

– High Tg (e.g., +15°C ~ +30°C):
The film is rigid, providing superior compressive strength and abrasion resistance, but flexibility is sacrificed. Such mortars cannot accommodate deformation and are more prone to cracking under load. These are often found in acrylic or styrene‑acrylic copolymers.

What Do the Data Say?

A well‑known study compared two EVA‑based RDPs with Tg values of ‑10°C and +10°C in a standard cement mortar. The results were striking: the ‑10°C Tg sample showed significantly higher elongation at break and impact resistance, along with much better adhesion to wooden substrates.

This clearly proves that the lower the Tg, the better the flexibility and crack‑bridging ability of the mortar.

How to Choose the Right Tg for Your Application?

The golden rule is simple: match the Tg to your specific end‑use.

– Tile adhesives – require flexible, anti‑slip performance; choose VAE‑based RDP with Tg 0°C ~ 10°C.
– External thermal insulation (EIFS) basecoats – need to resist thermal movement; select Tg ‑5°C ~ +5°C.
– Self‑leveling compounds – prioritise hardness and wear resistance; opt for Tg +10°C ~ +20°C.
– Repair mortars – demand high adhesion and low shrinkage; pick Tg 0°C ~ +10°C.

Don’t Forget the Minimum Film‑Forming Temperature (MFT).

Tg is not the only parameter to consider. You must also check the minimum film‑forming temperature (MFT). If the ambient temperature falls below the MFT, the RDP cannot form a continuous film – and all its benefits are lost. For winter construction, always select a grade with MFT below 5°C.

Conclusion

Tg is not a meaningless number on a datasheet. It is the decisive factor that controls the flexibility of cement mortar. Low Tg gives you softness and crack resistance; high Tg delivers hardness and strength. By precisely matching the Tg to your application, you can design a mortar system that is durable, crack‑free, and reliable. Next time you choose an RDP, start with Tg – get that right, and the rest will follow.