The Fundamental Difference: Tg vs. MFFT
When selecting Redispersible Polymer Powder (RDP) for various applications, we will always find two key indices: Glass Transition Temperature Tg and Minimum Film Forming Temperature MFFT. What do these two indicate? This passage will discuss their definition and connections.
Selecting the optimal RDP is critical for the success of any dry-mix mortar formulation. While both Tg and MFFT are closely related to the polymer’s thermal behavior, they govern two entirely different aspects of mortar performance: Tg dictates the final mechanical properties of the cured mortar, and MFFT governs the feasibility of the application on site. Understanding this difference is essential for formulation chemists and application engineers.
Defining Glass Transition Temperature Tg
Definition Tg: The temperature at which a polymer’s physical properties change from a hard, brittle, or “glassy” state to a softer, more rubbery or “plastic” state.
Tg is an intrinsic property of the polymer and determines the final hardness and flexibility of the polymer film. Tg is primarily determined by the chemical structure and molecular weight of the polymer chain. It is a thermodynamic property often measured by professional techniques like Differential Scanning Calorimetry (DSC).
Tg’s Impact on Mortar Performance (Final Properties)
- High Tg RDP: High Tg RDP yields mortars with higher hardness and wear resistance but potentially increased brittleness6. This is preferred for applications demanding high compressive strength and abrasion resistance, such as self-leveling compounds or floor screeds.
- Low Tg RDP: Low Tg RDP imparts better flexibility and crack resistance to the mortar. These products provide excellent elongation and deformation accommodation, making them mandatory for materials subject to high stress, like waterproofing membranes and flexible tile adhesives.
Defining Minimum Film Forming Temperature MFFT
Definition: MFFT: The lowest temperature at which a liquid polymer dispersion (like a paint or adhesive) will coalesce to form a continuous, coherent film.
MFFT focuses more on the practical film-forming capability during construction.
MFFT is a critical indicator of the polymer’s practicality during construction. It represents the kinetic energy required for the polymer particles, which are released when the RDP is mixed with water, to deform, pack closely, and fuse into a continuous, coherent network. This process is called Coalescence.
MFFT’s Role in Construction Feasibility
If the ambient temperature is below the MFFT, the polymer particles cannot deform and coalesce, preventing the RDP from effectively forming a film and creating a continuous polymer network. This leads to a significant drop in mortar performance (e.g., adhesive strength, flexibility).
This is a critical risk factor: A mortar applied below its RDP’s MFFT will likely suffer from poor cohesion, low adhesion, and severe dusting, regardless of how good the polymer’s final Tg properties are.
The Interplay: How Formulation Controls the Tg /MFFT Relationship
| Feature | Tg (Glass Transition Temperature) | MFFT (Minimum Film Forming Temperature) |
| Nature | Intrinsic, Thermodynamic Property | Application-related, Kinetic Property |
| Governs | Final hardness, flexibility, and mechanical strength | On-site film formation and product effectiveness |
| Primary Influence | Polymer’s chemical composition | Tg, Surfactants, Protective Colloids, Water |
While MFFT is always near the Tg of its polymer, the two are not always equal.
The Influence of Formulation on MFFT
MFFT is influenced by surfactants, protective colloids, water, and other factors in the emulsion.
For example, for polymers with a certain polarity, the MFFT of the emulsion may be lower than the Tg; for non-polar or very low polarity polymers, the MFFT may be higher than the Tg.
This is where Celotech’s expertise in RDP formulation is key: We utilize specialized additives (like coalescing aids or plasticizers) to finely tune the MFFT. This allows us to supply a polymer with a high performance-grade Tg (e.g., 15C° for high bond strength) that still possesses a low MFFT (e.g., 5C° to ensure effective film formation even in cooler construction environments.
Celotech RDP Selection Guide: Tg and MFFT in Specific Mortar Types
When selecting Redispersible Polymer Powder (RDP) for dry-mix mortars, the Glass Transition Temperature Tg and the Minimum Film Forming Temperature MFFT are key parameters. Simply put, Tg primarily determines the hardness and flexibility of the polymer film after the mortar cures, while MFFT relates to whether the polymer can effectively form a film during application, thereby performing its function.
The table below summarizes the typical considerations for Tg and MFFT of RDP in different mortar applications, providing a quick overview20:
| Application Scenario | Tg Preference | MFFT Requirement | Key Performance Considerations |
| Tile Adhesives | Medium to High Tg (e.g., 0°C ~ 20°C) | Usually not a primary focus, but must ensure effective film formation above MFFT | Balancing adhesive strength (original, after water immersion, after heat aging) with flexibility (to accommodate substrate and tile deformation) |
| External Rendering / Insulation Systems | Medium Tg (e.g., -5°C ~ 10°C) | Must adapt to outdoor application conditions, ensuring film formation at lower temperatures | Crack resistance, adhesive strength (e.g., to insulation boards), impact resistance (with typical polymer content around 3-4%, Tg correlates with impact resistance) |
| Fillers / Putties | Low to Medium Tg (e.g., -10°C ~ 5°C) | Sufficiently low to ensure good film formation at various application temperatures, preventing dusting | Cohesive strength, sandability, crack resistance; requires a balance of hardness and toughness
|
| Waterproofing Mortars / Sealing Materials | Low Tg (e.g., -20°C ~ -5°C) | Sufficiently low to ensure the formation of a dense, waterproof film under various conditions | High flexibility, high elongation, ability to accommodate deformation (at high dosage, low Tg directly contributes to flexibility)
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Application Deep Dive (Waterproofing): For waterproofing applications, the high dosage of RDP combined with a very low Tg -20C°ensures maximum flexibility and bridging of dynamic cracks. Here, Celotech must guarantee the MFFT is also extremely low to allow construction crews to work in typical cool outdoor conditions while ensuring the formation of that crucial, continuous waterproof film.
Conclusion
The selection of Redispersible Polymer Powder (RDP) hinges on the accurate evaluation of two fundamental parameters: Tg and MFFT. Tg is the intrinsic property that defines the mortar’s final, cured mechanical performance, governing its hardness and flexibility. MFFT is the dynamic property that defines the construction window, ensuring the polymer can effectively function under application conditions.
At Celotech, we leverage our specialized knowledge in polymer chemistry and formulation to offer RDP grades with precisely controlled Tg and MFFT values. Whether you require high Tg for durable flooring or low Tg low MFFT for flexible waterproofing, our technical team ensures you select the RDP that optimizes both final performance and on-site workability.
