Flexible Tile Adhesive: What is the Recommended RDP Dosage?

Flexible tile adhesive is a cement-based adhesive formulated to accommodate movement between the tile and substrate while maintaining sufficient bonding performance.

Flexibility is particularly important for large-format porcelain tiles, underfloor heating, exterior applications, tile-on-tile installation, and substrates that may experience thermal or dimensional movement.

Under EN 12004 and EN 12002, S1 and S2 indicate deformable adhesive performance. S1 requires a transverse deformation of at least 2.5 mm, while S2 requires at least 5 mm. Therefore, S2 represents a higher level of deformability than S1.

Unlike conventional tile adhesives, flexible systems need to withstand greater movement without losing adhesion or developing excessive cracking. This is one of the main reasons why they generally require a higher level of polymer modification.

How Much RDP Is Used in Tile Adhesive?

There is no fixed RDP dosage specified by EN 12004 for C1, C2, S1 or S2 adhesives.

The standard classifies the final performance, while RDP dosage is a formulation parameter determined by the tile adhesive manufacturer.

As a practical formulation reference, RDP content generally increases as the required performance becomes more demanding:

*These are indicative formulation ranges for laboratory trials, not mandatory EN dosage requirements. Actual RDP dosage depends on polymer grade, cement content, aggregate grading, cellulose ether, water demand and the target performance.

Why Does Flexible Tile Adhesive Need More RDP?

The main reason is that RDP provides polymer modification to the cementitious matrix.

After the dry mortar is mixed with water, the polymer particles redisperse throughout the system. During drying and curing, they form a polymer phase within the hardened adhesive.

This polymer phase can improve adhesion, cohesion and flexibility.

For a conventional tile adhesive, the primary objective may simply be to achieve the required bond strength. When the target changes to S1 or S2, the adhesive must also tolerate significantly greater deformation.

Consequently, the formulation generally requires a stronger polymer contribution.

40–50 kg/t RDP: Why Is This Common?

40–50 kg of RDP per ton is equivalent to: 4–5% RDP in the dry formulation.

This is a reasonable range for many S1-oriented or flexible tile adhesive formulations.

For example:

1,000 kg dry mortar + 50 kg RDP = 5% RDP

The polymer content at this level can provide a substantial improvement in the flexibility, adhesion and cohesion of the cured adhesive.

This is particularly valuable when the adhesive needs to accommodate movement caused by: thermal expansion + substrate movement + moisture variation + large-format tiles

Therefore, the 40–50 kg/t figure often seen in flexible tile adhesive formulations is not excessive. It reflects the higher polymer demand of a more demanding adhesive system.

Why Can RDP Reach 6–8%?

The dosage can become much higher when the target changes from ordinary adhesion to high deformability.

At 6% RDP: 60 kg RDP / ton = 6% polymer
At 8% RDP: 80 kg RDP / ton = 8% polymer

Such levels may be used for high-performance flexible or highly deformable formulations, particularly when targeting S2-type performance or demanding applications.

An S2 adhesive must achieve at least 5 mm transverse deformation. This is considerably more demanding than an S1 adhesive, which requires at least 2.5 mm.

Higher polymer modification can therefore help the adhesive accommodate larger movement without losing its integrity or bond to the substrate.

What Does RDP Do in Flexible Tile Adhesive?

RDP is not added only to increase flexibility. Its contribution extends across several important properties of the cured adhesive.

Higher adhesion: Particularly useful for porcelain and other low-absorption tiles.
Better cohesion: Helps reduce internal weakness in the hardened adhesive.
Improved deformability: Allows the adhesive to tolerate movement between tile and substrate.
Better crack resistance: Polymer modification can improve the resistance of the cured adhesive to stress.
Improved durability: Important in systems exposed to thermal or moisture fluctuations.

This combination of properties makes RDP one of the key ingredients in flexible tile adhesive formulation.

Does More RDP Always Mean Better Tile Adhesive?

No.

Increasing RDP from 4% to 8% does not automatically transform a tile adhesive into a better or more flexible product.

The final performance depends on the interaction between RDP, cement, aggregate, filler, cellulose ether, water and curing conditions.

For example, HPMC or MHEC has a different role from RDP. Cellulose ether primarily controls water retention, rheology, workability and open time, while RDP contributes more directly to polymer modification, adhesion and deformability.

The aggregate system also matters. Poor particle packing can negatively affect the performance even when the polymer content is high.

Therefore, the objective should not be to maximize RDP content. The goal is to find the right polymer type and dosage for the required performance level.

How Should Manufacturers Optimize RDP Dosage?

A practical formulation strategy is to start from the desired classification and application.

For conventional C1 and C2 tile adhesives, a relatively moderate polymer content may be sufficient. When moving toward C1TES1 or C2TES1, the formulation generally requires a stronger polymer contribution.

For high-flexibility or C2TES2-oriented systems, manufacturers can investigate higher RDP contents, potentially reaching the 6–8% range.

The final dosage should always be confirmed through practical performance testing rather than determined by RDP percentage alone.

JINJI CHEMICAL: RDP Solutions for Flexible Tile Adhesive

For manufacturers developing flexible tile adhesive, the challenge is not simply finding an RDP with a high polymer content. The more important question is whether the polymer is suitable for the required adhesion, flexibility and deformability of the final formulation.

JINJI CHEMICAL supplies RDP and cellulose ether products for cement-based construction materials, providing formulation options for different tile adhesive requirements.

Our RDP solutions can be used together with HPMC and MHEC to develop balanced tile adhesive systems in which polymer modification, water retention, rheology and workability are considered together.

For manufacturers working on flexible, S1 or high-performance tile adhesive, JINJI CHEMICAL focuses on helping customers select a suitable product combination rather than simply increasing additive dosage.

Conclusion

RDP plays an increasingly important role as tile adhesive performance requirements become more demanding.

While conventional C1 and C2 systems can often be formulated with relatively moderate polymer contents, flexible systems require greater polymer modification to accommodate substrate and tile movement.

For this reason, 4–5% RDP is commonly considered for many flexible or S1-oriented formulations, while 6–8% RDP may be investigated for highly deformable or S2-oriented systems.

However, these percentages are formulation references, not EN classification requirements. The final classification depends on the measured performance of the complete adhesive system.

Whether you are developing a C2TES1, C2TES2 or other high-flexibility tile adhesive, JINJI CHEMICAL can help you evaluate suitable RDP and cellulose ether solutions for your formulation.

Contact JINJI CHEMICAL to discuss your flexible tile adhesive requirements and find the right polymer solution for your application.