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A tile adhesive passes the dry adhesion test without difficulty.

But after water immersion, the bond strength drops significantly.

Does this mean more cement is needed?

Not necessarily.

Dry adhesion and wet adhesion are affected by different parts of the formulation. A mortar may develop good initial cement strength while still having poor resistance to long-term water exposure.

Redispersible polymer powder plays an important role in improving adhesion, flexibility, and durability in polymer-modified tile adhesive.

After mixing with water, the polymer redistributes through the mortar and forms a polymer phase as the material dries. SidleyChem’s RDP range is specifically used in tile adhesive and other dry-mix mortar systems to improve adhesion and flexibility.

If wet adhesion is poor, check:

• RDP type;
• polymer dosage;
• compatibility with the cement;
• curing time before immersion;
• water resistance of the selected polymer.

Water dosage is another important factor.

Excess water may make the tile adhesive easier to spread, but after curing, that water leaves behind additional pores.

A more porous mortar can allow water to penetrate more easily and may weaken the bond interface during immersion.

When comparing formulations, always keep the water dosage consistent.

If one sample requires significantly more water to reach the same working consistency, this difference should be considered when interpreting the adhesion result.

HPMC/MHEC helps tile adhesive retain water long enough for proper cement hydration and also improves workability and open time. SidleyChem’s construction-grade cellulose ethers are specifically designed for cement-based tile adhesive applications.

However, the cellulose ether system still needs to be balanced.If dosage is too low, rapid water loss can reduce cement hydration and bond development.

If the dosage is too high, the mortar may become overly viscous or contain more entrained air, which can also affect density and final strength. The goal is sufficient water retention without creating an unnecessarily high air content or viscosity.

Do not immerse specimens too early.

Before exposure to water, the mortar needs enough time for:

• cement hydration;
• polymer film development;
• bond formation at the tile and substrate interface.

If the curing period is shortened, the specimen may enter the water before the mortar has developed enough resistance.

When comparing two formulas, use exactly the same curing time, temperature, humidity, tile type, and substrate.

Otherwise, the results may reflect different test conditions rather than formulation quality.

A simple comparison can be: Sample A: Current formulation,Sample B: Higher RDP dosage,Sample C: Alternative RDP grade,Sample D: Adjusted water / cellulose ether balance. Keep all other raw materials and test conditions unchanged.

Compare:

• dry bond strength;
• tensile adhesion strength after conditioning;
• failure location;
• mortar density;
• workability.

Good dry adhesion does not automatically guarantee good wet adhesion.

A large drop in tile adhesive bond strength after water immersion may be related to:

• unsuitable RDP;
• insufficient polymer dosage;
• excessive mixing water;
• high mortar porosity;
• poor curing;
• unbalanced HPMC/MHEC dosage.

The best approach is to evaluate dry and wet adhesion together.

For tile adhesive formulators, the target is not simply high initial bond strength—it is stable adhesion under the actual service conditions the product is expected to face.