How to Choose RDP (Redispersible Polymer Powder) — A Complete Selection Guide

Quick Answers

1. What Is RDP and Why Does It Matter?

Redispersible Polymer Powder is a dry powder made by spray-drying a polymer emulsion. When mixed with water in a cement-based mortar, the powder particles redisperse into a polymer emulsion. As the cement hydrates and water evaporates, the polymer particles coalesce into a continuous film that forms a flexible network throughout the hardened cement matrix.

What RDP does for your mortar:- Increases tensile adhesion strength — the primary driver of C1→C2 upgrade- Adds flexibility — allows the mortar to absorb thermal and structural movement without cracking- Improves water resistance — the polymer film blocks capillary water ingress- Enhances workability — smoother, creamier mortar

What RDP does NOT do:- It does not replace cement. Strength still comes from cement hydration.- It does not replace HPMC. Water retention and open time are HPMC's job.- Increasing dosage of the wrong grade does not fix a grade selection error.

2. The Two Most Important Numbers: Tg and MFFT

2.1 Glass Transition Temperature (Tg)

Tg is the temperature at which the polymer transitions from a hard, glassy state to a soft, rubbery state. In practical terms:

The key trade-off: Lower Tg gives more flexibility but less compressive strength. Higher Tg gives more strength but less flexibility. The correct Tg depends on what your application needs — not on what is "better."

2.2 Minimum Film Forming Temperature (MFFT)

MFFT is the lowest temperature at which the polymer can form a continuous film. Below MFFT, the polymer particles remain as separate spheres — they never coalesce, and the RDP contributes nothing to mortar performance.

This is why MFFT matters for construction: if you apply tile adhesive at 5°C but the RDP has MFFT of 8°C, the polymer never forms a film. You effectively have no RDP in the mortar, regardless of dosage.

CHARING RDP products are formulated so that MFFT is below the typical application temperature range. Always check both Tg AND MFFT when comparing RDP grades — a low Tg is useless if MFFT is too high for the expected application conditions.

3. How to Choose RDP by Application

3.1 Tile Adhesive

Why C1 and C2 use different Tg:

C1 works on porous ceramic tiles where mechanical anchoring contributes to adhesion. A semi-flexible RDP (Tg 3-12°C) provides enough flexibility without sacrificing strength.

C2 must bond to non-porous porcelain where there is zero mechanical anchoring — all adhesion comes from the polymer film. The film must also flex to accommodate the thermal expansion difference between tile and substrate. This requires a flexible RDP (Tg -8°C or lower).

Why dosage increases from C1 to C2:

C2 requires tensile adhesion of 1.0 MPa — double C1's 0.5 MPa. This cannot be achieved by simply increasing dosage of the C1 RDP grade. The grade AND the dosage must both change. RD 7015H at 40 kg/t will not pass C2 water immersion — the polymer film is too stiff. RD 8080F at 15 kg/t will not pass C1 tensile — there is not enough polymer film formed.

3.2 EIFS / ETICS

EIFS base coats experience the widest temperature swings of any mortar application — from -20°C in winter to +70°C on a dark-coloured facade in summer. The polymer film must remain flexible across this entire range. RD 8080F at Tg -8°C provides this.

3.3 Waterproofing Mortar

For waterproofing, two properties matter: low Tg for flexibility AND hydrophobic treatment for water resistance. Standard RDP films soften under prolonged water exposure. Hydrophobic RDP maintains film integrity when wet.

3.4 Self-Leveling Compound

Self-leveling uses rigid RDP at low dosage. The goal is to add just enough polymer to prevent cracking from plastic shrinkage — not to add flexibility. A flexible RDP in SLC would reduce compressive strength without benefit.

3.5 Wall Putty

3.6 Cement Render and Repair Mortar

4. CHARING RDP Product Reference

5. Frequently Asked Questions

What is the difference between rigid and flexible RDP?

Rigid RDP (Tg >15°C) forms a hard polymer film that gives high compressive strength but cracks under movement. Flexible RDP (Tg <0°C) forms a soft, elastic film that absorbs movement but sacrifices some strength. The choice depends on the application: self-leveling needs rigid, C2S1 needs flexible, C1 needs semi-flexible.

Can I use the same RDP for all my products?

No. Using one grade across all products is a common cost-reduction approach that produces compromised performance. RD 7015H and RD 7003M work for C0/C1 and interior putty. RD 8080F is needed for C2 and EIFS. RD W8050 is needed for waterproofing. Each application has a Tg and dosage requirement that a single grade cannot cover.

Does increasing RDP dosage compensate for using the wrong grade?

No. Grade and dosage are independent variables. A rigid grade at 5% dosage does not produce the same flexibility as a flexible grade at 3% — because film flexibility is determined by the polymer chemistry, not the quantity. Select the correct grade first, then optimize dosage.

What happens if RDP dosage is too low?

Below 12 kg/t in C1, tensile adhesion after water immersion and freeze-thaw drops below 0.5 MPa. In C2, below 25 kg/t will not reach 1.0 MPa. The polymer film is simply not continuous enough — there are gaps where the brittle cement matrix is exposed.

What happens if RDP dosage is too high?

Compressive strength decreases. The mortar becomes overly flexible and may deform under load. Cost increases without proportional performance gain. For C2S1, 40 kg/t of RD 8080F is typically the optimal ceiling — beyond this, additional polymer does not improve transverse deformation.

How do I check RDP quality before bulk purchase?

Request a batch certificate (COA) covering: Tg, ash content (≤12%), moisture content (≤2%), and redispersibility. Test a sample in your own formulation: mix a small batch, cure, and check film quality. Ash content above 12% means the product contains more filler than active polymer — you are paying for limestone, not RDP.

Why does my C2 fail the water immersion test?

Three things to check: (1) RDP dosage — below 25 kg/t will not pass. (2) RDP grade — RD 7015H (Tg 12°C) is too stiff for C2 water immersion. Switch to RD 8080F or RD W8050. (3) Silica fume — add 15-40 kg/t to densify the matrix.

What RDP for outdoor applications in cold climates?

For exterior applications where winter temperatures drop below 0°C, use RDP with Tg below -5°C. The polymer film must remain flexible at service temperature — a Tg of 12°C means the film is glassy and brittle below that temperature. RD 8080F (Tg -8°C) or RD W8050 (Tg -11°C) are suitable for exterior use in most climates.

6. Quick Reference — Application vs Product

Note: All dosage recommendations are starting points. Cement quality, sand gradation, and local climate affect the optimal dosage. Laboratory trials with your specific raw materials are essential.

For technical inquiries: info@sdcharing.com

Shandong Charing Industry Co., Ltd.

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