RDP in Dry-Mix Mortars — Re-emulsification, Flexibility and Adhesion
Redispersible polymer powder (RDP) is a spray-dried polymer latex that, when mixed with water in a dry-mix mortar, re-emulsifies to form a polymer film throughout the cured cement matrix. This co-matrix of polymer and calcium silicate hydrate (C-S-H) gives RDP-containing mortars their characteristic combination of flexibility, tensile adhesion, and reduced water absorption that pure cement mortars cannot achieve.
How RDP Works — Re-emulsification and Film Formation
During dry-mix mortar production, polymer latex is spray-dried with a protective colloid (typically polyvinyl alcohol) to produce a free-flowing powder. When the mortar is mixed with water on site:
- Water contacts the RDP particle, dissolving the protective PVA shell
- The polymer latex re-emulsifies and disperses throughout the alkaline cement paste
- As the mortar dries and cement hydrates, the polymer particles coalesce to form a continuous film
- This polymer film bridges microcracks, absorbs deformation energy, and improves adhesion to the substrate
The minimum film formation temperature (MFFT) of the RDP is a critical parameter. At application temperatures below the MFFT, the polymer does not form a coherent film and the performance benefits are lost. For most construction applications, RDP grades with MFFT ≤ 5°C are preferred to allow application in cooler weather.
RDP Polymer Types — VAE vs VAc-VeoVa
| Type | Monomer basis | Tg (°C) | MFFT | Water resistance | Best for |
|---|---|---|---|---|---|
| VAE | Vinyl acetate–ethylene | 0 to +5 | ≤ 5°C | Good | Tile adhesive, EIFS, plasters |
| VAc-VeoVa | Vinyl acetate–vinyl versatate | +5 to +15 | ≤ 5°C | Excellent | Exterior renders, weatherproof systems |
| Acrylic-VAE | Acrylic–vinyl acetate–ethylene | -5 to +5 | ≤ 0°C | Very good | Flexible waterproofing, membranes |
For standard tile adhesive and general dry-mix mortar: VAE grade is the standard choice. For external applications or where superior water resistance is required: VAc-VeoVa provides better hydrophobicity through the VeoVa comonomer's branched ester groups.
Key Performance Benefits of RDP
Tensile adhesion strength
RDP at 2–4% increases tensile adhesion of tile adhesive from ~0.5 N/mm² (cement only) to ≥1.0 N/mm² (C2 class, EN 12004). The polymer film bridges the adhesive-tile interface, distributing stress over a larger area.
Flexibility and crack bridging
The polymer phase absorbs differential movement between tile and substrate. Critical in systems with underfloor heating, large-format tiles, or substrates subject to thermal cycling. RDP at 4% reduces mortar E-modulus by 30–50%.
Water absorption reduction
Polymer film coating reduces capillary water absorption of the cured mortar. Particularly important in wet-area tile installations (bathrooms, swimming pools). Tested per EN 1015-18.
Workability and open time
RDP improves mortar workability and spreadability through a lubrication effect. Does not significantly extend open time (that is HPMC's role), but reduces drag during trowel application.
Dosage by Application
| Application | RDP dosage (% by dry wt) | Target standard | Notes |
|---|---|---|---|
| Tile adhesive C1 | 1–2% | EN 12004 C1 | Basic polymer-modified |
| Tile adhesive C2 | 2–4% | EN 12004 C2 | Full polymer-modified |
| Tile adhesive C2TE | 3–5% | C2T (extended) + E (deformable) | Large format, underfloor heating |
| Wall putty | 1–3% | EN 15824 | Flexibility and adhesion |
| EIFS base coat | 3–6% | EN 13499 | Crack bridging essential |
| Flexible waterproofing | 5–10% | EN 14891 | High polymer content for class B |
Source RDP from Raykem
VAE and VAc-VeoVa grades. Multiple Tg options. 25 kg bags. Stocked in Dubai — UAE, Saudi Arabia and GCC supply.
View RDP Product → Powder System