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:

1. Water contacts the RDP particle, dissolving the protective PVA shell
2. The polymer latex re-emulsifies and disperses throughout the alkaline cement paste
3. As the mortar dries and cement hydrates, the polymer particles coalesce to form a continuous film
4. 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

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