Semi-flexible and flexible polymer powders

Redispersible polymer powders are grouped by how much flexibility they give the mortar. The two groups you meet most often in construction are semi-flexible and flexible polymer powders. Both are made the same way; the difference is how hard or soft the polymer is. For the mortar, this means how much hardness you give up and how much flexibility you gain.

Semi-flexible polymer powders

Semi-flexible polymers give the mortar adhesion and some flexibility, but they don't give up much of the hardened mortar's compressive strength or surface hardness. They are preferred where the mortar will face load and abrasion more than movement.

Properties

• Balance of hardness and flexibility. Doesn't make the mortar fully flexible, but gives it enough flexibility to resist cracks from shrinkage and small movements.
• Strength. Keeps the mortar's compressive strength and abrasion resistance; the surface stays hard.
• Temperature behaviour. The glass transition temperature (Tg) is higher. It softens less in the heat, but loses its flexibility sooner at low temperatures.

Production

Made the same way as flexible polymers: monomers are polymerised in water by emulsion polymerisation, a protective colloid is added and the emulsion is turned into powder by spray drying. The monomer ratio sets the hardness. The share of hard monomers such as vinyl acetate is kept high, so the Tg goes up.

Applications

• Standard ceramic tile adhesives. Raises bond strength; enough for adhesives where deformability is not required.
• Repair mortars and screeds. Raises adhesion and flexural strength while keeping surface hardness and abrasion resistance.
• ETICS adhesive mortars. Strengthens the insulation board's grip on the wall.

Flexible polymer powders

Flexible polymers let the hardened mortar stretch and bend without breaking. They are used where the substrate moves because of thermal movement, vibration or drying shrinkage, and in products that need crack bridging.

Properties

• High flexibility. The mortar can deform more without breaking and can bridge hairline cracks.
• Low hardness. The polymer film is soft. In return, the mortar's compressive strength and surface hardness drop somewhat.
• Flexibility in the cold. Because the Tg is low, it stays flexible at low temperatures too.

Production

The production route is the same: emulsion polymerisation, protective colloid and spray drying. The flexibility comes from the polymer's own structure, not from a plasticiser added afterwards. The share of ethylene or soft acrylic monomers is raised, the Tg drops and the polymer becomes internally flexible.

Applications

• Flexible ceramic tile adhesives. In S1 and S2 class (deformable) adhesives, especially on underfloor-heated floors, on exterior facades and with large-format tiles.
• Waterproofing mortars. Gives brush-applied waterproofing products the ability to bridge cracks.
• Flexible joint fillers and renders. Reduces cracking and detachment on surfaces that move with temperature.

Which one to choose?

If you mainly need strength, surface hardness and abrasion resistance from the mortar, semi-flexible polymer is the better fit. If the substrate moves or crack bridging is needed, for example in a flexible tile adhesive or a brush-applied waterproofing, go for flexible polymer. We can help you work out which one, and how much of it, suits your formulation.