Sikaflex®-521 UV

Weather-resistant STP adhesive and sealant with a wide adhesion spectrum

Sikaflex®-521 UV is a weather-resistant, one-component STP adhesive and sealant with good adhesion to a wide range of substrates, such as metals, ABS, PC, FRP and wood, where no special pre-treatment is usually required. This universal product is suitable for both interior and exterior waterproofing.


Resistant to aging and weathering


Good adhesion on a variety of substrates without special pretreatment


Free from isocyanate and solvents


Product advantages

  • Resistant to aging and weathering
  • Good adhesion on a variety of substrates without special pretreatment
  • Free from isocyanate and solvents
  • Recoatable
  • Sandable
  • Low odour


Sikaflex®-521 UV adheres well to a variety of substrates and is suitable for permanent, elastic sealing and bonding. Suitable substrates including wood are metals, primers and lacquers (two-component systems), ceramic materials and plastics. This universal product is suitable for waterproofing indoors and outdoors. Observe manufacturer’s recommendations before using Sikaflex®-521 UV on materials susceptible to stress cracking. Pre-tests must be carried out with original materials to avoid stress cracking. This product is only suitable for experienced users. To ensure adhesion and material compatibility, preliminary tests must be carried out with original materials under the relevant conditions.

Product details

Product features

Chemical base Silane terminated polymer (STP)
Colour (CQP001-1) White, grey, black
Curing mechanism Moisture-curing
Density before curing Depending on colour 1.4 kg/l
Stability Good
Application temperature Environment 5 to 40 °C
Skin formation time (CQP019-1) 30 minutes
Through-curing speed (CQP049-1) See diagram 1
Volume change (CQP014-1) -2 %
Hardness Shore A (CQP023-1 / ISO 7619-1) 40
Tensile strength (CQP036-1 / ISO 527) 1.8 MPa
Elongation at break (CQP036-1 / ISO 527) 400 %
Tear resistance (CQP045-1 / ISO 34) 5.5 N/mm
Operating temperature (CQP513-1) -50 to 90 °C
4 Hours 140 °C
1 Hour 150 °C
Durability (CQP016-1) Cartridge / Bag 12 Months
Hobbock / barrel 9 Months

Technical information

Sikaflex®-521 UV cures by reaction with air humidity. At low temperatures the water content of the air is lower and the cross-linking reaction is somewhat slower, see diagram 1.

Sikaflex®-521 UV is generally resistant to fresh water, sea water, dilute acids and dilute alkalis; temporarily resistant to fuels, mineral oils, vegetable and animal fats and oils; not resistant to organic acids, glycol, concentrated mineral acids and alkalis or solvents.


Surface pretreatment

The adhesive surfaces must be clean, dry and free from grease, oil and dust. The surface preparation depends on the specific condition of the substrate and is decisive for a durable bond. Instructions for surface pretreatment can be found in the current Sika® pretreatment table. The information contained there is based on experience and must always be verified by preliminary tests with original materials.


Sikaflex®-521 UV can be processed between 5 °C and 40 °C. Changes in reactivity and application properties must be taken into account. The optimum temperature for substrate and sealant is between 15 °C and 25 °C.

Apply Sikaflex®-521 UV with a suitable cartridge/bag gun or pump unit. For information on the selection and installation of a suitable pump system, please contact System Engineering at Sika Industry.


Smoothing must be carried out before the sealant skin forms. For smoothing we recommend Sika® smoothing agent N. Other smoothing agents must be checked for their suitability.


Uncured Sikaflex®-521 UV can be removed from tools and equipment with Sika® Remover-208 or other suitable solvents. Cured material can only be removed mechanically. Hands/skin must be washed immediately with suitable cleaning cloths (e.g. Sika® Handclean) or industrial hand cleaners and water. Do not use solvents on the skin!