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Sikasil WS-305 CN Silicone:   


Sikasil WS-305 CN is a neutral-curing silicone sealant with a high movement capability and excellent adhesion to a wide range of substrates.

Sikasil WS-305 CN is manufactured in accordance with ISO 9001 quality assurance system and the responsible care program.


Product Benefits


Meets requirements of ISO 11600 25 LM F & G, ASTM C 920 (class 50), TT-S00230C, TT-S001543A

Outstanding UV and weathering resistance

Adheres well to many substrates including glass, metals, coated and painted metals, plastics and wood


Areas of Application


Sikasil WS-305 CN can be used for weatherproofing and sealing applications where durability under severe conditions is required.

Sikasil WS-305 CN is particularly suited as a weather seal for curtain walling and windows.

This product is suitable for professional experienced users only. Tests with actual substrates and conditions have to be performed to ensure adhesion and material compatibility.


Cure Mechanism


Sikasil WS-305 CN cures by reaction with atmospheric moisture. The reaction thus starts at the surface and proceeds to the core of the joint. The curing speed depends on the relative humidity and the temperature (see diagram 1 below). Heating above 50 C to speed up the vulcanization is not advisable as it may lead to bubble formation. At low temperatures the water content of the air is lower and the curing reaction proceeds more slowly.

Diagram 1:Curing speed 1C-Sikasil


Application Limits


Most Sikasil WS, FS, SG, IG, WT, AS and other engineering silicone sealants manufactured by Sika are compatible with each other.Sikasil WS and FS sealants are compatible with SikaGlaze IG sealants. All other sealants have to be approved by Sika before using them in combination with Sikasil WS-305 CN. Where two or more different reactive sealants are used, allow the first to cure completely before applying the next.

Do not use Sikasil WS-305 CN on pre-stressed polyacrylate and polycarbonate elements as it may cause environmental stress cracking (crazing).

The compatibility of gaskets, backer rods and other accessory materials with Sikasil WS-305 CN must be tested in advance.

Joints deeper than 15 mm should be avoided.

The above information is offered for general guidance only. Advice on specific applications will be given on request.


Method of Application


Surface preparation


Surfaces must be clean, dry and free from oil, grease and dust.

Advice on specific applications and surface pretreatment methods is available from the Technical Service Department.




After suitable joint and substrate preparation, Sikasil WS-305 CN is gunned into place. Joints must be properly dimensioned as changes are no longer possible after construction. For optimum performance the joint width should be designed according to the movement capability of the sealant based on the actual expected movement. The minimum joint depth is 6 mm and a width / depth ratio of 2:1 must be respected. For backfilling it is recommended to use closed cell, sealant compatible foam backer rods e.g. high resilience polyethylene foam rod. If joints are too shallow for backing material to be employed, we recommend using a polyethylene tape. This acts as a release film (bond breaker), allowing the joint to move and the silicone to stretch freely.

For more information please contact our Technical Department.


Tooling and finishing


Tooling and finishing must be carried out within the skin time of the adhesive.

When tooling freshly applied Sikasil WS-305 CN press the adhesive to the joint flanks to get a good wetting of the bonding surface.




Uncured Sikasil WS-305 CN may be removed from tools and equipment with Sika Remover-208 or another suitable solvent. Once cured, the material can only be removed mechanically.

Hands and exposed skin should be washed immediately using Sika Handclean Towel or a suitable industrial hand cleaner and water. Do not use solvents.




Sikasil WS-305 CN cannot be overpainted.


Further Information


Copies of the following publications are available on request:

Material Safety Data Sheet


Packaging Information


300 ml


600 ml


Chemical base

1-C silicone

Colour (CQP1 001-1)

300 ml cartridge: Black, Grey, Transparent, White

600 ml sausage: Black

Cure mechanism


Cure type


Density (uncured) (CQP 006-4) transparent

all other colours

1.03 kg/l approx.

1.49 kg/l approx.

Non-sag properties (CQP 061-4 / ISO 7390)

< 2 mm approx.

Application temperature

5 - 40 C

Skin time2 (CQP 019-2)

35 min. approx.

Tack-free time2 (CQP 019-1)

180 min. approx.

Curing speed (CQP 049-1)

See diagram 1

Shore A-hardness (CQP 023-1/ISO 868)

15 approx.

Tensile strength (CQP 036-1/ISO 37)

1.0 N/mm2 approx.

Elongation at break (CQP 036-1/ISO 37)

900% approx.

Tear propagation resistance (CQP 045-1/ISO 34)

4 N/mm approx.

100% modulus (CQP 036-1/ISO 37)

0.4 N/mm2 approx.

Movement accommodation capability (ASTM C 719)

+/- 50%

Thermal resistance (CQP 513-1)
Short term
4 hours
1 hour


180 C approx.

200 C approx.

220 C approx.

Service temperature

-40 to 150 C approx.

Shelf life (storage below 25 C) (CQP 016-1)

12 months


1) CQP = Corporate Quality Procedure                2) 23 C (73 F) / 50% r.h.



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