Selecting the Right Sealant Technology for the Job

What are the important questions to ask yourself when making a seal?

To help you determine the right sealant technology for each job (silicone, polyurethane, MSP (modified-silane polymer), butyl, etc.), you must first determine which properties are essential, which provide advantages but are not crucial, and which are secondary.

Below are a few of the questions to ask yourself:

  •   Is impermeability to liquids or gases necessary? Although all four technologies offer impermeability to liquids, only butyl sealants are impermeable to gases and vacuum.
  •   What is the maximum temperature the seal will have to withstand? If the surrounding temperatures will be more than 90°C (194°F) at all times, only silicone sealants are suitable.
  •   Is the substrate hard to bond? All four technologies offer a broad spectrum of adhesion. However, experience shows that MSP’s adhere better to certain plastics. Furthermore, only silicone will bond to silicone surfaces.
  •   Is a high level of mechanical strength necessary? Some polyurethane adhesive sealant offer mechanical strengths of up to 10 MPa. MSP XS55 has a mechanical strength of up to 6 MPa. Most silicones and butyls have a lower mechanical strength.
  •   Will the seal be painted or coated? If the answer is yes, MSPs are the right choice. Do not use silicones.
  •   Is a clear seal needed? If the answer is yes, the Dow Corning range has many clear sealants. Merbenit offers only one clear MSP(Merbenit TS40).
  •   Is a food-grade sealant necessary? If the answer is yes, use an NSF and/or FDA approved silicone.
  •   Will the seal be exposed to chemicals?Always find out what the chemical is (organic solvent, acid, base, etc.) to check for compatibility. Silicones stand up best to chemical attack in general.
  •   Will the seal be exposed to UV light? Again, silicones offer the best UV resistance. MSP’s offer good resistance (particularly Merbenit UV27 ) while polyurethanes degrade rapidly if they are not treated with a UV inhibitor.

Challenges in the application process must also be considered:

  •   Can the adhesive be applied at ambient temperature? Again, silicones, MSP’s and polyurethanes are RTV (room temperature vulcanizing). Only one butyl adhesive sealant (Totalseal TS 1127) cures at cold temperatures. The other products are hot-melt adhesives.
  •   Is a self-leveling adhesive needed? We carry several self-leveling silicones. The only self-leveling MSP we carry is Merbenit 2K20 (two-part). None of the products in the polyurethane or butyl ranges are self-leveling.
  •   Is a two-part adhesive needed? If the setting time is long or the adhesive sealant cannot be exposed to air moisture (enclosed housing, thick layer of adhesive sealant, etc.), a two-part adhesive may be a wise choice. Dow Corning two-part silicones are available only in bulk, whereas Merbenit and Le Joint Français two-part products are available in cartridges.
  •   Is high initial tack necessary? If the answer is yes, opt for a MSP or a butyl adhesive.

Summary table

Silicone PU MSP Butyl
Impermeable to gases No No No Yes
Maximum temperature 90°C (194°F) 90°C (194°F) 90°C (194°F) 90°C (194°F)
Spectrum of adhesion Wide Wide Very wide Wide
Mechanical strength Up to 3 MPa Up to 10 MPa Up to 6 MPa Low
Paintable No Yes Yes Yes
Clarity Several grades No TS40 No
Food contact Several grades No No No
UV resistance Very good Very poor Average (UV 27) Average
Chemical resistance Excellent Poor Average Average
RTV curing Yes Yes Yes No (except TS1127)
Self-leveling Several grades No 2K20 No
Two-part Bulk only Several grades 2K10/2K20 No
Initial tack No No Yes Yes

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