MONJU Materials Blog

Silanes vs Siloxanes: A Primer

If your job is to specify long-lasting protection for concrete or natural stone, you're familiar with siloxane sealers.  They're affordable and don't alter surface appearance much.  You may have also heard of silanes, siloxane's older, more expensive brother.  Chances are, given enough budget, you prefer an invisible, deep penetrating sealer that lasts more than just a few years, but the cost of specifying silanes made siloxanes more attractive.  

Good news, we offer the world's most specified line of silanes at prices that approach high-quality siloxanes.  Check out this quick primer on the pros and cons of both products and call us if you have any questions.  We're here to help you make the right decision.

PROPERTIES OF SILOXANE SEALERS INCLUDE:

Pros

  • Film-forming = excellent surface protection against driving rain
  • Excellent initial beading effect
  • Low water absorption coefficient, about 0.1kg/m2 h0.5
  • Siloxanes are less expensive than pure silanes

Cons

  • Non-penetrating, protection against water ingress is dependent on a thin film.  However, a siloxane diluted with silane will penetrate more than a pure siloxane.
  • Larger molecule size = less reactivity to mineral surfaces.  However, a high-quality siloxane should form a semi-permanent bond to mineral surfaces.
  • Larger molecular size = more obstruction of pores = less vapour permeability
  • Surface film is susceptible to UV attack which will break down even the best siloxane sealers in 2-5 years max.  Beading effect will usually last a year or so.
  • Will impact the surface appearance, less so if the siloxane is diluted.
  • Not compatible with subsequent surface coatings.

PURE SILANES SUCH AS PROTECTOSIL HAVE THE FOLLOWING PROPERTIES:

Pros 

  • Small molecular size (about a twentieth of siloxane molecules) = deep penetration, and high reactivity to mineral substrates.
  • Small molecular size = high vapour permeability
  • Silanes form a permanent chemical bond to the substrate, whereas films such as siloxanes depend on a combination of chemical and mechanical bonds.  The latter of which are less reliable over the long-term.
  • No surface film, so the protective barrier is not susceptible to UV or abrasion
  • Very low water absorption coefficient, about 0.07kg/m2 h0.5.  In general will reduce water uptake by 90% compared to untreated surfaces.
  • Does not alter surface appearance
  • Does not rule out subsequent coatings such as surface protection or paint.

Cons

  • Silanes are more expensive than siloxanes (typically 2-3x).  However, our prices are comparable to high-quality siloxanes.
  • Typically higher VOCs than siloxanes, less so when diluted with water.
  • No film = no surface protection against the effects of abrasion
  • Silanes tend to react slowly (4-14 days depending on humidity) and spread deeply, leaving the surface less protected.  This is why two coats are required to achieve saturation.

In short, specifiers will choose pure silanes when the budget allows because of their superior protection.  Going with a siloxane has some short-lived advantages, but doesn't offer the long-term protection silanes do.  

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