Knauf's Moisture-Resistant Core: Silicone vs Wax Emulsion
Two chemistries, two failure modes. A side-by-side reading of the public hydrophobization literature.
Water-resistant gypsum board — Type H in EN 520, Type WR in ASTM C1396 — has effectively converged on two hydrophobization chemistries: silicone (PMHS / PDMS emulsions) and paraffin wax emulsion. Knauf and Saint-Gobain Placo public patents over the last fifteen years map a clear technical preference for silicone above the 4.5% absorption threshold; the cost-driven plants in Asia have stayed with wax. Both work. They fail differently.
Silicone (PMHS / PDMS)
Poly(methylhydrogensiloxane) hydrolyzes at the gypsum crystal surface, releasing hydrogen and grafting a hydrophobic methyl-silicon shell onto the dihydrate. The bond survives 28-day soak tests and aged cycling. Failure modes are typically: insufficient activation (emulsion pH below 4), foam incompatibility (high-active PMHS depresses surface tension and collapses unstable foam), or kiln overdrying (silicone shell forms but is starved of the trace water it needs to crosslink completely).
Wax emulsion
Paraffin wax sits in the pore network as a physical barrier. Cheaper. Faster to qualify. Fails by migration: at elevated service temperatures, the wax can travel through the core and produce surface blooms. Below 4% absorption is achievable but tight.
What a plant manager should ask
Three questions before changing suppliers: - What is your emulsion's PMHS active content, and how does it respond at our slurry pH? - Do you have foam-compatibility data on our exact surfactant? - What is your recommended kiln residual-moisture target for hydrophobization completion?
If a sales rep cannot answer the third question, they are selling you the bottle, not the result.