AbstractColloidal silica in the particle size range 1.4–10 nm was extracted in to tetrahydrofuran (THF). Alkali silicate solutions with SiO2: M2O (M = Li, Na, K, Cs) ratios ranging from 3:1 to 20:1 were used as source of silica particles in the size range 1.4–4.4 nm whereas commercial silica sols were used for particles in the range 5–10 nm. Films of polyester‐ and polyether‐based polyurethane‐containing colloidal silica were prepared and their mechanical properties measured. The reinforcing effect increased with increasing silica content and showed a maximum between 1.5 and 2.5 nm for polyether‐based polyurethane and between 4 and 6 nm for polyester‐based polyurethane. The area under the hysteresis loops of the stress–strain curves also showed maximum in the same particle size ranges for the two types of polyurethane. Reinforcement mechanisms are discussed in terms of interactions between small particles and hard or soft segments of t
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