Silicone stabilizers for rigid polyurethane or polyisocyanurate foams

A process for producing a rigid polyurethane or polyisocyanurate foam, comprising reacting at least one isocyanate component, at least one polyol component, at least one foam stabilizer, at least one urethane and/or isocyanurate catalyst, and water and/or blowing agents, wherein said at least one foam stabilizer includes a polyether siloxane of formula (I),

• R—Si(CH3)2—O—[—Si(CH3)2—O—]n—[—Si(CH3)R1—O—]m—Si(CH3)2—R2 (I)
• where
• R and R2 are methyl,
• R1 in each occurrence is the same or different and represents —(CH2)x—(O)z—(CH2—CHR′—O)y—R″,
• R′ in each occurrence is the same or different and represents —H, —CH3, —CH2CH3 or phenyl,
• R″ in each occurrence is the same or different and represents —H or -alkyl,
• n+m+2=15 to 40,
• m=0 to 2,
• x=2 to 15,
• y=1 to 40,
• z=0 to 1
• where the (CH2—CHR′—O) units can be the same or different,
• and that for z=0 the requirement is that R″ contains at least 3 carbon atoms, although z=0 applies to not more than 70 mol % of the R′ radicals in the siloxane,
• wherein on average, the average being a number average averaged over all compounds of formula (I), from 10 to 45 mol % of the R″ radicals are not hydrogen radicals, and the remaining proportions of the R″ radicals are hydrogen atoms, and wherein on average m=0 to 2, n+m+2=15 to 40, x=3 and y=5 to 25.