Method for producing isocyanate-terminated siloxanes
Isocyanate-functional or protected isocyanate-functional organopolysiloxanes are prepared directly by reaction of a siloxane bearing at least one silicon-bonded hydroxyl group with an alkoxy-functional α-silane containing an isocyanate group or blocked isocyanate group.
1. A process for preparing isocyanate-terminated siloxanes comprising reacting at least one Si-OH-terminated, branched or unbranched siloxane (A) comprising units of the formula (I):
R a R 1 b (R 2 O) c SiO 4-a-b-c/2 (I),
where
R is a monovalent, SiC-bonded, substituted or unsubstituted hydrocarbon radical optionally interrupted by oxygen atoms,
R 1 is an Si-bonded hydroxyl group,
R 2 are identical or different and are each a monovalent, substituted or unsubstituted hydrocarbon radical optionally interrupted by heteroatoms,
a is 0, 1, 2 or 3,
b is 1, 2 or 3 and
c is 0, 1, 2 or 3,
with the proviso that the sum a+b+c≦3 and at least one unit having b different from 0 is present,
with at least one optionally protected isocyanate-functional α-silane of the formula:
(R 2 O) n R m Si—CH 2 —NR 3 (II),
where
R 3 is CO or H—CO—Z,
where Z is a monovalent, substituted or unsubstituted hydrocarbon radical optionally interrupted by heteroatoms,
n is 1, 2 or 3 and
m is 0, 1 or 2,
with the proviso that the sum m+n=3.
2. The process for preparing isocyanate-terminated siloxanes of claim 1 , wherein methoxydimethyl(methylisocyanato)silane is used as an α-silane.
3. The process for preparing isocyanate-terminated siloxanes of claim 1 , wherein the reaction of the siloxanes (A) with the α-silanes of the formula (II) is carried out in the presence of a Brönsted acid.
4. The process for preparing isocyanate-terminated siloxanes of claim 3 , wherein the Brönsted acid is phosphoric acid, monoisopropylphosphate, dibutylphosphate, toluene-sulfonic acid, methylsulfonic acid or any mixture thereof.
5. The process for preparing isocyanate-terminated siloxanes of claim 3 , wherein the Brönsted acid is used in an amount, based on the weight of siloxane (A), of from 0.01 to 1% by weight.
6. The process for preparing isocyanate-terminated siloxanes of claim 4 , wherein the Brönsted acid is used in an amount, based on the weight of siloxane (A), of from 0.01 to 1% by weight.
7. The process for preparing isocyanate-terminated siloxanes of claim 1 , wherein the reaction of the siloxanes (A) with the α-silanes is carried out at a temperature of from −20° C. to 100° C.
8. The process for preparing isocyanate-terminated siloxanes of claim 1 , wherein the reaction of the siloxanes (A) with the α-silanes is carried out at an applied vacuum of from 0.1 mbar to 800 mbar.