I agree with Dave Peritt - if they are dying at -20, don't freeze them.
(Is your freezer a frost-free type? If so, be aware they cycle from 0 tp
-20 every few hours to defrost). As long as they are sterile / preserved,
they should be good for many months.
If the antibody is denaturing even under mild storage conditions, consider
adding glycerol to 20%, or trehalose to 10%. These reagents can help
stabilize antibodies.