Process for preparation of improved semipermeable composite membranes

In a process for producing a semipermeable composite membrane which comprises forming on a microporous substrate a thin layer of polymeric material comprising a polyamino polymer containing at least 1 milliequivalent, per gram of the polymer in the dry state, of active amino groups selected from primary amino groups and secondary amino groups, and thereafter interfacially crosslinking a surface portion of said thin layer with a crosslinking agent comprising a cyclic compound having at least two functional groups (a) selected from the group consisting of carbonyl halide, sulfonyl halide, isocyanate and acid anhydride groups;

• the improvement wherein
• (1) said polymeric material contains dispersed therein a polyfunctional aliphatic compound having at least two functional groups (b) which are selected from the group consisting of --CH(OH)CH.sub.2 X.sub.1, --COOA.sub.1, --NHCOOA.sub.2, ##STR86## and active halomethyl groups, in which X.sub.1 and X.sub.2 each represent a halogen atom, A.sub.1 and A.sub.2 each represent a hydrogen atom or an organic radical capable of being split off together with the oxygen atom to which it is bonded, and A.sub.3 represents a trivalent or tetravalent saturated aliphatic groups having 2 to 5 carbon atoms, and are substantially incapable of reacting with the primary or secondary amino groups in said polymer at a temperature at which the interfacial crosslinking is carried out, but capable of reacting easily with either the primary or secondary amino groups or both in said polymer at a temperature at least 30.degree. C. higher than said crosslinking temperature, and
• (2) the interfacially crosslinked thin layer is heated to a temperature at which said polyfunctional compound reacts with the primary or secondary amino groups or both in said polymer.