Heat-activatable, storage-stable polyurethane powders

A process for forming a heat-activatable polyurethane powder which comprises the steps of (1) emulsifying with the aid of a surfactant a polymeric glycol or a mixture thereof with a low molecular weight diol in an inert organic liquid, (2) contacting the emulsion with an organic diisocyanate, (3) allowing the diisocyanate to react with the polymeric glycol or the mixture thereof with said low molecular weight diol in the emulsion to form an NCO-terminated polyurethane dispersed in said inert liquid, (4) contacting the resulting dispersion with a monofunctional isocyanate blocking agent, (5) permitting the aforesaid monofunctional isocyanate blocking agent to react with not more than about half of the --NCO groups of the dispersed polyurethane to form a heat-activatable adduct, (6) contacting the resulting dispersion with a difunctional active hydrogen compound and then reacting a sufficient amount of the latter with the residual --NCO groups of said polyurethane to provide the polyurethane with free active hydrogen-containing groups, and (7) separating the dispersed polyurethane as a finely divided powder, the particles of said powder providing about stoichiometric amounts of blocked --NCO groups and of active hydrogen-containing groups, whereby when the powder is heated the --NCO groups which are blocked will be regenerated and react with said free active hydrogen-containing groups.

In a process for forming a heat-activatable polyurethane powder composed of a mixture of two separate types of particles, one type having blocked --NCO groups, and the second having terminal active hydroxyl-containing groups, so that the said mixture provides chemically equivalent amounts of each type, the improvement comprising forming the first type of particles by the steps of

• 1. mixing a polymeric glycol, or a mixture thereof with a diol and a monofunctional isocyanate blocking agent in an inert organic liquid in which said polymeric glycol, or the mixture thereof with said diol is insoluble and forming an emulsion thereof with the aid of the surfactant,
• 2. contacting the resulting emulsion with an organic diisocyanate,
• 3. allowing the diisocyanate to react with the polymeric glycol, or the mixture thereof with said diol, and with said monofunctional isocyanate blocking agent to form a dispersion of polyurethane containing blocked --NCO groups and
• 4. separating the resulting product as a finely divided powder, characterized in that when the powder is heated some --NCO groups will be regenerated.

A process of forming a heat-activatable polyurethane powder adapted for use in applying a flexible film to a substrate and characterized by low-melt viscosity upon initial melting and in which powder the particles are essentially bead-like in form, comprising

• 1. adding to a solution of non-ionic surfactant in n-heptane a mixture of poly(tetramethylene ether) glycol, 1,4-butane diol, and epsilon-caprolactam and agitating so as to form an emulsion of the glycol, diol, and the epsiloncaprolactam in the n-heptane,
• 2. adding to the resulting emulsion 4,4'-methylene bis(cyclohexyl isocyanate) with further agitation and in an amount which is in stoichiometric excess of that required to react with the components of the emulsion, initiating a reaction between said isocyanate and said emulsion components to form a dispersion of polyurethane beads having both terminal --NCO groups and --NCO groups which have reacted with said epsilon-caprolactam to form blocked --NCO groups,
• 3. contacting the aforesaid dispersion with 1,6-hexamethylenediamine with agitation and for a time and temperature sufficient to permit it to react with said terminal --NCO groups, and
• 4. recovering the resultant product as a finely divided powder which provides about equal amounts of terminal amino groups and blocked --NCO groups, said powder being in the form of beads which are adapted for use in powder coating processes.