Attrition resistant controlled release fertilizers

An attrition resistant, controlled release, granular fertilizer particle composition, comprising:

• (a) a central mass of urea containing between 26 and 53 percent NH.sub.2 functional groups;
• (b) a base coating, surrounding and chemically bonded to the central mass and consisting of substituted ureas formed by reacting a molecular excess of polymeric diphenylmethane diisocyanate, amounting to between 0.5 and 1.5 percent of the fertilizer particle and containing between 30 and 33 percent NCO, with the NH.sub.2 functional groups from the central mass of urea;
• (c) a water insoluble sealing layer, surrounding and chemically bonded to the base coating, formed by the reaction and polymerization of the excess polymeric diphenylmethane diisocyanate of the base coating, with an amount of polyalkyleneterephthalate polyester polyol to produce a NCO to OH moiety ratio between 1.1 and 1.8, the polyol exhibiting a hydroxyl number between 225 and 350, and containing between 5 and 15 percent trialkanolamine and less than 0.1 percent water;
• (d) between 1 and 5 water insoluble coatings, surrounding the sealing layer, each coating amounting to between 0.5 and 1.5 percent of the fertilizer particle, the coatings consisting of polymeric diphenylmethane diisocyanate and polyalkyleneterephthalate polyester polyol applied in alternate layers to produce a NCO to OH moiety ratio in the water insoluble coatings between 1.0 and 1.5.

A stepwise method of producing an attrition resistant particulate fertilizer, comprising:

• (a) surrounding and chemically bonding urea particles containing between 26 and 53 percent NH.sub.2 functional groups, with a base coating consisting of substituted ureas, by applying a molecular excess of polymeric diphenylmethane diisocyanate, amounting to between 0.5 and 1.5 percent of the fertilizer particle and containing between 30 and 33 percent NCO, to a mobile mass of the particles of fertilizer at a temperature between 80.degree. and 120.degree. C. and holding for a period of time between 2 and 10 minutes;
• (b) surrounding and chemically bonding the base coating with a water insoluble sealing layer by reaction and polymerization of the excess polymeric diphenylmethane diisocyanate of the base coating with an amount of polyalkyleneterephthalate polyester polyol to produce a NCO to OH moiety ratio between 1.1 and 1.8, the polyol containing between 5 and 15 percent triethanolamine and less than 0.1 percent water and exhibiting a hydroxyl number between 225 and 350, by applying the polyol to a mobile mass of base coated particles at a temperature between 100.degree. and 150.degree. C. and holding for a period of time between 2 and 10 minutes;
• (c) surrounding the sealing layer with between 1 and 5 water insoluble coatings, each coating amounting to between 0.5 and 1.5 percent of the fertilizer particles, by applying alternate layers of polymeric diphenylmethane diisocyanate and anhydrous polyalkyleneterephthalate polyester polyol with a NCO to OH moiety ratio between 1.0 and 1.5 to a mobile mass of the fertilizer particles at a temperature between 100.degree. and 150.degree. C. and holding for a period of time between 2 and 10 minutes.