Method for the synthesis of an auxetic polyurethane foam with a defined cell structure and auxetic polyurethane foam obtainable by the method

A method to prepare an auxetic polyurethane foam with a defined cell structure in a single foaming process, comprising the steps of:

• a. mixing a polyol reagent and a foaming reagent to form a homogeneous reaction mixture suitable for forming a homogeneous auxetic polyurethane foam, such that the foaming reagent facilitates the formation of auxetic polyurethane foam by enabling the reaction mixture to expand and polymerize;
• b. mixing an isocyanate with the homogeneous reaction mixture, the isocyanate being selected and provided in an amount to cause the homogeneous reactive mixture to undergo foaming and polymerization to form a polyurethane network having closed cells and/or partially closed cells and/or pre-stressed ribs that contribute to contraction during cooling and enable the formation of a defined cellular structure with buckled ribs, delaying a cure of the reactive mixture and increasing the elasticity of the reactive mixture;
• c. permitting the reactive mixture of step (b) to expand within a mold and/or on a conveyer, wherein the expansion forms a polymer network that includes closed cells that contract as the polymer network cools;
• d. allowing the expanded reaction mixture at least partly to cool within the mold and/or on the conveyor to induce contraction at least partially caused by negative pressure from the trapped gases in the closed cells, and/or applying external compressive forces to the expanded mixture during or after contraction;
• wherein the contraction and/or the compression promotes cell buckling and changing of the initial cellular structure to a defined cellular structure with the closed cells having cellular ribs or struts protruding inward and/or outward prior to finalization of chemical cross-linking and physical curing;
• and
• e. after the contraction and/or the compression, allowing the compressed and/or contracted reaction mixture of step (d) to cure, wherein the curing process causes the defined cellular structure to form a defined auxetic polyurethane foam structure characterized by a Poisson's ratio in the range of −3 to 0.