## 1 Introduction  
Polyurethanes (PUs) are polymers composed of a soft segment and a hard segment prepared by the reaction between hard isocyanates and soft polyols. Polypeptides of the hard segments form strong hydrogen interactions with each other. In contrast, soft segments comprising linear long-chain polyols have flexible properties. The physical properties of synthesized polyurethanes depend on the reaction ratio between isocyanate and polyol and the types of reactants, such as polyols, chain extenders, or isocyanates. Therefore, a variety of synthetic polyurethanes prepared with various combinations of reactants have been widely adopted in automotive interior materials, adhesives, coatings, shoes, clothing, etc.  
However, the low thermal stabilities of polyurethanes limit their use in high-temperature processes or applications. Furthermore, polyurethanes begin to decompose at temperatures >180 \({}^{\circ}\)C, resulting in the production of toxic gases, such as HCN or CO, that are harmful to the human body or environment [1; 2; 3; 4].  
Recently, researchers have been interested in introducing flame retardants into polyurethanes to compensate for the low thermal stability. The flame retardants can be classified into two types:an additive type, in which a flame retardant is physically added to the polymeric resin, and a reactive type, in which a flame retardant reacts with reactant monomers, forming polymers. In general, the most often used additive flame retardants need to be highly miscible with the raw materials and not deteriorate the mechanical properties of the produced polymers nor produce any toxic gases.  
Since environmental regulations prohibit the use of halogenated flame retardants, phosphoric flame retardants have attracted attention as one alternative. In particular, the flame retardancy of polyurethane is improved by introducing a monomer having flame retardancy into the main chain, using a method of encapsulating the flame retardant at the end of the polyurethane or by using organic-inorganic additives [5; 6; 7; 8].  
Epoxy resins are thermosetting polymers constructed of three-dimensional network structures after a curing reaction with hardeners in the presence of catalysts. Epoxy resin derivatives can exhibit various physical properties by reacting with different hardeners, and they are widely used in coatings, adhesives, and moldings because the polymerized matrices provide excellent thermal and mechanical properties and chemical resistance [9; 10]. However, the high brittleness of cured epoxy polymers promotes easy crack propagation. Therefore, numerous scientists have reported methods to reduce this brittleness by adding thermoplastic resin, rubber resins, or elastomeric polyurethanes [11; 12]. Specifically, polyurethanes in epoxy compositions, which are known to be phase separated after curing, have been added to absorb external impact [13; 14].  
In this report, a phosphoric monomer was reacted with an epoxy resin to prepare a reactive polyol. The polyol mixed with a polyether polyol in a 1:1 ratio was reacted with 2 mol of isophorone diisocyanate to enhance thermal resistance. In addition, the prepared polyurethane was added to epoxy compositions to investigate physical changes, where bisphenol A epoxy resin and dicyandiamide (dicy) were stoichiometrically blended. The effects of the prepared polyurethane addition to the epoxy compositions were further analyzed with tensile and flexural strength measurements, the Izod impact method, thermogravimetric analysis, and scanning electron microscopy.