Flame-retardant urethane resin composition

A method for producing a flame retarding composition comprising:

• preparing an additive by mixing red phosphorus and a bromine-containing flame retardant;
• preparing a mixture by mixing a polyol compound, a trimerization catalyst, a foaming agent, a foam stabilizer, and the additive;
• and
• adding a polyisocyanate compound into the mixture to form a urethane resin, wherein
• the trimerization catalyst is at least one selected from the group consisting of a nitrogen-containing aromatic compound, a carboxylic acid alkali metal salt, a tertiary ammonium salt, and a quaternary ammonium salt,
• the bromine-containing flame retardant is at least one selected from the group consisting of hexabromobiphenyl and decabromobiphenyl,
• an amount of the red phosphorus is within a range of 3 parts by weight to 18 parts by weight based on 100 parts by weight of the urethane resin,
• an amount of the trimerization catalyst is within a range of 0.6 parts by weight to 10 parts by weight based on 100 parts by weight of the urethane resin, and an isocyanate index of the urethane resin is 400 to 1,000.