Flame-retardant coatings for rigid polyurethane foam based on mixtures of polysaccharides and polyborate

New types of flame-retardant coatings for rigid polyurethane foam (RPUF) are developed using mixtures of amorphous sodium polyborate (SPB) and various polysaccharides. Based on our previous research reporting that the RPUF coated with a mixture of SPB and starch shows high flame retardancy, polysaccharides such as carboxymethyl cellulose, hydroxyethyl cellulose, glucomannan, 2-hydroxypropyl guar gum (HPG), and gellan gum are used instead of starch. By coating each mixture on the surface, the RPUF (10 mm thickness) endures the premixed flame of butane gas burner with length of 100 mm for more than 12 min, and the backside temperatures remain within the range of 100–160°C. The high flame retardancy is successfully achieved with lower adhesive amounts of the mixtures (8.9–19.1 mg/cm²) than that of the starch/SPB mixture (51.3 mg/cm²). Water resistance is also substantially improved by using gellan gum, CMC, or glucomannan with NaOH. The elution ratio when immersed in water for 12 h is significantly suppressed to 4.8% using the gellan gum/SPB mixture compared with 80.1% using the starch/SPB mixture. The differential thermal analysis and thermogravimetry of the coating mixtures and the scanning electron microscope observations of combustion residues suggest the flame-retardant mechanism that a carbonaceous foam layer is produced from polysaccharides by the action of SPB foam layer and both of the foam layers protect inside from heat and oxygen.