Flame-retardant coatings for rigid polyurethane foam based on mixtures of polysaccharides and polyborate Isao Tsuyumoto Ó American Coatings Association 2020 Abstract 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 re- search 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 retar- dancy is successfully achieved with lower adhesive amounts of the mixtures (8.9–19.1 mg/cm 2 ) than that of the starch/SPB mixture (51.3 mg/cm 2 ). Water resis- tance 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 thermogravime- try of the coating mixtures and the scanning electron microscope observations of combustion residues sug- gest the flame-retardant mechanism that a carbona- ceous 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. Keywords Flame retardant, Polyurethane, Foam, Borate, Polysaccharide, Coating Introduction Polyurethane foam (PUF) is widely used for various applications such as heat insulators in buildings and refrigerators, cushions in cars, trains and upholstered furniture, and mattresses. Flame-retardant treatments of PUF are essential for fire prevention because it easily catches fire and goes up in flames due to high contact area with air. As for PUF, halogenated phosphate esters such as tris (1-chloro-2-propyl) phos- phate (TCPP), tris (1,3-dichloro-2-propyl) phosphate (TDCP), and tris (2-chloroethyl) phosphate (TCEP) have been extensively used as the flame retardants, though their restrictions are recommended by the European Commission (EU) because of a potential carcinogenicity risk for infants. 1–3 Concerns regarding human health are increasing because toxic compounds may be released in fire, disposal, and normal use of the halogenated flame retardants, and thus many research- ers have been intensively investigating nonhalogenated flame retardants. 4–6 Red phosphorus has been reported to impart high flame retardancy specifically to poly- urethane foam, while red phosphorus itself is a highly combustible hazardous material. 7–9 Expandable gra- phite, 10–12 layered silicate, 13,14 and melamine 15 have also been investigated for flame retardants of polyur- ethane foams. In most of these studies, the flame retardants were mixed with polyurethane foams as additives or incorporated into the raw material monomers as functional groups by reaction. Impregnation of sodium borate solution is a well- known technique to impart flame retardancy to cellu- lose-based materials such as wood, paper, and cotton. The solubility of sodium borate has been reported to rise one order of magnitude by adjusting Na/B ratio to 0.22, 16 and flame-retardant wood and rigid polyur- ethane foam were successfully prepared by impregna- tion of the concentrated sodium borate (sodium polyborate, SPB) solution to meet the Japanese I. Tsuyumoto (&) Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi, Ishikawa 9218501, Japan e-mail: tsuyu@neptune.kanazawa-it.ac.jp J. Coat. Technol. Res. https://doi.org/10.1007/s11998-020-00390-9