Abstract
In order to explore highly efficient flame-retardant rigid polyurethane foam (RPUF), phosphorus/nitrogen compounds and expandable graphite (EG) were successfully incorporated into RPUF by a free one-spot method. The combustion results showed that the fire safety of the RPUF samples was remarkably improved by the addition of phosphoric/nitrogen compounds and EG. With the incorporation of 22.4 wt.% phosphorus/nitrogen compounds and 3.2 wt.% EG, the RPUF composites achieved UL-94 V-0 rating. Besides, the total heat release and total smoke release of RPUF composites were reduced by 29.6% and 32.4% respectively, compared to those of the pure RPUF sample. PO• and PO2• together with nonflammable gaseous products were evolved from phosphoric/nitrogen compounds in the gas phase, which quenched the flammable free radicals in the matrix and diluted the concentration of combustible gaseous products generated from PRUF during combustion. The compact char residues which acted as excellent physical barriers were formed by catalysis of EG and phosphoric/nitrogen compounds in the condense phase. The fire hazard of RPUF was significantly reduced by the synergistic effect of phosphorus-nitrogen compounds and EG. This work provides a promising strategy to enhance the fire safety of RPUF.
Highlights
Polyurethanes (PUs) have been widely utilized in various fields including paints, construction, adhesives, refrigeration, aeronautics, medical devices, sealants and oil pipeline [1,2,3,4].Rigid polyurethane foam (RPUF) is one of the most extensively applied PU materials due to its excellent mechanical and adiabatic properties, such as high compressive strength, light weight, construction convenience, low water absorption, low thermal conductivity and so on [5,6,7]
The first decomposition stage is attributed the degradation of PU chains to form the monomeric precursor such as polyhydric alcohols, isocyanate, amines, etc., while the second degradation stage is caused by the decomposition of the substituted urea, which results from the reaction of carbodiimide with water vapor or alcohol [41,42]
The char yield of RPUF-2 is markedly increased by 520% compared to that of pure RPUF, which is attributed to the outstanding catalytic charring of carboxyethyl phenylphosphinic acid glycol ester (CEPPG)
Summary
Polyurethanes (PUs) have been widely utilized in various fields including paints, construction, adhesives, refrigeration, aeronautics, medical devices, sealants and oil pipeline [1,2,3,4]. 2-carboxyethyl phenylphosphinic acid glycol ester (CEPPG), melamine phosphate (MP) and melamine phosphite (MPi) were efficient P/N flame retardants, which were endowed with the high content of nitrogen and phosphorous elements [27,28,29] Those P/N flame retardants produce slight and flurry char residues during combustion, which leads to the poor smoke suppression. Expandable graphite (EG), acting as an excellent halogen-free flame retardant with a highly carbon source, generates a worm-like char layer during its combustion. This compact and firm char layer has a distinguished obstructing effect on smoke and heat. This work provides a potential method to improve the fire safety of RPUF by combining P/N flame retardants with EG
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