Study Of Flame Retardancy Of Phosphorus-Containing Flame-Retardant Lignin-Based Epoxy Thermosets

In the synthetic process of epoxy resins,lignin can be used as a green alternative to petroleum-based phenolic compounds（e.g.,bisphenol A）due to its rich phenolic structures.In this paper,a kind of liquid lignin-based epoxy resin（LPG-ER）was synthesized by chemical crosslinking under acidic conditions using lignin and phenol as substrates and glyoxal as crosslinking agent,and its mechanical properties and combustion behaviors were systematically investigated.Similar to traditional epoxy resins,lignin-based epoxy resins have the potential of flammability in practical applications,the main reasons for their flammability include:（1）its chemical structure mainly consists of carbon,hydrogen and oxygen;（2）the lack of necessary flame-retardant factors;（3）the reduction of char graphitization degree.Phosphorus-containing flame retardants can be used as efficient flame retardant additives for LPG-ER because of their excellent gas-phase quenching and solid-phase promotion effects.In this paper,three multifunctional phosphorus-containing flame retardants were successfully prepared,including（A）bi-hydroxyl-bi-9,10-dihydro-9-oxa-10-phosphaphe-nanthrene-10-oxide（DOPO）,（B）nitrogen/phosphorus/sulfur-containing,and（C）nitrogen/phosphorus-modified Co-MOF（metal-organic framework）flame retardants.These materials follow the design concept from simple（containing only phosphorus elements）to complex（containing nitrogen,phosphorus and sulfur elements）,and then from organic to organic/inorganic.Bi-hydroxyl-bi-DOPO material（A）contained good pyrolytic volatility（⁵ wt%residual char）with subsequent low addition in LPG-ER.When the addition amount of A was 4 wt%,the resulting flame-retardant epoxy cured product（LPG-ER-4）had an LOI value of 35.2%and achieved UL-94 V-0 level.In addition,total heat release（THR）and average value of effective combustion heat（Av-EHC）of LPG-ER-4 all decreased compared with those of LPG-ER,indicating the effective suppression of burning intensity.Char formation also increased from 27.8 to 29.1 wt%,and the degree of char graphitization was effectively improved（I_D/I_G decreased from 4.13 to 3.04）.Nitrogen/phosphorus/sulfur-containing material（B）tended to undergo self-condensation to form char（⁴2.5 wt%）.When B was added at 10 wt%,the LOI value of the resulting flame-retardant epoxy cured product（LPG-ER-10）was 34.5%,and reached UL-94 V-0 level.Due to the presence of abundant phosphorus/nitrogen/sulfur-containing groups,the inhibition effect on the burning intensity of LPG-ER was more obvious（THR and Av-EHC decreased by 34.4%and 18.2%,respectively）.Co component in the structure of nitrogen/phosphorus-modified Co-MOF material（C）further promoted char formation ability,so it had the highest char formation（55.9 wt%）.The LOI value of the resulting flame-retardant epoxy cured product（LPG-ER-6M）was 34.8%and reached UL-94 V-0 level when the addition of C was 6 wt%.Due to the presence of Co component,the improvement on char quality was the most obvious（I_D/I_G decreased from 4.14 to 2.74）.In addition,the catalytic oxidation ability from Co component further reduced the content of carbon monoxide and improved fire safety.In summary,functionalities（or flame-retardant mechanisms）of three phosphorus-containing flame retardants are mainly reflected in the following aspects:（A）phosphorus-containing components can be thermally decomposed into PO?and PO₂?groups,thus effectively capturing H?and OH?groups produced by epoxy resins combustion to achieve gas-phase quenching effect（reducing heat release）;（B）the thermal degradation of nitrogen/sulfur-containing components can produce inert gases,which can play a dilution effect on burning flame（reducing flame intensity）;（C）phosphorus/nitrogen and Co elements promote char formation via solid-phase promotion and catalytic charring effects,which can effectively isolate combustion reaction from internal epoxy matrix and suppress smoke generation by both increasing the mass and quality of residual char.From personal perspective,nitrogen/phosphorus-modified Co-MOF flame retardant（C）can be more promising due to its lower addition amount,excellent gas-and solid-phase flame-retardant performance,and the higher improvement on fire safety.