Synthesis And Application Of A Novel Sulfur-Nitrogen Synergistic Flame Retardants And Tensile Enhancer For Waterborne Polyurethane

Polymeric materials are widely used in daily life due to their light weight and high strength.As the fifth largest plastic,the versatility of polyurethane(PU)has been found in many fields such as the construction,coating and leather because of its good chemical stability,mechanical and processing properties.However,the risk of PU flamability shadows its applications in the daily life,hence the modification of flame-retardant becomes one of the key topics in the fabrication of PU products.Previously,series of effective flame-retardants,e.g.the halogenated and phosphoric compounds,have been developed.However,nowadays,it is going more and more serious due to the world-wide anxieties to the ecological toxicities of halogenated and phosphoric compounds.Accordingly,the alternatives are called prospectively.Meanwhile,the deterioration of PU mechanical properties accompanying with the modification of flame retardant is always perplexing.Therefore,the development of alternative compound is urgent,possessing the property of flame retardibility and simultaneously tensile enhancability.In this paper,sulfur was selected as the main element instead of the toxic phosphor element which acted as the char formation catalyst in the form of sulfuric acid during combustion,and carboxymethyl cellulose(CMC)was chosen as the char source of intumescent flame retardant,which is an environmentally friendly cellulose derivative with abundant sources and low price.Combining with melamine(ME)and formaldehyde(FA),a new type of S/N synergistic flame retardant(SNFR)was synthesized,which was expected to simultaneously enhance the mechanical properties of PU by reactions between the residue groups of isocyanate and amino/hydroxyl groups.Fourier transform infrared spectroscopy(FTIR),thermogravimetric analysis(TG)and scanning electron microscopy(SEM)were employed for characterization of SNFR.WPU composited with SNFR and EG was evaluated by FTIR,tensile test(TT),TG,vertical burning test(VBT),SEM and hydrophilic and hydroscopic tests.Research to develop new methods and new materials for polymer green flame retardant technology,the main results are as follows:(1)Optimization of the reaction conditions of CMC,ME,SS and FA by orthogonal experiment method: 0.7% CMC and molar ratio of ME:SS:FA=1:3:5 was determined at 50 ℃and 8 h.TG showed that SNFR was quite thermally stable.(2)SNFR and WPU were blended with different ratios to prepare S/N flame-retardant waterborne polyurethane(SN-PU).It was observed that SNFR was effective to improve the mechanical properties of WPU coating film.For example,once adding 8 wt % SNFR,the tensile strength and module of WPU coating film increased from 3.25 and 1.71 MPa of the pure WPU coating film to 11.88 and 3.05 MPa,respectively.The flame retardancy of PU film was improved,e.g at 6 wt % SNFR,the flame retardant performance reached the UL-94 V-2level,but it was not further improved by adding 8 wt % SNFR.It indicated that SNFR was not so powerful in the flame retardant.SEM showed that the surface of SN-PU char was porous.This result proved that SNFR was the intumescent type.(3)Based on the above results,EG was added into SN-PU to promote the flame retardant.The experimental results showed that the flame retardancy was greatly elevated as EG was added,but the tensile strength deteriorated whilst the module increased.For example,once adding 2 wt % EG in WPU with 8 wt % SNFR,the flame retardant performance of SN-EG-PU reached the UL-94 V-0 level.Comprehensively,the optimal recipe of flame retardant WPU was determined,i.e.SN8-EG3 WPU.Using this recipe,the tensile strength and module of WPU coating film were 5.0 MPa and 16.91 MPa,respectively.SEM exhibited that the surface of char after burning was a "worm-like" densely porous layer of char,the internal structure remains intact,and the "fly ash" phenomenon was improved.