| Hydroxymethyl melamine,short for MOH,belongs to the series of melamine derivatives,is obtained by addition reaction of melamine and formaldehyde.It can be used for curing agent of coating,adhesive,fabric finishing agent,etc.Because MOH is easy to self-crosslinking under the influence of acid and heat,it is not easy to store,which greatly limits its application.Aiming at the difficulties of self-crosslinking and poor storage stability of hydroxymethyl melamine in water,this paper proposes a new method of freeze-drying to prepare hydroxymethyl melamine.For the first time,it was used as a physical crosslinking agent to enhance the mechanical properties of waterborne polyurethane and as a carbonizing agent to prepare expansive flame-retardant epoxy resin.The main work of this paper and the main research results obtained are as follows:（1）Taking melamine and 37%formaldehyde solution as the main raw materials,the effect of formaldehyde and melamine molar ratio,reaction temperature,reaction time and reaction pH value on the structure of hydroxymethyl melamine solution（MOH-L）was studied to find out the optimal reaction conditions.Then using the vacuum freeze drying technology to remove the water in the MOH-L,obtain methylol melamine powder（MOH-S）.The chemical structure of MOH-S was characterized by FI-IR,1H NMR,13C NMR and elemental analysis.The results show that MOH-S is a derivative of melamine with the main structure of hydroxymethyl melamine.MOH-S showed excellent storage stability without self-crosslinking after 3 months of storage.Dicyclohexylmethane diisocyanate（HMDI）,hexamethylene diisocyanate（HDI）and polypropylene glycol（PTMG-2000）as the basic raw materials for the preparation of anionic waterborne polyurethane（WPU）.After dissolving MOH-S in water and dispersing it into WPU emulsion,the particle size of the emulsion increases,but it will not break the emulsion after centrifugation,showing excellent emulsion stability.Removing the solvent water,the WPU/MOH-S composite film is obtained,and the composite film is analyzed by Fourier transform infrared spectroscopy（FTIR）,water absorption,contact angle,scanning electron microscope（SEM）,mechanical performance test,and thermogravimetric analysis（TGA）.The results show that the addition of MOH can significantly improve the mechanical properties of WPU without causing phase separation.At the same time,also gives the film solvent resistance and transparency.When the amount of MOH-S was 30 wt.%,the tensile strength of the composite membrane was the strongest,from 8.5 MPa to 23 MPa.（2）Comparing the thermal decomposition behavior of methylolmelamine and melamine under nitrogen atmosphere,it was found that MOH-S showed a surprising amount of residue,melamine was completely decomposed at 340°C,the residue amount was only 0.6%,and MOH-S at 340°C residue amount was as high as 72.1%,heating to 680°C can also obtain 19.7%residue amount.Therefore,MOH-S was used as a carbonforming agent to form an intumescent flame retardant system with ammonium polyphosphate（APP）to improve the intumescent flame retardant efficiency of EP/APP.The flame retardation and thermal stability of EP/15%（APP+MOH-S）3:1:1 composites were measured by limit oxygen index（LOI）,vertical combustion（UL-94）,Cone calorimetry（Cone）and thermogravimetric analysis（TG）.The results showed that 15 Wt.%APP/MOH-S with a mass ratio of 3:1 made the composite reach the grade UL-94 V-0 and the oxygen index reached 32.5%.In addition,compared with the traditional carbonizing agent pentaerythritol（PER）,MOH-S showed faster carbonizing speed and higher carbon layer expansion.Therefore,EP/15%（APP+MOH-S）3:1 has lower heat release rate（PHRR）,smoke release rate（TSR）and fire spread index（FIGRA）,indicating that MOH-S epoxy resin as a carbonizing agent has better fire safety.In addition,the gas-phase and condensed-phase components of EP/15%（APP+MOH-S）3:1:1 were analyzed by TG-FTIR,FTIR,SEM and EDX,and the flame retardant mechanism was proposed. |
