| Melamine formaldehyde(MF)foam is a kind of intrinsic fire-retardant material with sound absorption,heat resistance and thermal insulation properties.It has broad application prospects in the fields of construction,transportation and aviation.However,MF foam has problems of high brittleness and low strength,and it is easy to release free formaldehyde during use,which greatly limits its practical application.MF foam is prepared by microwave foaming of MF resin solution.Thus,the properties of MF foam are determined by molecular structure of MF resin and cell structure of the foam.This thesis has carried out the research from modification of resin matrix and control of cell structure.Firstly,the reaction characteristics of melamine and formaldehyde were systematically studied.The modification effect and mechanism of formaldehyde scavengers were investigated and the reaction process was optimized to prepare MF resin with low formaldehyde and high storage stability.Then the effect of urea on the curing kinetics of MF resin was investigated.And the process parameters of microwave foaming were further optimized to prepare lightweight and highly elastic MF foam.The specific results are as follows:(1)The preparation process of MF resin is mainly divided into two stages:one is the methylolation reaction between melamine and formaldehyde in the initial stage of the reaction,the other is the polycondensation reaction between methylol melamine in the later stages of the reaction.When the degree of polycondensation reached a certain equilibrium at 3 h,the introduction of 10%urea could reduce the free formaldehyde content of MF resin solution to 0.65%and extend the storage time to 60 days.Compared with thiourea and caprolactam,the addition of urea not only consumes unreacted free formaldehyde but also produces a more stable structure(-CH2-NH-)than the methyl ether bond,which reduces the possibility of formaldehyde emission.At the same time,it also reduces the number of reactive functional groups(hydroxymethyl groups)and improves the storage stability of the MF resin solution.(2)The curing process of MF resin mainly includes the condensation and crosslinking between imino and methylol groups.The self-crosslinking reaction rate between methylol groups is faster at a low temperature,but the activity of imino groups is higher than that of methylol groups at a high temperature which causes an increase in the rate of polycondensation between the imino and methylol groups.The modification of urea can increase the content of imino groups and decrease the number of methylol groups of MF resin.As a result,the curing rate is slow below 50? and the curing reaction is easier to occur above 50?.Besides,the activation energy of modified resin decreases with the increase of the conversion rate,which is consistent with the characteristics of the autocatalytic reaction.(3)A lightweight and high elastic MF foam is obtained by microwave foaming method with the MF resin solution.In the process of microwave foaming,the process parameters such as resin concentration,microwave temperature and time,foaming auxiliary agents and so on were adjusted to balance foaming and solidification.The modification of urea on MF resin not only reduces the formaldehyde content of the foam to 0.08 mg/g,but also increases the carbon chain length between the triazine rings and improves the compressibility of the foam.The addition of 15%urea can increase the compression strength of MF foam by 126%and the elastic recovery rate by 47%.During the microwave foaming process,the addition of methanol could further increase the elastic recovery of MF foam to 80%. |
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