Preparation And Properties Of Low Water Absorption And Insulated Rigid Melamine Foam

Rigid melamine foam is a kind of intrinsic flame-retardant foam material synthesized by melamine and formaldehyde in alkaline aqueous solution and then prepared by foaming process.Since there are still unreacted hydrophilic groups（-NH-,-OH）in the melamine foam matrix,it is easy to form hydrogen bonds with water molecules in the air,which can increase the hydrophilicity of the foam matrix.In addition,the closed cell content of melamine foam is low,and the open-cell structure of foam cannot effectively hinder the penetration of water into the cells.The thermal conductivity of the thermal insulation material will increase under the influence of long-term moisture penetration.With the effect of the freeze-thaw cycle（freezing at low temperature,melting at room temperature）,the moisture stored in the foam makes the foam material deform and break,and the bonding force between the plate and the steel becomes worse,which affects the mechanical properties and service life of the foam.In this paper,by improving the hydrophobicity of the melamine resin matrix and increasing the closed cell content of the foam,a low water absorption and thermal insulation rigid melamine foam is prepared,which is expected to be used as a refrigerated thermal insulation material.The main research contents are as follows:（1）In the synthesis stage of melamine resin prepolymer,glutaraldehyde is used as a small-molecule hydrophobic modifier to"insert""-CH₂CH₂CH₂CH₂CH₂-"hydrophobic flexible alkyl chain into the polymer molecular chain to obtain modified melamine resin prepolymer with improved hydrophobicity,which was further foamed to prepare glutaraldehyde modified rigid melamine foam.（2）In the synthesis stage of the melamine resin prepolymer,n-butanol is used as a small molecule hydrophobic modifier,and the"-CH₂CH₂CH₂CH₃"hydrophobic flexible alkyl chain is introduced to the side end of the polymer molecular chain.N-butanol participates in etherification and caps part of the active groups（-CH₂OH）,which is helpful to increase the storage stability of the resin,reduce the content of hydrophilic groups of-OH,improve the hydrophobicity of the resin,and obtain the modified melamine resin prepolymer,which was further foamed to prepare n-butanol modified rigid melamine foam.（3）The hydrophobicity and storage stability of the above two types of modified melamine resin prepolymers and the water absorption,apparent density,cell morphology,and compressive strength of the modified melamine foam obtained were studied.The results showed that the storage stability of n-butanol modified melamine prepolymer was better,and the comprehensive properties of the two types of modified melamine foams were similar,but the water absorption of n-butanol modified melamine foam was lower.（4）Composite modified rigid melamine foams were prepared by adding four types of solid fillers of nano-silica,micro-silica,Na⁺-MMT,and Aerosil R974 in n-butanol modified melamine resin prepolymer.First,the morphology of the foams prepared above were analyzed,and it was determined that the cell size of the composite modified rigid melamine foam prepared by nano and micro silica was more uniform.Then,the effects of different contents of nano and micro silica on foam morphology,water absorption and closed cell content were mainly studied.Finally,a composite modified rigid melamine foam with significantly reduced water absorption and increased closed cell content was obtained.The best preparation process and formula of the modified rigid melamine foam is:the viscosity of the n-butanol modified prepolymer ranges from 30 Pa·s to 50 Pa·s,2wt%foam stabilizer of silicone oil based on the mass of the prepolymer,1wt%emulsifier of dodecylbenzene sulfonic acid,5wt%foaming agent of n-pentane,0.7wt%curing agent of hydrochloric acid,1wt%nano silica based on the mass of melamine,and foaming at 75℃in the oven for 40 min,then curing at 120℃for 6 h.The best comprehensive properties of the obtained composite modified rigid melamine foam are:apparent density of 53kg/m³,compressive strength of 252.2k Pa,water absorption of 11.26%,thermal conductivity of 0.039W/（m·K）,average cell the diameter is 160μm and the limiting oxygen index is 32.5%.