Preparation Of A Novel PUR Hot Melt Adhesive And Its Mechanism Of Moisture Curing

Polyester/polyether glycols and diisocyanates are used as the main raw materials to prepare moisture-curing polyurethane(PUR)hot-melt adhesive by bulk polymerization.The effects of reaction temperature,reaction time,raw material ratio and other factors on the properties of PUR hot melt adhesive are investigated.The optimum synthesis conditions for PUR hot melt adhesive are obtained:the reaction temperature,pressure and time are 358K,50Pa and 2h respectively,the molar ratio of glycol PBA/PPG was 6:4,and the isocyanate is 4,4'-Methylene diphenyl diisocyanate.The molar ratio of isocyanate group to hydroxyl group is 2,the tackifier is acrylate resin,the amount of catalyst,chain extender are 0.8wt%,1.0 wt%,respectively.In order to improve the initial viscosity of PUR hot melt adhesive,pentaerythritol diacrylate PEDA is synthesized by p-toluene-sulfonic acid(PTSA)catalyzed direct esterification of pentaerythritol(PER)with acrylic acid(AA).The quantitative and qualitative analysis of the obtained product is carried out by HPLC,FT-IR and 1H NMR.The mechanism and kinetic of esterification of PEDA are studied.A kinetic model is developed to describe the variation of the concentration of each component in the esterification system with time.The kinetic parameters are determined by Global Optimization Method-Genetic Algorithm procedure with an error minimization algorithm.Supported catalyst(PTSA-Silica)is prepared by loading PTSA on silica gel,and PEDA is synthesized by direct esterification using PTSA-Silica as a catalyst.A series of PUR hot melt adhesive(a=0?10wt%)with different PEDA content are prepared with PEDA as modifier.The effect of PEDA on the rheological properties,thermal stability and mechanical properties of PUR are investigated by dynamic mechanical analysis,TGA and mechanical tests.The results show that as the a value increases,the PUR hot melt adhesive maintains the interval of Newtonian fluid characteristics,and its viscosity ?*,storage shear modulus G',melt viscosity ? and tensile strength all show an increasing trend,but the elongation at break slightly decreased.The results show that the PUR hot melt adhesive has the best performance such as open time,adhesive strength and melt viscosity when the amount of PEDA is 7wt%.However,when the a value is larger than 7wt%,the resulting product(P10)could not be used as PUR,because its set time is too short for proper application.The reactive polyurethane hot melt adhesive(HMPUR)is synthesized by bulk polymerization using two kinds of polyester diol,a polyether diol,MDI and PEDA as the main raw materials.The HMPUR sample is pyrolyzed at three characteristic temperatures of 553 K,723 K and 873K according to the results of thermogravimetric analysis(TG).On this basis,pyrolysis-gas chromatography/mass spectrometry(Py-GC/MS)is used to study the thermal degradation behavior of PUR hot melt adhesive,and the qualitative and quantitative data of the pyrolysis products at the above three characteristic temperatures are obtained.The mechanism of cleavage reaction is proposed.More than 20 characteristic volatile pyrolysis products are identified by on-line mass spectrometry.The rationality of the above-mentioned HMPUR pyrolysis reaction mechanism is verified by analyzing the pyrolysis products.The moisture curing behavior of PUR hot melt adhesive is studied to provide theoretical guidance for industrial operations.The moisture-curing kinetic experiments for the film of HMPUR with 2.0 mm thickness are carried out at different temperatures(283,293,303 and 313K)and 85%relative humidity by monitoring the mass changes of the film to investigate the moisture curing process of HMPUR.A diffusion-reaction assumption is proposed to describe the mechanism of moisture-curing process and three kinetic models are established to correlate the above moisture-curing kinetic data:(i)rapid reaction;(ii)zero-order reaction;(iii)first-order reaction.On this basis,three non-steady-state moisture curing kinetic models are established and the above experimental data are correlated.The results show that the first-order reaction kinetic model has the smallest average relative deviation and can be used to describe the reaction mechanism and kinetic of the moisture curing behavior.Based on this model,the water concentration distribution in the HMPUR film at different time is predicted and the depth of water diffusion(x*)at ? is defined.The depth of water diffusion corresponding to five specific moisture cure time(102s,103s,104s,105s,and 106s)is calculated.The results show that when the moisture curing time is greater than 105s,the x*value is almost constant(1.082?1.606mm),which indicates that the hypothesis about the non-steady-state kinetic model is reasonable.According to the Arrhenius equation,the activation energy(Ek)and diffusion activation energy(ED)are 9.52kJ/mol and 28.45kJ/mol,respectively,indicating that the influence of temperature on the diffusion process is greater than that of the reaction process.