Experimental And Kinetic Modeling Study On The Polyaddition And Interfacial Hydrolysis Of Isocyanate In Miniemulsion Dispersion

Polyurethane(PU) is a most versatile polymer material,and there has been a strong preference for waterborne PUs for the environmental concern.Waterborne PUs are commonly prepared by the incorporation of ionic groups into the polymer structure to be dispersed well in the water phase,i.e.self-emulsification process.But the hydrophilic segments of waterborne PUs adversely affect water and oil repellency of coated films.The synthesize PU in the water phase directly.Since isocyanate is sensible to water,some attention should be paid to the isocyanate-water reaction in miniemulsion polymerization.In this paper,waterborne PUs were synthesized by in-situ miniemulsion polymerization of diisocyanates with hydrophobic diols to make up the flow of conventional method,and the competition of isocyanate-hydroxyl reaction with the isocyanate-water reaction is the focus.A kinetic model was developed considering the diffusion through the formed polymer,interfacial hydrolysis(side reaction) and inner polyaddition(main reaction) to analyze the influence of experimental parameters on the concentration ratio of urethane groups to urea groups of the resulted polymer.The hydrophobe in miniemulsion suppresses the mass exchange between the different oil droplets by osmotic force,so that the polymerization is initiated in each small stabilized droplet making the water-sensible monomers' polymerization possible theoretically.Isocyanates hydrolysis in monomer miniemulsion has been studied experimentally and theoretically.A kinetic model has been developed considering the diffusion through the formed polymer and interfacial hydrolysis in monomer miniemulsion.Both experimental results and reported data from literatures fit the kinetic model well.It has been found that the hydrolysis of aromatic isocyanate with high reactivity is a fast reaction,while the hydrolysis of aliphatic isocyanate with moderate reactivity is a reaction-controlled process.The hydrolysis of aliphatic isocyanate at different temperatures was found to be a reaction-controlled process.A correlation between IR spectra and the urea fraction of waterborne PUs was proposed to investigate the hydrolysis during polymerization.The concentration ratio of urethane groups to urea groups indicate the competition of polyaddition and hydrolysis during miniemulsion polymerization The individual bands in 1600～1800cm^-1 were resolved by employing Fourier self-deconvolution and Gaussian curve-fitting techniques,and the concentration ratios of urethane groups to urea groups were determined from ¹H NMR.A good correlation was evidenced between IR and ¹H NMR measurements.Isophorone diisocyanate(IPDI) was chosen as the isocyanate monomer.The competition of polyaddition with hydrolysis was investigated experimentally and theoretically.The polyaddition of IPDI with hydrophobic diol has been performed in miniemulsion to obtain waterborne PUs.FTIR has characterized the concentration ratios of urethane groups to urea ones.It has been found that the high reactivity of diol and the high hydrophilicity of vinyl monomer could favor the polyaddition.It was also shown that the hydrolysis could be restrained by decreasing temperature and the amount of catalyst.Based on the experimental data,a kinetic model was developed considering the diffusion through the formed polymer,interfacial hydrolysis and inner polyaddition in the droplet.The model describes the monomer concentration,polymer membrane thickness and the concentration ratios of urethane groups to urea ones at different reaction times.The experimental results fit the kinetic model well,and theoretical model predicts the effects of the reactant and several experimental factors,such as temperature,catalyst and solvent on the competition of the main reaction with the side reaction.There are some agreements in the prediction with the experimental results. Therefore,this kinetic model can be used to analyze the competition of polyaddition and hydrolysis during the preparation of waterborne PUs by miniemulsion polymerization.