Study On Preparation And Crystallization Behavior Of High-solid Content Waterborne Polyurethane

In this paper, several series of high-solid content waterborne polyurethanes (WPUs)were successfully synthesized by using polyethylene glycol adipate diol (PBA),1,4-butanediol(BDO), N-(2-aminoethyl)-amino ethane sulphonated sodium (AAS),hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) as the main rawmaterials, and the crystallization behavior of WPUs were studied in detail.The high-solid content waterborne polyurethanes were prepared by different synthesistechniques. Firstly, WPU emulsions with50%solid content were synthesized on the basis ofacetone process and pre-polymer mixing process. The influence of acetone and R value(NCO/OH value) on emulsification process and stability of WPU emulsions were studied. Theresults showed that the proper dosage of acetone was1.25times of resin (mass) when R wasconstant, and the R value should be less than2.5when dosage of acetone was constant inorder to acquire stable emulsions. Secondly, WPU emulsions with high-solid content (greaterthan50%) were synthesized by internal-external emulsification process, and the influences ofemulsifiers on emulsification and solid content of emulsions were studied. The resultsindicated for the internal emulsification process, the viscosity of WPU emulsion was362mPa s, and the phase-inversion time was34s, but for the internal-external emulsificationprocess, when the different emulsifiers of OP-10, SDS and LAS were introduced, thephase-inversion time could reduce to21s,29s and31s, respectively, and the correspondingviscosity of WPU emulsions decreased to117mPa s,255mPa s and267mPa s. Finally,WPU emulsions with high solid content (greater than50%) and low viscosities (less than200mPa s) were prepared on the basis of batch emulsification by using different dihydric alcohols(such as PBA2000, PBA300, PCDL2000and PCDL3000) and chain extender (such and BDO,BDAand EDA).The crystallinity evolution of soft segments during the synthesis of polyester-based WPUwas studied by DSC、XRD、POM、SEM and TGA. The results showed that crystallinity of softsegments decreased from100%to56.77%after pre-polymerization, increased to71.25%afterhydrophilic chain-extending reaction, then decreased to56.98%after emulsification, andfinally increased to58.74%after the secondary chain-extending reaction. Moreover, the crystalline dimensions of the soft segment decreased from30μm to1μm during thesynthesis process of WPU. The fracture mechanisms of the intermediate films variedgradually from brittle failure to ductile fracture. The variation tendency of thermostabilities ofthe intermediates obtained in each step of the synthesis process was in accordance with thevariation tendency of the crystallinity of the soft segment.The crystallinity and thermostability of WPU films synthesized by different molecularweight of soft segment, different kinds and contents of hard segment and different hydrophilicchain extender were studied by DSC and TGA. The results showed the WPU withhigh-molecular weight PBA3000as the soft segment, regular structure HDI as the hardsegment, AAS as hydrophilic chain extender and paraffin as the accelerant had bettercrystallinity and thermostability. The structure and crystallinity of WPU containing carboxylicand sulfonic groups were characterized by DSC, XRD and AFM. The results showed that withthe increase of AAS/DMPA molar ratio, the crystallinity of WPU film increased, the visiblelight transmittance reduced between300~500nm. Moreover, the heat resistance of WPU filmwas improved with the increase of crystallinity.The crystallization behaviors of WPU films under isothermal and non-isothermalconditions were characterized by DSC, and their crystallization kinetics were investigated byAvrami equation and Mo Zhishen equation. The results showed that the non-isothermalcrystallization kinetic parameters F(T) increased from2.34K min1.34to7.59K min0.36，withthe increase of relative crystallinity from0.1to0.7, which suggested that enhancement ofactivation temperature could improve the crystallization rate and the initial bonding strengthof WPU adhesive. The appropriate activated temperature was70℃considering performancerequirement of WPU adhesives and energy consumption. The isothermal crystallizationkinetics results of WPU films indicated the correspondence could be found betweenisothermal crystallization kinetic parameters t1/2of WPU films and the open time of WPUadhesive. The larger the t1/2of WPU films was, the longer the open time of WPU adhesivewould have.