Preparation, Structure And Properties Of UV-Curable Waterborne Hyperbranched Polyurethane Acrylate Dispersions

The UV curable waterborne coating has both the advantages of waterborne coating andUV curable coating, such as environmental, wide applicability, good performance of film, etc.It becomes an inevitable trend of coating. At present, waterborne polyurethane-acrylate(WPUA) coating has attracted more and more attentions due to their good comprehensiveperformance. However, the shortcomings of common linear waterborne polyurethane-acrylate(LWPUA), including relatively high viscosity, low solid content and crosslinking density, aswell as insufficient hardness and thermal properties, limit its future applications.Hyperbranched polymer (HBP) has many attractive features, such as low viscosity, highsolubility and reactivity, good compatibility, owing to its possesses a kind of uniquespherical-multibranched structure. Therefore, HBP can be applied to prepare UV-curablewaterborne polyurethane-acrylate coatings in order to relieve the contradiction between thesolid content and viscosity of LWPUA. However, there are few references about HBP used asprepolymer in the system of UV-curable WPUA at present. In this work, a series ofUV-curable waterborne hyperbranched polyurethane acrylate dispersions (WHBPUAD) wereprepared by endcapping of the hyperbranched ester with different number of hydrophilic andacrylic groups by the special design of the polymer structure. The dispersion has good storagestability, higher solid content, relatively lower viscosity, and the UV cured film had excellentcomprehensive performance such as higher hardness, good flexibility and thermal stability.The main research contents and results are listed as following:Firstly, a series of UV-curable waterborne hyperbranched polyurethane acrylatedispersions were prepared by a three-step procedure based on isophorone diisocyanate (IPDI),hyperbranched polyester (HBP), maleic anhydride (MA) and2-hydroxyethyl acrylate (HEA).The structure of WHBPUADs was characterized by Fourier transform infrared spectroscopy(FTIR),1H nuclear magnetic resonance spectroscopy (1H-NMR), differential scanningcalorimeter (DSC) and gel permeation chromatography (GPC). The effects of the types ofcatalysts and their concentrations, reaction temperature and time on the conversion werestudied by FTIR and chemical titration. The results showed that the target production wasobtained successfully and the optimal condition was that the isocyanate adduct of IPDI/HEA(Prepolymer I) was prepared with0.5wt.%DBTDL as catalyst at5℃for150min；Prepolymer II was prepared by mono-esterification reaction from HPB and MA with4wt.%DMAP as catalyst at50℃for300min; and then the Prepolymer II reacted with Prepolymer Iat70℃for300min to prepare WHBPUA. The glass transition temperatures (Tg) of WHBPUAD with different raw materials’ proportion were about30℃, which were close witheach other. The trend of relative molecular mass was agree with that of the theoretical valuebut all were lower in quantity, and the molecular weight distribution (MWD) became broadwith the hard segment content increase.Secondly, laser particle size analyzer and rotational rheometer were used to investigatethe stability, the dispersity and the steady rheological behaviors of WHBPUADs. The effectsof many factors including solid content, concentration of hydrophilic groups, and degree ofneutralization on the dispersity and rheological behaviors were studied. The flow curves ofWHBPUADs were fitted mathematically by using Cross model, Carreau model,Krieger-Dougherty (K-D) equation and so on. The maximum volume fraction (m) and themaster curve of the different solid content were obtained. It was found that WHBPUAD wasquite stable which could store for at least1year in the dark. Meanwhile, the system havegood dispersibility. Most of the particle diameters were less than100nm, and increased withthe concentration of hydrophilic groups and degree of neutralization. The WHBPUAD is anon-Newton pseudoplastic fluid with shear thinning characteristic. The shear thinningphenomenon becomes more obvious with the increase in solid content, concentration ofhydrophilic groups, and degree of neutralization. The viscosity of WHBPUAD was lower thanthat of the linear waterborne polyurethane acrylate (LWPUA). At the same shear rate, theviscosity of WHBPUAD increased with increasing solid content and degree of neutralizationbut decreased with increasing concentration of hydrophilic groups. The value of mwas0.71which was obtained by the K-D equation. The Cross model, Carreau model and Power modelequation all could be used to fit the rheological behavior of WHBPUAD, but the later was lessaccurate.Thirdly, the photopolymerization kinetics of WHBPUAD was monitored by FTIR. Thedrying process and the influences of molecular structure on the photopolymerization kineticsand properties of WHBPUAD films were discussed. The results showed that most water ofthe wet film could be remove within2minutes at the temperature of80℃and theconcentration of hydrophilic groups could significantly prolong the drying time. Based on theproperties of the selected photoinitiator, the suitable drying condition was at50℃for2to10minutes under a vacuum. The results of FTIR showed that the double bond conversion (τ) andphotopolymerization rate (Rp) were affected by the concentration of double bond andviscosity of WHBPUADs. The UV-curable systems with higher double bond concentrationand lower viscosity led to higher τ and Rp. The maximum τ and Rpcould reach to93%and71 mmol·g-1s-1, respectively. UV-cured films were found to exhibit superior mechanicalproperties. It shows that all of the coating formulations passed the tests of adhesion with0grade, impact with50kg.cm, and flexibility tests with1mm mandrel, respectively. Thependulum harnesses were improved with the increase of double bond concentration.WHBPUAD12-4has the best mechanical properties with the maximum pendulum harnessesof0.70%.Fourth, the effect of different ingredient on the thermal stability of WHBPUAD filmswas studied by TG, and the kinetic parameters and dynamical equations of the thermaldecomposition of WHBPUAD were obtained. Then, the pyrolytic process of WHBPUAD wasstudied by TGA-FTIR and Py-GC/MS. The possible pyrolysis mechanism was studied. Theresults indicated that the WHBPUAD films had good thermal stability without obvious weightloss peaks before170℃. All the WHBPUAD films with different ingredient had similarprocesses of thermal decomposition, which appeared four obvious weight loss regions. Theresults of thermal decomposition activation energy which obtained by the Horowitz-Metzgerequation could well reflect the relationship between the intrinsic thermal stability and themolecular structure. The sample with higher proportion of soft segment in the structure wouldhave higher thermal decomposition activation energy and thermal stability. The possiblepyrolysis mechanism was as following: in the first step of degradation, the peripheral esterbonds of the WHBPUAD molecule were broken and the mainly evolved gases were CO2, MAand TEA. In the second step, its degradation was resulted from the hard segment and thedegradation products involved CO2and HEA; In the third step, the degradation was related tothe soft segment and the mainly degradation products were CO2, HEA, IPDI, unsaturatedalcohol, unsaturated acid, ester and so on; Finally, for the fourth degradation stage, a muchmore complex mixture of products was identified, including heterocyclic, olefinic aldehyde,enol, olefin, acrylic, acrylate and their derivatives, most probably originating from the core ofB-H20.