外文文献翻译：紫外线固化水性丙烯酸酯涂料

外文文献翻译学 院 轻工学院 专 业 印刷工程 导 师 聂义然 学 生 户忠赫 学 号 201110830491 2015年3月20日UV-Radiation Curing of Waterborne Acrylate Coatings AbstractA kinetic study of the ultrafast curing of water-based acrylate resins upon UV irradiation was conducted by means of infrared spectroscopy. Under intense illumination, the crosslinking polymerization was found to occur in the driedfilm within less than one second to generate a totally insoluble polymer. The influence of a number of critical factors on the polymerization rate and cure extent has been investigated, namely the type of photoinitiator, the chemical structure of the acrylate functionalized oligomer, and the sample temperature. Coatings obtained from emulsions undergo a faster and more extensive polymerization than coatings obtained from dispersions, because of a greater molecular mobility in the soft low-modulus polymer formed. Very hard coatings were produced by the UV curing of dispersion-type acrylate resins, especially when the UV irradiation was performed at 80C on the sample emerging from the drying oven. Tg values up to 120C were reached when such resins were UV-cured at ambient temperature as 1-mm thick plates, because of the large amount of heat released during such ultrafast polymerization. UV-cured coatings made of aliphatic polyurethane-acrylates proved to be very resistant to accelerated weathering in the presence of adequate light stabilizers. Light-induced polymerization of multifunctional monomers or oligomers, also called UV-radiation curing, is one of the most efficient methods to synthetize rapidly highly cross linked polymer networks at ambient temperature. Upon intense illumination, a solvent-free acrylic resin can thus be transformed within afraction of a second into a solid polymer, totally insoluble in the organic solvents and very resistant to heat and mechanical treatments. Because of its distinct advantages, this environment-friendly technology has found a large variety of industrial applications, mainly as fast-drying protective coatings, printing inks, adhesives, and composites, as well as in photolithography to produce printing plates, microcircuits, and optical disks.UV-curable resins typically consist of a photo initiator, afunction alized oligomer, and a monomer serving as a reactive diluent to adjust the formulation viscosity. The Photo initiated cross linking-polymerization process can be represented schematically as follows: The multifunctional acrylate monomers commonly used as diluents still have a strong odor and may cause eyeand skin irritation. Moreover, they are enhancing the shrinkage process which yields internal stresses, and they may be responsible for curling and poor adhesion. Waterbased UV-curable systems appear as a promising alternative to overcome these drawbacks, water being used as the only diluent. The formulation viscosity can, thus, be reduced to the precise level required for spray or rolling application, simply by adjusting the water content. Moreover, water-based UV-cured coatings have been shown to combine the flexible properties of high molecular weight polymers with the hardness of cross linked acrylate polymers. The potential of water-based resins and their performance in UV-radiation curing has already been investigated. They proved particularly well suited to be used as screen inks and protective coatings for plastics, paper, and wood. We report here a new study on the high speed UV-curing of some commercial water-based acrylate coatings, by focusing on the influence of the photo initiator and the functionalized oligomer on the polymerization kinetics, namely cure speed and cure extent. The effect of the kind of water-based resin used (dispersion or emulsion) on the visco elastic properties of the UV-cured polymer will also be investigated, as well as the correlation existing between the degree of conversion and the polymer properties, in particular its hardness.EXPERIMENTALMaterialsThe UV-curable waterborne formulations used in this study consisted of aqueous emulsions or dispersions of acrylate functionalized oligomers containing a water soluble or water dispersible radical-type photo initiator. The compatibility of the initiator with the aqueous formulation and with the dried coating is essential to achieve its uniform distribution in the sample. Two types of photo cleavable photo initiators were used in this study:(1) Oil-soluble photoinitiators which are partly soluble in water: Darocur 1173 and Irgacure 2959 from Ciba Specialty Chemicals, and Lucirin TPO-L from BASF;（2） Oil-soluble photoinitiators which were dispersed in water: Irgacure 819 DW from Ciba Specialty Chemicals, and Esacure KIP/EM from Fratelli-LambertiThe chemical formulas of these photo initiators are given in Figure . They consist either of hydroxyl phenyl ketones or of acylphosphine oxides, which generate upon UV-exposure benzoyl radicals and either alkyl radicals or phosphinoyl radicals,.RespectivelyThe waterborne resin was made of a short acrylate endcapped polymer chain containing either a few carboxylate groups to be dispersible in water or an added emulsifier. The characteristics of the five acrylate oligomers tested, all from BASF, and their acrylate content are given below.The water content was 50 wt% for the emulsions and60 wt% for the dispersions.Drying and UV CuringThe formulation containing typically 1 wt% of photoinitiator was cast onto a barium fluoride crystal or a glass plate to obtain, after drying at 80C, a 20-m thickcoating. The sample was cured on a UV line (IST Minicure, 80 W/cm) at a speed ranging between 5 and 60 m/min, at an incident light intensity of 500 mW cm. The UV dose received by the sample at each pass was measured by radiometry (International Light IL-390 radiometer), its value ranging from 42 to 500 mJ cm2, depending on the web speed. The UV exposure was performed either at ambient temperature, or at 80C on the sample emerging from the UV oven. All the experiments were performed in the presence of air.AnalysisUpon UV-radiation curing of the dry film, the acrylate double bond disappeared rapidly, after being attacked by the initiator-free radicals, with formation of a tridimensional polymer network.The polymerization of the acrylate double bonds was followed by infrared spectroscopy through the decrease of the sharp band at 1410 cm1. The acrylate conversion (x) after a given exposure time (t) was determined from the ratio of the IR absorbance before and after UV irradiation: x = 1 (A1410)t/(A1410)0. The hardness of the UV-cured polymer was evaluated by monitoring the damping of the oscillationsof a pendulum placed onto a glass plate coated with a 50-m thick film (Persoz hardness). Persoz values typically range from 50 sec, for soft elastomeric materials,up to 350 sec, for hard and glassy polymers. Viscoelastic characteristics of the UV-cured polymer were determined by dynamic mechanical thermal analysis (elastic modulus and relaxation temperature) on 1 mm thick samples. From the variation of the storage module (E) and of the tensile loss factor (tan ) with the temperature, values of the Young modulus and of the glass transition temperature, Tg, were obtained.RESULTS AND DISCUSSIONDrying of Water-based Acrylate ResinsAs water is being removed during drying of the waterborne coating, the polymer micelles will assemble with each other to form a uniform film by coalescence. By the end of the drying stage, the milky aqueous dispersion has been transformed into a clear coating, which needs to be cured to become chemically and mechanically resistant. The drying step is kinetically controlled by a number of factors, such as the sample temperature, the film thickness, and the atmosphere humidity. The loss of water upon drying was followed either by gravimetry or by IR spectroscopy (OH band at 3500 cm1), and found to give concordant results. Figure 2 shows the water release profiles obtained by the two methods for 50-m thick wet films dried at ambient temperature. A faster drying was systematically observed with the emulsion rather than with the dispersion: after one hour, the residual water content of the dried film was measured to be 2 wt% for the emulsion E-1, compared to 8 wt% for the dispersion D-1. This is much too long for most industrial applications, so the temperature has to be raised to speed up the drying process. A 20-fold increase in the drying rate was achieved by operating at 80C, as shown by the water release profiles reported in Figure 2 for both emulsion- and dispersiontype waterborne formulations. The influence of the temperature on the drying process was quantified by measuring the initial rate of water loss (Table 1). By operating at 80C, it took only one minute to release 95% of the water in the emulsion heated at 80C, and two minutes for the dispersion, compared to 15 minutes and two hours, respectively, at ambient temperature. In further experiments, the waterbased coatings were dried at 80C for five minutes or at ambient temperature for a few hours, so as to contain less than 2 wt% remaining water.Influence of the Photoinitiator on the UV CuringThe photoinitiators (PI) selected are partly soluble in water or consist of an aqueous dispersion; thus, they were added directly to the water-based resin before drying and UV curing. Figure 3 shows the influence of the photoinitiator (1 wt%) on the polymerization profiles of the formulation E-1 exposed to intense UV radiation (500 mW cm2) at ambient temperature. The following PI ranking was obtained:Darocur 1173 Esacure KIP Irgacure 819 DW Lucirin TPO-L Irgacure 2959Similar results were obtained with the Laromer dispersions,but the polymerization proceeded less extensively(40% conversion) because of mobility restrictions in the dry film. A faster and more complete curing was achieved by performing the UV exposure at 80C. Table 2 reports the conversion values reached after one pass at a speed of 5 m/min (0.43 J cm2) for the 25 formulations UV-cured at 80C. For the aromatic dispersions (D-2 and D-3), the somewhat better performance of the acylphosphine oxides can be explained by the strong absorbance of these resins in the 250-300 nm wavelength range. The resulting radiation filter effect will be less pronounced with these photoinitiators where the absorbance extends up to 400 nm rather than with the hydroxyl phenyl ket ones which absorb precisely in the 250-300 nm wavelength region. The UV-cured dispersions, which give very hard polymers, contain a certain amount of residual acrylate double bonds (15% for Irgacure 2959 in sample D-1), a quantity which can be somewhat reduced upon further UV exposure. Complete polymerization was achieved for the coating E-1, whatever the photoinitiator, because the Tg of the fully cured polymer is well below 80C. The acylphosphine oxide photoinitiators undergo a fast photolysis upon UV exposure, as shown in Figure 4 for Irgacure 819 DW, which is essentially gone at a UV dose of 0.4 J cm2. The slower PI loss observed in the two aromatic dispersions was attributed to a stronger radiation filter effect of the resin (UV absorbance of 2.5 below 300 nm). It is worth mentioning that a fast curing of coating E-1 was achieved even by lowering the Irgacure 819 DW concentration down to 0.1 wt%, an acrylate conversion of 80% being already reached for a UV-dose of 50 mJ cm2 (Figure 5). The levelling off observed upon further exposure is due to a complete consumption of the photoinitiator at that stage. When UV-cured coatings are used for outdoor applications, their weathering resistance needs to be increased by the addition of light stabilizers. While HALS radical scavengers were shown to have no detrimental effect on the curing process(nitroxyl radicals are not formed in the O2-depleted sample undergoing polymerization), UV absorbers do slow down the photopolymerization by competingwith the photoinitiator for the capture of the incident photons. Such radiation filter effect is less pronounced for acylphosphine oxide PIs than for hydroxyl phenylketone PIs, as shown in Table 3 which reports the conversion values reached after UV curing at 80C for unstabilized and stabilized coatings (1 wt% Tinuvin 292 + 2 wt% Tinuvin 400). The final conversion of the UV-cured coatings was hardly affected by the presence of the UV absorber when Irgacure 819 DW was used as photoinitiator. Stabilized UV-cured water-based coatings that had an aliphatic polyurethane backbone were found to exhibit an outstanding resistance to accelerated weathering, and they are therefore particularly well suited to protect organic materials against sunlight during outdoor exposure(see below).Influence of the Resin on UV CuringThe chemical structure of the acrylate functionalized oligomer, as well as the type of water-based system (emulsion or dispersion), will affect both the polymerization kinetics and the properties of the UV-cured polymer. The molecular mobility in the dried film and the glass transition temperature of the cross linked polymer will determine the polymerization rate and the final cure extent, respectively.Figure 6 shows the polymerization profiles of the five resins selected upon UV exposure at 80C in the presence of 1 wt% Irgacure 2959. The fastest and most complete polymerization occurs for the soft emulsion based coatings (E-1 and E-2), while 50% conversion was hardly reached with the aromatic dispersion-based coatings (D-2 and D-3). The temperature was found to have a very pronounced effect on the UV curing of the aliphatic polyurethane dispersion (Laromer LR-8949), the final conversion passing from 25 to 82% when the sample temperature was raised from ambient to 80C. The same trend was observed by using Irgacure 819 DW as photoinitiator and by monitoring the polymerization in real time by infrared (RTIR) spectroscopy (Figure 7). Table 4 reports the values of the resin reactivity (maximum slope of the polymerization curves recorded) and the conversion reached after a five-second UV exposure at 25, 50, or 80C. The more complete polymerization achieved in online UV curing was attributed to a greater increase in the sample temperature due to the faster exothermal reaction. It is possible to speed up the polymerization of the dispersion-based coatings without raising the temperature, either by performing the UV exposure in a 100% humid atmosphere (plasticizing effect of the water absorbed) or by adding an acrylate monomer (15 wt% tripropy leneglycol diacrylate), which acts as a reactive plasticizer. Figure 8 shows such effects in the case of the sample D-1 which was UV-cured online at ambient temperature. Performing the UV curing in an inert atmosphere to suppress O2 inhibition provides only a marginal improvement because of the slow diffusion of oxygen in a solid film, as shown in previous studies.Properties of UV-Cured Waterborne CoatingsThe waterborne coatings examined in this study were found to become completely insoluble in the organic solvents after UV exposure (0.5 J cm2), as expected from the high cross link density of the tridimensional polymer network formed. The dispersion-type resins, which yield glassy polymer materials upon UV curing, proved to be more resistant to staining than the emulsion-type resins, which give more elastomeric materials. However, they exhibit a more pronounced hydrophilic character, due to the presence of the carboxylic acid group (or carboxylate anion) required to achieve an homogeneous dispersion of the resin in water before UV curing. The contact angle of a droplet of water (w) was in the range of 2647 for the UV-cured coatings D-3, D-2, and D-1, compared to values of 60 and 74 for the coatings E-2 and E-1, respectively (Table 5). From the value of w and that of the contact angle of tricresyl phosphate (nonpolar solvent), we have calculated the dispersion and polar component of the surface energy (D and P) by using Youngs equation.Figure 9 shows, as histograms, the values obtained for the five UV-cured coatings. It clearly appears that the polymers obtained from emulsion-type formulations are less hydrophilic (low p) than those obtained from dispersion-type formulations. This result is in full agreement with our previous results on UV-cured waterborne acrylic coatings, where the polar component of the surface energywas found to increase regularly with the acid content of the functionalized oligomer Because of their hydrophilic character, the dispersiontype UV-cured coatings will pick up water when they are placed in a humid environment. This process can be followed quantitatively through the increase of the infrared band at 3500 cm1 assigned to OH groups in UV-cured polymers placed for 90 min in a 100% humid atmosphere. From these values, the actual amount of water absorbed by the coating can be calculated. It was found to reach values up to 10 wt% for the sample with the highest acid content. As expected, the absorption of moisture caused a substantial softening of the water-based UV-cured coatings. The value of the Persoz hardness decreased as increased amounts of water were absorbed, as shown in Figure 10. Fortunately, this water uptake is completely reversible, the water being rapidly removed when the sample was placed in a dry atmosphere, so that it recovered itsoriginal hardness. When the acrylate double bonds underwent polymerization upon UV exposure, the hardness of the coating increased steadily, with the formation of a chemically resistant material. For the emulsion-type samples, the tacky film obtained after drying was transformed within less than one second into a low-modulus soft polymer. In this respect, it should be mentioned that Laromer PE 55 W was successfully u