UV-radiation curing of waterborne acrylate coatings.
A kinetic study of the ultrafast curing of water-based acrylate Noun 1. acrylate - a salt or ester of propenoic acid
salt - a compound formed by replacing hydrogen in an acid by a metal (or a radical that acts like a metal) resins upon UV irradiation irradiation /ir·ra·di·a·tion/ (i-ra?de-a´shun)
2. the dispersion of nervous impulse beyond the normal path of conduction.
3. was conducted by means of infrared spectroscopy spectroscopy
Branch of analysis devoted to identifying elements and compounds and elucidating atomic and molecular structure by measuring the radiant energy absorbed or emitted by a substance at characteristic wavelengths of the electromagnetic spectrum (including gamma ray, . Under intense illumination, the crosslinking polymerization polymerization
Any process in which monomers combine chemically to produce a polymer. The monomer molecules—which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. was found to occur in the dried film within less than one second to generate a totally insoluble insoluble /in·sol·u·ble/ (in-sol´u-b'l) not susceptible of being dissolved.
Not soluble. 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 oligomer /ol·i·go·mer/ (ol´i-go-mer) a polymer formed by the combination of relatively few monomers.
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 80[degrees]C on the sample emerging from the drying oven. [T.sub.g] values up to 120[degrees]C were reached when such resins were UV-cured at ambient temperature Outside temperature at any given altitude, preferably expressed in degrees centigrade. as 1-mm thick plates, because of the large amount of heat released during such ultrafast polymerization. UV-cured coatings made of aliphatic aliphatic /al·i·phat·ic/ (al?i-fat´ik) pertaining to any member of one of the two major groups of organic compounds, those with a straight or branched chain structure.
adj. polyurethane-acrylates proved to be very resistant to accelerated weathering in the presence of adequate light stabilizers.
Keywords: Photoinitators, FTIR FTIR Fourier Transform Infrared (spectroscopy)
FTIR Frustrated Total Internal Reflection
FTIR Fourier Transfer Ir , ATR ATR Achilles tendon reflex, see Ankle reflex , polymerization, acrylics, polyurethanes polyurethanes (pŏl'ēyr`əthānz), group of plastics that may be either thermosetting or thermoplastic. Polyurethane can be made into both flexible and rigid foams. , UV, EB, radiation cure, hardness, scratch resistance, water-based, weatherability, reaction kinetics kinetics: see dynamics.
Kinetics (classical mechanics)
That part of classical mechanics which deals with the relation between the motions of material bodies and the forces acting upon them.
Light-induced polymerization of multifunctional monomers or oligomers, also called UV-radiation curing, is one of the most efficient methods to synthetize Syn´the`tize
v. t. 1. To combine; to unite in regular structure. rapidly highly crosslinked polymer networks at ambient temperature. (1-7) Upon intense illumination, a solvent-free acrylic resin can thus be transformed within a fraction 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 A lithographic technique used to transfer the design of circuit paths onto printed circuit boards as well as the circuit paths and electronic elements of a chip onto a wafer's surface.
A photomask is created with the design for each layer of the board or wafer (chip). to produce printing plates, microcircuits, and optical disks. (5,7,8)
UV-curable resins typically consist of a photoinitiator, a functionalized oligomer, and a monomer monomer (mŏn`əmər): see polymer.
Molecule of any of a class of mostly organic compounds that can react with other molecules of the same or other compounds to form very large molecules (polymers). serving as a reactive diluent diluent /dil·u·ent/ (dil´oo-int)
1. causing dilution.
2. an agent that dilutes or renders less potent or irritant.
Serving to dilute.
n. to adjust the formulation viscosity. (1) The photoinitiated crosslinking-polymerization process can be represented schematically as follows:
The multifunctional acrylate monomers commonly used as diluents still have a strong odor and may cause eye and skin irritation skin irritation,
n reaction to a particular irritant that results in inflammation of the skin and itchiness. . Moreover, they are enhancing the shrinkage Shrinkage
The amount by which inventory on hand is shorter than the amount of inventory recorded.
The missing inventory could be due to theft, damage, or book keeping errors. process which yields internal stresses, and they may be responsible for curling and poor adhesion. Water-based 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 crosslinked acrylate polymers A group of polymers which could be referred to as plastics generally. They are noted for their transparency and resistance to breakage when compared to conventional window glass. Commonly called as acrylics or polyacrylates, acrylate polymers. . (9) The potential of water-based resins and their performance in UV-radiation curing has already been investigated. (9-16) 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 photoinitiator 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 emulsion: see colloid.
Mixture of two or more liquids in which one is dispersed in the other as microscopic or ultramicroscopic droplets (see colloid). Emulsions are stabilized by agents (emulsifiers) that (e.g. ) on the viscoelastic Adj. 1. viscoelastic - having viscous as well as elastic properties
natural philosophy, physics - the science of matter and energy and their interactions; "his favorite subject was physics" 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.
The UV-curable waterborne formulations used in this study consisted of aqueous aqueous /aque·ous/ (a´kwe-us)
1. watery; prepared with water.
2. see under humor.
adj. emulsions or dispersions of acrylate functionalized oligomers containing a water soluble or water dispersible radical-type photoinitiator. 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 photocleavable photoinitiators were used in this study:
(1) Oil-soluble photoinitiators which are partly soluble in water: Darocur 1173 and Irgacure 2959 from Ciba Specialty Chemicals “Ciba” redirects here. For the pre-1971 company, see Novartis.
Ciba Specialty Chemicals is a chemical company based in and near Basel, Switzerland. It was formed as the non-pharmaceuticals elements of Novartis were spun out in 1997, following the merger in the , and Lucirin TPO-L from BASF BASF Bar Association of San Francisco (since 1872; San Francisco, California)
BASF Badische Anilin und Soda Fabrik (German chemical products company)
BASF Builders Association of South Florida ;
(2) Oil-soluble photoinitiators which were dispersed in water: Irgacure 819 DW from Ciba Specialty Chemicals, and Esacure KIP/EM from Fratelli-Lamberti.
The chemical formulas of these photoinitiators are given in Figure 1. They consist either of hydroxyphenylketones or of acylphosphine oxides, which generate upon UV-exposure benzoyl radicals Noun 1. benzoyl radical - the univalent radical derived from benzoic acid
chemical group, radical, group - (chemistry) two or more atoms bound together as a single unit and forming part of a molecule and either alkyl radicals Noun 1. alkyl radical - any of a series of univalent groups of the general formula CnH2n+1 derived from aliphatic hydrocarbons
alkyl, alkyl group
chemical group, radical, group - (chemistry) two or more atoms bound together as a single unit and forming part or phosphinoyl radicals, respectively.
The waterborne resin was made of a short acrylate end-capped polymer chain containing either a few carboxylate carboxylate,
n a carboxylic acid salt, ester, or ion. groups to be dispersible in water or an added emulsifier emulsifier /emul·si·fi·er/ (e-mul´si-fi?er) an agent used to produce an emulsion.
An agent used to make an emulsion of a fixed oil. . 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 and 60 wt% for the dispersions.
Drying and UV Curing
The formulation containing typically 1 wt% of photoinitiator was cast onto a barium fluoride Barium fluoride (BaF2) is a chemical compound of barium and fluorine, also known as Barium(II) fluoride. It is a solid which can be a transparent crystal. Applications crystal or a glass plate to obtain, after drying at 80[degrees]C, a 20-[micro]m thick coating. 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.sup.-2]. The UV dose received by the sample at each pass was measured by radiometry Radiometry
A branch of science that deals with the measurement or detection of radiant electromagnetic energy. Radiometry is divided according to regions of the spectrum in which the same experimental techniques can be used. (International Light IL-390 radiometer radiometer (rā'dēŏm`ətər), instrument for detection or measurement of electromagnetic radiation; the term is applied in particular to devices used to measure infrared radiation. ), its value ranging from 42 to 500 mJ [cm.sup.-2], depending on the web speed. The UV exposure was performed either at ambient temperature, or at 80[degrees]C on the sample emerging from the UV oven. All the experiments were performed in the presence of air.
Upon 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 tri·di·men·sion·al
Of, relating to, or having three dimensions. polymer network.
The polymerization of the acrylate double bonds was followed by infrared spectroscopy through the decrease of the sharp band at 1410 [cm.sup.-1]. The acrylate conversion (x) after a given exposure time (t) was determined from the ratio of the IR absorbance absorbance /ab·sor·bance/ (-sor´bans)
1. in analytical chemistry, a measure of the light that a solution does not transmit compared to a pure solution. Symbol .
2. before and after UV irradiation: x = 1 - ([A.sub.1410])[.sub.t]/([A.sub.1410])[.sub.0]. The hardness of the UV-cured polymer was evaluated by monitoring the damping damping
In physics, the restraint of vibratory motion, such as mechanical oscillations, noise, and alternating electric currents, by dissipating energy. Unless a child keeps pumping a swing, the back-and-forth motion decreases; damping by the air's friction opposes the of the oscillations oscillations See Cortical oscillations. of a pendulum placed onto a glass plate coated with a 50-[micro]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 Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. Techniques include:
or elastic constant
In materials science and physical metallurgy, any of various numbers that quantify the response of a material to elastic or springy deflection. and relaxation temperature) on 1 mm thick samples. From the variation of the storage module (E) and of the tensile tensile,
adj having a degree of elasticity; having the ability to be extended or stretched. loss factor (tan [delta]) with the temperature, values of the Young modulus and of the glass transition temperature The glass transition temperature is the temperature below which the physical properties of amorphous materials vary in a manner similar to those of a solid phase (glassy state), and above which amorphous materials behave like liquids (rubbery state). , [T.sub.g'] were obtained.
RESULTS AND DISCUSSION
Drying of Water-based Acrylate Resins
As 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 coalescence /co·a·les·cence/ (ko?ah-les´ens) the fusion or blending of parts.
a fusion or blending of parts. . By the end of the drying stage, the milky milky (mil´ke)
1. having the appearance of milk; whitish, cloudy, fluid.
2. filled with or consisting of milk or a milklike fluid. 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 gra·vim·e·ter
1. An instrument used to measure specific gravity.
2. An instrument used to measure variations in a gravitational field. or by IR spectroscopy (OH band at 3500 [cm.sup.-1]), and found to give concordant results. Figure 2 shows the water release profiles obtained by the two methods for 50-[micro]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 80[degrees]C, as shown by the water release profiles reported in Figure 2 for both emulsion- and dispersion-type 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 80[degrees]C, it took only one minute to release 95% of the water in the emulsion heated at 80[degrees]C, and two minutes for the dispersion, compared to 15 minutes and two hours, respectively, at ambient temperature. In further experiments, the water-based coatings were dried at 80[degrees]C for five minutes or at ambient temperature for a few hours, so as to contain less than 2 wt% remaining water.
[FIGURE 1 OMITTED]
Influence of the Photoinitiator on the UV Curing
The 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 [cm.sup.-2]) at ambient temperature. The following PI ranking was obtained:
Darocur 1173 < Esacure KIP < Irgacure 819 DW < Lucirin TPO-L < Irgacure 2959
Similar 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 80[degrees]C. Table 2 reports the conversion values reached after one pass at a speed of 5 m/min (0.43 J [cm.sup.-2]) for the 25 formulations UV-cured at 80[degrees]C. 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 hydroxyphenylketones 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 [T.sub.g] of the fully cured polymer is well below 80[degrees]C.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
The acylphosphine oxide photoinitiators undergo a fast photolysis photolysis
Breakdown of molecules into smaller units via absorption of light. Flash photolysis, an experimental technique developed by Manfred Eigen, Ronald George Weyford Norrish, and George Porter, studies short-lived chemical intermediates formed in many photochemical upon UV exposure, as shown in Figure 4 for Irgacure 819 DW, which is essentially gone at a UV dose of 0.4 J [cm.sup.-2]. 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 [cm.sup.-2] (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 (17) (nitroxyl radicals are not formed in the [O.sub.2]-depleted sample undergoing polymerization), UV absorbers do slow down the photopolymerization by competing with the photoinitiator for the capture of the incident photons. Such radiation filter effect is less pronounced for acylphosphine oxide PIs than for hydroxyphenylketone PIs, as shown in Table 3 which reports the conversion values reached after UV curing at 80[degrees]C 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 polyurethane
Any of a class of very versatile polymers that are made into flexible and rigid foams, fibres, elastomers (elastic polymers), surface coatings, and adhesives. 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 (18-22) (see below).
[FIGURE 4 OMITTED]
Influence of the Resin on UV Curing
The 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 crosslinked 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 80[degrees]C 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 80[degrees]C.
The same trend was trend was observed by using Irgacure 819 DW as photoinitiator and by monitoring the polymerization in real time by infrared (RTIR RTIR Round the Island Race (UK) ) 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 80[degrees]C. The more complete polymerization achieved in online UV curing was attributed to a greater increase in the sample temperature due to the faster exothermal exothermal /exo·ther·mal/ (ek?so-ther´mal) exothermic.
marked or accompanied by the evolution of heat; liberating heat or energy. 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% tripropyleneglycol diacrylate), which acts as a reactive plasticizer plas·ti·ciz·er
Any of various substances added to plastics or other materials to make or keep them soft or pliable.
plasticizer or -ciser
Noun . 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 [O.sub.2] inhibition provides only a marginal improvement because of the slow diffusion of oxygen in a solid film, as shown in previous studies. (14,21)
[FIGURE 5 OMITTED]
Properties of UV-Cured Waterborne Coatings
All the waterborne coatings examined in this study were found to become completely insoluble in the organic solvents after UV exposure (0.5 J [cm.sup.-2]), as expected from the high crosslink 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 hydrophilic /hy·dro·phil·ic/ (-fil´ik) readily absorbing moisture; hygroscopic; having strongly polar groups that readily interact with water.
adj. character, due to the presence of the carboxylic acid carboxylic acid: see carboxyl group.
Any organic compound with the general chemical formula −COOH in which a carbon (C) atom is bonded to an oxygen (O) atom by a double bond to make a carbonyl group (−C=O; see group (or carboxylate anion A carboxylate anion is an ion with negative charge that contains the group -COO−. It is the conjugate base of a carboxylic acid.
Carboxylic acids dissociate into a carboxylate ion and a positively-charged hydrogen ion (proton) much more readily than alcohols ) required to achieve an homogeneous dispersion of the resin in water before UV curing. The contact angle of a droplet droplet
very small drop of fluid.
the finite particles of matter which are transmitted from animal to animal. of water ([[theta Theta
A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option. ].sub.w]) was in the range of 26[degrees]-47[degrees] for the UV-cured coatings D-3, D-2, and D-1, compared to values of 60[degrees] and 74[degrees] for the coatings E-2 and E-1, respectively (Table 5). From the value of [[theta].sub.w] and that of the contact angle of tricresylphosphate (nonpolar nonpolar
not having poles; not exhibiting dipole characteristics. solvent), we have calculated the dispersion and polar component of the surface energy ([[gamma].sub.D] and [[gamma].sub.P]) by using Young's equation. (23)
[FIGURE 6 OMITTED]
Figure 9 shows, as histograms, the [gamma] values obtained for the five UV-cured coatings. It clearly appears that the polymers obtained from emulsion-type formulations are less hydrophilic (low [[gamma].sub.P]) than those obtained from dispersion-type formulations. This result is in full agreement with our previous results on UV-cured waterborne acrylic coatings, (14) where the polar component of the surface energy was found to increase regularly with the acid content of the functionalized oligomer.
Because of their hydrophilic character, the dispersion-type 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 [cm.sup.-1] 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 its original hardness. (14)
[FIGURE 7 OMITTED]
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 used as a UV-adhesive to assemble two glass plates coated with this tacky resin. An effective bonding was achieved by a short exposure to either UV radiation or simply to sunlight, with Irgacure 819 DW used as the photoinitiator. For the dispersion-type samples, the dry film was moderately hard, and it became substantially harder upon UV curing. Figure 11 shows the hardening of E-1 and D-2 coatings upon UV exposure at ambient temperature, together with the concomitant drop of the acrylate double bond content.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
The Persoz hardness of the emulsion-type coating is on the order of 70 sec after UV curing, while that of the dispersion-based coating increased from an initial value of 155 sec to over 300 sec after a one second UV exposure. The corresponding difference in molecular mobility fully accounts for the well-contrasted curing behavior of these two types of water-based systems. As expected, increasing the curing temperature to 80[degrees]C generated harder polymer materials due to a more complete curing, as shown in Figure 12. The Persoz hardness reached values up to 350 sec for the UV-cured D-1 coating, almost as much as for mineral glass (400 sec). To demonstrate the correlation that exists between the cure extent and the polymer hardness, we have plotted, in Figure 13, the Persoz hardness versus the acrylate conversion for the five water-based resins exposed to UV radiation at 25[degrees]C. It clearly appears that the final conversion was considerably lower for the hard polymers (dispersion) than for the soft polymers (emulsion). It can be seen that the E-2 sample showed a distinct behavior, high conversions (80%) being reached in a relatively hard material (Persoz 275 sec). It could be due to the higher crosslink density of the UV-cured polymer, as well as to a greater mobility of the oligomer chain, which would allow a more extensive curing. Figure 14 shows similar hardness versus conversion plots for samples UV-cured at 80[degrees]C, immediately after emerging from the drying oven. The behavior is the same as at 25[degrees]C, with a clear distinction between emulsion- and dispersion-type formulations and the expected increase in hardness and cure extent.
[FIGURE 10 OMITTED]
The viscoelastic properties of the UV-cured waterborne polymers have been studied by dynamic mechanic analysis (DMA (1) (Digital Media Adapter) See digital media hub.
(2) (Document Management Alliance) A specification that provides a common interface for accessing and searching document databases. ) on 1-mm thick samples. Figure 15 shows some typical profiles recorded for the storage modulus and the tensile loss factor of the D-3 sample UV-cured at ambient temperature (UV dose of 1.2 J [cm.sup.-2]) when heated up to 160[degrees]C. From these curves, a [T.sub.g] value of 118[degrees]C and a Young's elastic modulus of 3973 MPa were obtained for this dispersion-type aromatic polyurethane-acrylate. As polymerization is not expected to proceed in the glassy state because of severe molecular mobility restrictions, the curing reaction usually stops once the glass transition temperature of the polymer formed reaches the temperature of the sample. The fact that a [T.sub.g] value as high as 118[degrees]C was obtained for a sample which was cured at ambient temperature can be explained by the exothermicity of the acrylate polymerization. The heat that evolved (~80 kJ [mol.sup.-1]) during the short UV exposure (two second) of the 1-mm thick sample caused a sharp rise of the temperature, which can reach up to 160[degrees]C upon intense illumination. (7)
[FIGURE 11 OMITTED]
Similar results were obtained with the aliphatic polyurethane-acrylate D-1 which is more flexible than D-3 due to its lower elastic modulus (E = 1565 MPa, [T.sub.g] = 96[degrees]C) but is still as hard (Persoz hardness of 350 sec). It should be emphasized that the properties of the final product will depend on the UV-curing conditions (i.e., light intensity, sample thickness). A clear illustration is provided by UV curing at ambient temperature of the emulsion-type aromatic polyester-acrylate E-1 which produces a soft elastomer elastomer (ĭlăs`təmər), substance having to some extent the elastic properties of natural rubber. The term is sometimes used technically to distinguish synthetic rubbers and rubberlike plastics from natural rubber. in the case of less than 100-[micro]m thick coatings, or a hard and stiff material ([T.sub.g] = 56[degrees]C, E = 1178 mPa, Persoz hardness of 200 sec) for 1-mm thick plates (Figure 16). It is still possible to obtain a low modulus elastomer with a 1-mm thick sample of E-1 by performing the UV-irradiation at low intensity (e.g., by sunlight), so as to allow the heat that evolved via the reaction to dissipate dis·si·pate
v. dis·si·pat·ed, dis·si·pat·ing, dis·si·pates
1. To drive away; disperse.
2. over a longer period of time. This example clearly shows that it is essential to precisely define the UV-curing conditions, because they may drastically affect the properties of the final product.
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[FIGURE 13 OMITTED]
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[FIGURE 15 OMITTED]
[FIGURE 16 OMITTED]
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[FIGURE 18 OMITTED]
[FIGURE 19 OMITTED]
Weathering Resistance of Waterborne UV-Cured Coatings
Protective coatings are commonly used to improve not only the surface properties of organic materials, but also their resistance to weathering in exterior applications. Therefore, such coatings must show durability against environmental factors through resistance to UV radiation, oxygen, moisture, pollutants pollutants
see environmental pollution. , and heat without delamination delamination /de·lam·i·na·tion/ (de-lam?i-na´shun) separation into layers, as of the blastoderm.
1. A splitting or separation into layers.
2. from the substrate. In this respect, UV-cured polyurethane-acrylate coatings containing a long lasting UV-absorber proved to be particularly effective in increasing the exterior durability of various polymer materials (i.e., wood, organic glasses, painted metals). (24-26)
The light stability of the five waterborne UV-cured acrylic coatings examined in this study was tested in a QUV-A accelerated weatherometer operated under wet cycle conditions (eight-hour UV exposure at 70[degrees]C followed by four hours in the dark at 50[degrees]C under 100% relative humidity relative humidity
The ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage. ). As expected, the aliphatic polyurethane-acrylate (sample D-1) proved to be the most resistant to weathering because of its low absorbance above 300 nm. The four samples containing aromatic structures were found to undergo yellowing and degradation leading to delamination within less than 500 hr of QUV-aging.
The loss of transparency as well as yellowing were evaluated by monitoring the absorbance at 420 nm of the D-1 sample upon QUV-A exposure, Irgacure 2959 (2 wt%) being used as the photoinitiator. The transmission of the 15-[micro]m thick UV-cured film was found to drop from an initial value of 98 to 90% after 3000 hr of QUV QUV Relative Magnetic Bearing (radiotelegraphy) exposure, while it did not change at all in the presence of a combination of UV absorber (2 wt% Tinuvin 400) and HALS radical scavenger (1 wt% Tinuvin 292), as shown in Figure 17. By contrast, the other coatings that contain aromatic structures were found to undergo rapid yellowing upon accelerated weathering, as shown in Figure 17 for the unstabilized D-2 sample.
The photodegradation process was followed by infrared spectroscopy, the main changes being observed in the 1250-1520 [cm.sup.-1] region (urethane urethane (yoor´ithān´),
n ethyl carbamate used as an anesthetic agent for laboratory animals, formerly used as a hypnotic in humans. group) and in the 2800-3000 [cm.sup.-1] region (CH group), as shown in Figure 18 for the unstabilized D-1 sample exposed to accelerated QUV-A weathering for 4000 hr. The addition of the two light stabilizers considerably reduces the degradation process, with the infrared spectrum Noun 1. infrared spectrum - the spectrum of infrared radiation
infrared, infrared frequency - the infrared region of the electromagnetic spectrum; electromagnetic wave frequencies below the visible range; "they could sense radiation in the infrared" remaining essentially unchanged after a 4000-hr QUV exposure. Figure 19 shows the decay profiles of the C-NH group (1522 [cm.sup.-1]) and CH group (2950 [cm.sup.-1]) for the unstabilized and stabilized D-1 samples. It is quite remarkable that the stabilized coating remained perfectly clear and glossy after as much as 4000 hr of wet cycle QUV-aging, without delamination. Moreover, the UV absorber was hardly consumed, as shown in Figure 20, thus, ensuring a long lasting screening of the most harmful UV radiation of sunlight (300-350 nm) and an effective protection of the polymeric polymeric /poly·mer·ic/ (pol?i-mer´ik) exhibiting the characteristics of a polymer.
1. Having the properties of a polymer.
2. substrate against photodegradation. Based on such performance, one can expect this water-based UV-cured coating to be successfully used in exterior applications, for which outstanding light stability and superior mechanical and surface properties are required to ensure a long-term protection of the coated material.
[FIGURE 20 OMITTED]
Water-based acrylate coatings can be cured by a short UV irradiation in the presence of hydroxyphenyl ketone ketone (kē`tōn), any of a class of organic compounds that contain the carbonyl group, C=O, and in which the carbonyl group is bonded only to carbon atoms. or acylphosphine oxide photoinitiators. As the crosslinking reaction proceeds in a solid material, it is highly dependent on the molecular mobility, which can be increased by raising the temperature or by operating in a humid atmosphere. Emulsion-based resins, which produce soft polymers, were found to cure faster and more extensively than dispersion-based resins, which produce hard polymers. The main advantage of using such water-based resins, besides the ability to easily control their viscosity through the solid content, is that the UV curing does not release volatile organic compounds volatile organic compound Environment Any toxic cabon-based (organic) substance that easily become vapors or gases–eg, solvents–paint thinners, lacquer thinner, degreasers, dry cleaning fluids and consumes little energy. Polymers with tailor-made properties can be designed by properly choosing the chemical structure of the functionalized oligomer, thus making them best suited for the considered application, mainly as protective coatings to improve the surface properties of a large variety of materials (i.e., wood, paper, plastics, metals), as well as their weathering resistance.
Characteristics of Acrylate Oligomers Tested E-1 Laromer PE55W Emulsion Aromatic polyester-acrylate 2.8 M/kg E-2 Laromer PE-22 WN Emulsion Aromatic polyester-acrylic 3.9 M/kg D-1 Laromer 8949 Dispersion Aliphatic polyurethane-acrylate 3 M/kg D-2 Laromer 8983 Dispersion Aromatic polyurethane-acrylate 1.4 M/kg D-3 Laromer 9005 Dispersion Aromatic polyurethane-acrylate -- Table 1 -- Influence of Temperature on the Drying of Waterborne Acrylic Resins. Wet Film Thickness: 50 [micro]m Temperature ([degrees]C) 25 80 Water loss rate Rate ratio (% [s.sup.-1]) (80[degrees]C/25[degrees]C) Dispersion 0.06 1.6 26 Emulsion 0.2 4 20 Table 2 -- Influence of the Photoinitiator (1 wt%) on the Cure Extent of Water-Based Acrylate Coatings after a UV-Dose of 0.46 J [cm.sup.-2]. Temperature: 80[degrees]C Photoinitiator D-1173 Esa KIP I-2959 TPO-L I-819 DW Acrylate conversion (%) D-3 32 42 40 45 40 D-2 49 52 52 50 56 D-1 70 80 84 68 68 E-2 78 73 81 77 79 E-1 100 99 100 100 98 Table 3 -- Influence of Light Stabilizers (1 wt% Tinuvin 292 + 2 wt% Tinuvin 400) on the UV Curing at 80[degrees]C of Water-Based Acrylate Coatings. Acrylate Conversion (%) Photoinitiator Laromer Resin Unstabilized Stabilized UV dose (J[cm.sup.-2]) 0.1 0.8 0.1 0.8 Irgaure 819 DW 1 wt% E-1 98 100 96 100 2 wt% D-1 78 81 70 80 2 wt% D-2 50 56 48 55 Irgacure 2959 2 wt% D-1 72 84 58 80 2 wt% D-2 50 56 10 45 Table 4 -- Influence of Temperature on the Reactivity of UV-Curable Water-Based Acrylate Resins (Irgacure 819-DW) = 1 wt%. RTIR Spectroscopy Online UV Curing Reactivity Conversion (%) Conversion (%) ([sec.sup.-1]) after 5 sec (1 J 1 J [cm.sup.-2] [cm.sup.-2]) Temperature 25 50 80 25 50 80 25 50 80 ([degrees]C) E-1 0.63 2 3.3 71 94 95 81 98 99.9 E-2 0.56 2 2.5 52 70 70 72 77 79 D-2 0.19 0.7 0.9 21 37 51 41 50 54 D-1 0.017 0.1 1.25 1.5 28 56 16 41 79 D-3 0.008 0.3 0.8 9.5 31 45 29 38 40 Table 5 -- Hydrophilic Characteristics of UV-Cured Water-Based Acrylic Coatings. Contact Angle of Water ([[theta].sub.w]) and of Tricresylphosphate ([[theta].sub.TCP]), Polar, and Dispersive Components ([[gamma].sub.p'] [[gamma].sub.d]) of the Surface Energy. UV Dose: 0.34 J [cm.sup.-2] 1% [[theta].sub.w] [[theta].sub.TCP] [[gamma].sub.p] 1-819 DW ([degrees]) ([degrees]) (mJ[m.sup.-2]) E-1 74 20 5.6 E-2 60 24 12.7 D-1 35 37 29.5 D-2 47 37 22.4 D-3 26 42 35.5 1% [[gamma].sub.d](mJ[m.sup.-2]) [gamma]total (mJ[m.sup.-2]) 1-819 DW E-1 40.1 45.7 E-2 39.1 51.8 D-1 34.5 64.0 D-2 34.5 56.9 D-3 32.4 67.9
One of the authors (I.L.) would like to thank the Ministere des Affaires Etrangeres for a research fellowship.
Part of this work was presented at the RadTech Europe Conference in Berlin, November 3-5, 2003.
Presented in part at the RadTech Europe Conference in Berlin, November 3-5, 2003.
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C. Decker and I. Lorinczova ([dagger]) -- Ecole, Nationale Superieure de Chimie de Mulhouse*
* Department de Photochimie Generale (CNRS CNRS Centre National de la Recherche Scientifique (National Center for Scientific Research, France)
CNRS Centro Nacional de Referencia Para El Sida (Argentinean National Reference Center for Aids) ), 3 rue Werner, 68200 Mulhouse, France.
[dagger] Universite de Bratislava (Slovaquie).