Photocuring of Waterborne Latex Paints Under Sun and Visible-Light Sources
A prerequisite is that the photoinduced curing must work with light of wavelengths above 380 nm because most paints contain a substantial amount of titanium dioxide (10-20% by weight), which strongly absorbs radiation of wavelengths lower than 380 nm and thereby prevents the initiation of the photocuring reaction. Some outdoor photocurable latex coatings are known, but until recently they have only worked with the UV-A part of the emission of sunlight by using benzophenone derivatives or cleavable UV photoinitiators. Recently, aqueous emulsions of phosphine oxides were able to cure clear waterborne coatings under visible light. However, in air and with the low light intensity available from sunlight, the photocuring must either occur after drying, in order to give good coating properties, and/or use a crosslinkable polymer mixture showing so-called "physical drying" (i.e. being tack-free before light irradiation).
We have found that an a -diketone, camphorquinone (CQ), used in the presence of a waterborne latex and a reactive plasticizer, is able to photoinitiate the curing of a pigmented waterborne paint under sunlight1.
In the present work, the ability of CQ to cure waterborne coatings under sunlight, in air, is described. The effect of the composition of the waterborne latex is presented. The film properties after light curing are discussed as a function of the structure of the reactive acrylic crosslinker. Moreover, the role of the structure of the reactive diluent is emphasized. Finally, the properties of a sunlight-cured paint are compared to the equivalent conventional paint containing VOCs1-3.
Experimental
Materials
The latex compositions are given in Table 1. They were synthesized by emulsion polymerization. PC1, for example, is based on ethylene (E)-vinyl acetate (VA) - (acetoacetoxy) ethyl methacrylate (AAEMA). The following monomers were used in the synthesis of the latex: IBMA (isobutylmethacrylate), ROA (hexa(propyleneglycol) acrylate), MMA (methyl methacrylate), BuA (butyl acrylate), GMA (glycidyl methacrylate), MAA (methacrylic acid), DCPA (dicyclopentenyl acrylate), DAEMA (N,N-dimethylaminoethyl methacrylate), VeoVA (vinyl versatate), AA (acrylic acid), EHA (2-ethylhexyl acrylate), VCH (vinylcyclohexane). The solids content of the latexes were about 50%. The crosslinkers used were trimethylolpropane triacrylate (TMPTA), dipentaerythrytol pentaacrylate (DiPEPA), polyurethane diacrylic oligomer and polyethylene glycol diacrylates (PEGDA): ethylene glycol diacrylate (EDGA), PEG 400 diacrylate (PEGDA1), diethylene glycol diacrylate (PEGDA2), PEG600 diacrylate (PEGDA3) and hydroxylated trimethylolpropane triacrylate (PEGDA4).Light Sources
Photocurable samples were exposed to the light of an Atlas Sunchex light box (Io=100 mW/cm2) or exposed to sunlight behind a glass window.
Sample Preparation
The photoinitiator is dissolved in the crosslinker before addition to the latex. The mixtures were kept for two days in the dark before use. Curing rates were followed by infrared spectroscopy on films obtained by casting the latex mixture onto a BaF2 crystal with a 36-µm bar coater. The films were dried in the dark for 12 hours at room temperature. Hardness and swelling were measured on films having 200-µm thicknesses before drying, cast onto glass plates. Films used for hardness and solvent resistance measurements were exposed for 90 minutes in the Sunchex lightbox. The curing experiments in the absence of air were conducted in laminate, which protected the latex film from air with a polypropylene film that inhibited the diffusion of oxygen into the coating during the curing time.
Apparatus and Methods
The kinetics of the curing reaction under light irradiation were studied by FTIR spectroscopy (Brucker IFS28 FTIR spectrophotometer) by following the decrease of the characteristic absorption bands of the acrylic double bond near 810, 1410 and 1637 cm-1 upon exposure to light. The degree of conversion was calculated from the ratio of the corresponding IR absorbance before and after light exposure. The shrinkage remained below 5% based on C-H peaks. The degree of swelling was measured by soaking the weighted sample in THF for 15 hours. The insoluble polymer was dried at 45 °C to constant weight. The hardness of the coating was evaluated by monitoring the damping time of the oscillation of a pendulum in seconds (Persoz hardness).
Results and Discussion
The UV-absorption spectra of camphorquinone (CQ) corresponds well to the visible emission of natural light and artificial visible lights like xenon arc lamps (Figure 1). The transmission window of CQ between 300-420 nm allows the use of a mixture of photoinitiators.
Sunlight Curing of a Latex/PEGDA Mixture in Air
A sample of 100 parts of a waterborne latex with 20 parts by weight of reactive crosslinker PEGDA1 and five parts of CQ was exposed to visible light with wavelengths above 300 nm. Upon irradiation the infrared absorption bands of PEGDA1 disappeared within a few minutes, showing that CQ works well as a photoinitiator in the coalesced solid medium (Figure 2). CQ is bleached, resulting therefore in a clear coating.Curing rates recorded in both air and in laminate of a latex/CQ mixture containing PEGDA1 show that oxygen quenching was not observed with several of our waterborne latexes (Figure 3 with PC1). A tack-free coating is obtained within minutes of exposure to sunlight in the presence of CQ. Among the a-diketones studied, CQ shows the fastest curing in air, reaching nearly 100% conversion of the acrylic bonds within two minutes of sunlight irradiation.
In addition to the disappearance of the acrylic moieties, the tacky coalesced film becomes hard and tack free, and both the hardness and the Tg increase. Moreover, the tacky, coalesced latex/PEGDA1 mixture is soluble in THF before exposure to sunlight, but after a few minutes of light exposure in the presence of CQ the film becomes insoluble in THF, proving that an effective crosslinking involving the polymer backbones of the coalesced latex occurs. The insolubilization of the clear latex exposed to sunlight is obtained with as low as 0.2 parts of CQ and 5 parts PEGDA.
Effect of the Latex Composition
The curing rates of the PEGDA in the presence of several latexes containing either vinyl acetate or acrylic monomers as the main constituent (Table 1) were measured (Figure 4).
The reactivity of CQ is affected by the structure of the waterborne latex and is almost inhibited in the presence of Int-3, but no clear structure/reactivity relationship could be found. Both cationic and anionic surfactants were used, and it was found that in both cases a crosslinking reaction was observed.
Effect of the Acrylic Crosslinker
The structure of the monomer is known to have a marked effect on the efficiency of CQ as photoinitiator, especially in air. The latex PC1 containing CQ was swelled with several acrylic crosslinkers and exposed to visible light. The properties of the light-exposed films were measured (Figure 5). It became obvious that, despite the fact that the acrylic double bonds disappear upon light exposure, the films obtained with polyurethane acrylic monomers remained soluble in THF. On the other hand, PEGDA with more than 1 PEG moiety or TMPTA allowed the cured films to become solvent-resistant in THF. Using low-molecular-weight PEGDA2 leads to harder films. With PEGDA1 or PEGDA3, the films are tack-free but remain soft. TMPTA gave good solvent resistance, but the tackiness was not as good as with a PEGDA crosslinker.PEGDA as a reactive plasticizer seems to be the best, while increasing the number of acrylic moieties only increases the solvent resistance in the case of the latexes Int-6 and Int-7 (Figure 6).
At least five parts by weight of PEGDA1 and 0.25 parts of CQ are needed to obtain a solvent-resistant coating (Figure 7), but for 100% conversion of the acrylic moieties at least one part of photoinitiator is needed.
However, the best coating properties are obtained when both a water-soluble (PEGDA1) and a non-water-soluble crosslinker (like DiPEPA) are used together (Figure 8). In each case, nearly 100% acrylic conversion is reached.
Properties of a Latex Grafted with Polyether Moieties (INT-12)
PEGDA crosslinker is very reactive toward CQ and more generally towards polypropylene oxide moieties and -O-CHR- or -S-CHR- moieties. Latexes having such functionality (for example through copolymerization or by grafting with ROA or more or less hydrolyzed glycidyl moieties) were prepared.It was observed that photocuring of TMPTA in the presence of CQ and the latex prepared with ROA (INT-12 and paints based on Latex C or F) was faster than in the presence of CQ and PC1, which was initially the most reactive latex (Figure 4). The hardness of the light-cured films increases a lot compared to a latex not containing such chemical structures (Figure 9). The cured coating containing the latex INT12 and PEGDA2 shows good hardness considering that the core structure of the latex was not optimized for taking into account the plasticizing effect of the reactive diluent PEGDA2 and could, therefore, be even higher. Moreover, in this case, the light-exposed films were tack-free even if TMPTA alone was used.
It is no longer required to use PEGDA-based crosslinker, which leads (except in the case of PEGDA2) to a soft material. The combination of a specific -X-CHR- latex shell and CQ leads to coatings having better properties than latex having only an acrylic or vinylacetate core.
Effect of Co-Initiators
Addition of small amounts (0.1 to 0.5 %) of N-phenylglycine or EDAB (4-ethyl-N,N-dimethylamino benzoate) increases the hardness and decreases the swelling. At higher concentrations, the properties of the coating became worse. The aliphatic amine MDEA had a negative effect.
Comparison of Standard Paints and Light-Curable VOC-Free Paints
Two series of paints consisting of 30% latex and 70% pigmented paste were prepared. One series was based on a vinyl acetate (VA)-based latex used mainly in indoor applications: latexes A, B and the paint based on latex C, which contains -O-CHR- moieties. The other series is based on acrylic monomers mainly used for outdoor coatings: latex D, E and the paint based on latex paint F, which contains -O-R- moieties. For each latex sample, the classic paints were prepared by adding 3-6% of VOC. The equivalent photosensitive paint formulation was prepared by replacing the VOC by 3% of a mixture of crosslinker (TMPTA or PEGDA2) and CQ (in each case as w/w 100:1).
After visible-light exposure, both the outdoor and indoor photocurable paints based on "standard" latex A, B, D and E containing no ether moieties appear to be as good as classical VOC-containing paints (Figure 10). Paints based on latex containing specific O-CHR- moieties reach hardness values above that obtained with common paints. Moreover, keep in mind that these results were obtained with a very low amount of crosslinker and CQ, and will be even better if the amount of crosslinker/CQ is increased. The optimum is near 7-10% with TMPTA as crosslinker.
Conclusions
By using a-diketones, and especially camphorquinone, white pigmented waterborne latex formulations based on acrylates or vinyl acetate for outdoor/indoor paints can be cured under visible light, including sunlight.We have designed a white-pigmented waterborne latex formulation based on acrylates or vinyl acetate for outdoor/indoor paint applications that is photocurable under conventional visible light or sunlight. The cured paint showed an excellent solvent resistance towards THF, and the light-cured paints exhibit a hardness up to 30% higher than comparable VOC-containing paints. The light-curable paints can be produced by simply replacing the VOC by a photocurable composition, which will simplify their industrial acceptance.
Acknowledgement
This work was founded by the European Community BRITE-EURAM III program, BRPR CT980646, VISLATEX project BE97-5112.
References
1 Bibaut-Renauld, C.; Burget, D.; Fouassier, J.P.; Varelas, C.G.; Thomatos, J.; Tsagaropoulos, D.; Ryrfors, L.O.; Karlsson, O.J. Use of a-diketones as Visible Photoinitiators for the Photocrosslinking of Waterborne Latex Paints. J. Polym. Sci., Part A: Polym. Chem., 40, 2002, 3171-3181.2 Patent (assigned to Interchem SA) pending.
3 Proceedings 7th Nurnberg Congress, 7-8 April 2003, 73-83, Nurnberg/Germany, Vincentz Publishing Hanover, Germany
This paper was presented at the 7th Nürnberg Congress, European Coatings Show, April 2003, Nürnberg, Germany.
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