Photolatent effect additives for coatings.UV light curing is one of the most economical and environmentally compatible curing technologies known. It has successfully enabled numerous industrial applications over recent decades. However, there still remain challenges that prevent penetration into broader areas of interest. For example, there are intrinsic limitations associated with radical photopolymerization, e.g., oxygen inhibition, insufficient curing of shadow areas, poor curing of thick or highly pigmented coatings, lack of adhesion on certain substrates, etc. Recent developments using photolatent additives have specifically targeted these deficiencies and thus support the expansion of the scope of UV curing technology in industrial applications. In addition, novel photo-induced effects have been developed, which open up new application areas that have been unexploited to date. [ILLUSTRATION OMITTED] INTRODUCTION Photolatent additives play a crucial role in the coatings industry. (1), (2) These are thermally stable or latent materials that become reactive after they absorb a photon. Photolatent initiators and catalysts are well known examples of these types of additives. (3), (4) Photoinitiators are required for photo-induced 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. , which can be used to cure coatings. The thermally stable photoinitiators serve as triggers and turn an otherwise dormant or latent system into an active one once they are activated by light. Using light as a trigger is an important way to gain better control in the way coatings are made and, in certain cases, how they can be used. Light also can be used to generate other effects as well. For example, photolatent colorants are colorless col·or·less adj. 1. Lacking color. 2. Weak in color; pallid. 3. Lacking animation, variety, or distinction; dull. See Synonyms at dull. and once activated by light they chemically change to produce new colors. Photolatent fragrances, as the name suggests, are additives that generate pleasant aromas or fragrances once they are photoactivated. The purpose of this article is to better understand what these photolatant additives are and where they can be used. Another aim of this work is to examine how these photolatent additives can bring value by addressing shortcomings A shortcoming is a character flaw. Shortcomings may also be:
NEW FREE RADICAL PHOTOINITIATORS Photoinitiators have traditionally been used to initiate the polymerization of vinyl groups (acrylates, methacrylates, and other unsaturated unsaturated /un·sat·u·rat·ed/ (un-sach´ur-at?ed) 1. not holding all of a solute which can be held in solution by the solvent. 2. denoting compounds in which two or more atoms are united by double or triple bonds. groups). (3), (4) More than 90% of light-cured coatings used commercially are produced by free-radical initiation (Table 1). Other photoinitiators, such as photoacid generators, can be used for carbocationic polymerization. A new and emerging class of photoinitiators is photolatent amines amines (  mēnz´),n.pl organic compounds that contain nitrogen. , which can potentially be used in many base-catalyzed coating systems.
Table 1--Technology Platforms Used in UV Curing
Radical Cationic Crosslinking Base-Catalyzed
Crosslinking Crosslinking
Resins Acrylates, Epoxides, Vinyl Epoxy
Maleates, Ethers Polyol/Isocyanate
Styrene Michael
addition
Initiators Aromatic Diaryl Iodonium Salts Latent or blocked
Ketones Triaryl Sulfonium amines
Salts
Nonionic photoacid
generators
Industries Coatings Electronic Materials, Coatings,
Electronic Inks, Adhesives Adhesives
Materials, Under Active
Inks Development
Adhesives
The dominant mode of action of the photoinitiator commonly used commercially is to initiate free-radical polymerization. The free-radical photoinitiators used generally undergo Type I photocleavage (bond homolysis) or Type II (hydrogen abstraction) to produce energetic free radicals. The radicals are short-lived chemical intermediates and are very reactive. These free radicals add to double bonds and cause polymerization and crosslinking to occur, as shown in Figure 1. [FIGURE 1 OMITTED] New photoinitiators in this class have been developed, (5), (6) which offer advanced control over the UV-curing process and also can minimize certain unwanted effects. Some of the salient problems seen with traditional free-radical photocuring can be summarized as follows: * Oxygen inhibition, especially at the coating's surface * Odor, associated with volatility of photofragments (such as benzaldehyde benzaldehyde (bĕnzăl`dəhīd) or benzenecarbonal (bĕn'zēnkär`bənəl), C6H5CHO, colorless liquid aldehyde with a characteristic almond odor. formation) from the photoinitiator * Unwanted color (i.e., yellowness) as a result of photocuring * Requirement for direct line-of-sight to ensure completeness of cure, which is especially important for 3-D objects or articles that have complex shapes * Inadequate through-cure, especially for pigmented or optically opaque coatings * Poor adhesion, which occurs from inefficient curing and/or from shrinkage-induced stress buildup build·up also build-up n. 1. The act or process of amassing or increasing: a military buildup; a buildup of tension during the strike. 2. during photocuring Oxygen inhibition is a perennial problem for photocuring of many coatings. This leads to blocking (sticking) and low mar resistance. A new photoinitiator was developed to address this problem. This photoinitiator is surface active, as shown in Figure 2. [FIGURE 2 OMITTED] This developmental photoinitiator is liquid and nonvolatile and has an intrinsically low surface tension (26 dynes/cm at 21[degrees]C), which forces it to the surface. Used at additive levels, it can serve as a good leveling/slip agent. Used in photocurable coatings, it leads to good surface curing. Specifically, the photoinitiator can be photoactivated with UV-A UV-A or UVA Noun ultraviolet radiation with a range of 320-380 nanometres light (it has a long wavelength absorption between 300-385 nm) and leads to low color coatings that show good slip properties. Because it is a photoinitiator, it grafts itself to the coating, rendering a permanent non-migratory slip property. The result is to generate low color coatings that have a surface that show good slip and high mar resistance. (7) Another development is multifunctional photoinitiators that exhibit high photocure efficiencies. Two examples are shown in Figure 3. [FIGURE 3 OMITTED] The [alpha]-hydroxy 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. (AHK AHK Die Deutschen Auslandshandelskammern (Germany) AHK Autohotkey (Windows open-source utility) AHK Acid House Kings (Swedish band) AHK Acylhomoserine Lactone AHK Auto Hot Key ) is a preferred photoinitiator for inkjet inks or coatings that contain high levels of acrylate Noun 1. acrylate - a salt or ester of propenoic acid propenoate salt - a compound formed by replacing hydrogen in an acid by a metal (or a radical that acts like a metal) monomers. Low viscosity resins that have high monomer monomer (mŏn`əmər): see polymer. monomer 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). content (such as inkjet inks) generally are more susceptible to [O.sub.2] inhibition. Thus, this photoinitiator can help where air inhibition can be a problem. The dual functionality also renders it nonvolatile and allows it to serve as a crosslinker. In addition, this photoinitiator also can serve a rather unorthodox role as a low color photosensitizer photosensitizer Oncology A substance that sensitizes an organism, cell, or tissue to light; an agent used in photodynamic therapy which, when absorbed by CA cells and exposed to light, is activated, killing cancer cells. See Photodynamic therapy. for diaryliodonium salts. The multifunctional phenylglyoxylates (Figure 3B) is also an important new liquid photoinitiator. It can be used to reduce color in Verb 1. color in - add color to; "The child colored the drawings"; "Fall colored the trees"; "colorize black and white film" color, colorise, colorize, colour in, colourise, colourize, colour final cured coatings. A preferred practice is to use it with photobleachable long wavelength absorbing aryl ar·yl n. An organic radical derived from an aromatic compound by the removal of one hydrogen atom. phosphine phosphine 1. PH3, a toxic war gas called hydrogen phosphide. 2. a coal tar dye; called Philadelphia yellow. oxide photoinitiators. This photoinitiator combination can be useful for obtaining good surface and bottom curing of thin and thick coatings. Typically, V+D V+D Voice Plus Data Communications lamps (which emit strongly between 300-450 nm) are used with this photoinitiator package for low color coatings over wood or plastic substrates. UV-CURABLE WATERBORNE COATINGS UV-curable waterborne coatings are becoming increasingly popular because they do not give off VOC-based solvents during curing. A number of photoinitiators are currently recommended, (8), (9) such as the water-dispersed bis-acyl phosphine oxide (BAPO BAPO British Association of Prosthetists and Orthotists BAPO Belgian Association of Pediatric Orthopaedics BAPO British Association for Paediatric Otolaryngology BAPO Belize Association of Producers Organization BAPO British Army Post Office ) and the AHK/BP eutectic blend. The eutectic blend is water dispersible into most common (meth meth n. Methamphetamine hydrochloride. )-acrylate functionalized polyurethane dispersions. It provides good surface cure and can be used in clear coatings. If the coating is pigmented and/or contains protective UV absorbers, then the water dispersed BAPO photoinitiator is also used. BAPO is an important photoinitiator that has proven itself in many demanding coatings, composites, and gel coat applications. The main reasons for BAPO's success is that it can use two photons sequentially, producing two different radical pairs (see Scheme 1). It possesses a long wavelength absorption that photobleaches during curing. Thus, BAPO is desirable for long wavelength (up to 430 nm) photocuring. It is especially useful for LED array photocuring. [ILLUSTRATION OMITTED] LED VISIBLE LIGHT CURING LED arrays were recently introduced for curing industrial coatings An industrial coating is a paint or coating defined by its protective, rather than its aesthetic properties, although it can provide both. The most common use of industrial coatings is for corrosion control of steel or concrete. and adhesives. LED arrays as currently designed for these applications have an output at 400 [+ or -] 20 nm. These quasi-monochromatic light sources differ from standard mercury lamps, which have broadband spectral output. LED arrays are attractive because they do not emit IR radiation (which may cause heating problems, as may be seen with Hg lamps) and they have a stable output. It was found (10) that when BAPO is used as the photoinitiator, up to 25 mil thick gel coats (containing 15 wt% Ti[O.sub.2]) are easily cured with the LED array, as shown in Figure 4. [FIGURE 4 OMITTED] This demonstration of deep through-curing of gel coats even with high filler shows the power of correctly matching the photoinitiator with the light source. The output of the LED array is "tuned" to the BAPO long wavelength absorption for efficient light coupling and photoactivation. The BAPO, in turn, photobleaches, permitting even deeper optical penetration into the gel coat. This combination results in surprisingly good through-cure, compared to when a mercury lamp is used. PLASMA UV-CURING FOR 3-D OBJECTS When coating 3-D objects, one is often faced with the problem of getting uniform light exposure over all coated areas. The object can contain hidden areas, cavities, or small orifices that light cannot reach, which will mean these areas will be uncured or at best only partially cured. An exciting new development that is capable of resolving this dilemma is to use UV light that is generated from plasma. (11), (12) A gas (He/[N.sub.2] mixture) is used to surround the part, then, by switching on a microwave, UV light is generated from it. In essence, the part is placed inside the lamp (Figure 5). [FIGURE 5 OMITTED] In this way, UV light is generated for curing in regions that are normally inaccessible. PHOTOLATENT ACID AND BASE CATALYSTS To expand the capability of light curing, new photo latent acid and base catalysts were developed, (13-16) as shown in Figure 6. These novel photolatent additives generate a long-lived catalyst, which is distinctly different from the short-lived free-radical initiators. [FIGURE 6 OMITTED] The resins they cure also are different. In the case of photolatent base catalysts, they are used to photo-activate the curing of conventional coatings that cure via condensation reactions, such as for polyurethanes or epoxy epoxy Any of a class of thermosetting polymers, polyethers built up from monomers with an ether group that takes the form of a three-membered epoxide ring. The familiar two-part epoxy adhesives consist of a resin with epoxide rings at the ends of its molecules and a curing based coatings as shown in Figure 7. [FIGURE 7 OMITTED] The photolatent base catalysts are a sharp departure from the more commonly used free-radical photoinitiators, which offer exciting possibilities to lower cost and increase productivity in those coating systems. Leading the design development of these photolatent base catalysts is the effort to engineer them to match particular resins. (13) As described in Figure 8, this match is done by first understanding the pKa threshold value needed to drive the chemistry. Higher pKa values above this threshold generally lead to faster rates of catalysis catalysis Modification (usually acceleration) of a chemical reaction rate by addition of a catalyst, which combines with the reactants but is ultimately regenerated so that its amount remains unchanged and the chemical equilibrium of the conditions of the reaction is not . Similarly, when the pKa value is lower than the threshold, the reaction rate is diminished. Unlocking the power of the photolatent base catalysts is then reduced to tailoring the initial pKa of the photolatent form as well as adjusting the pKa jump that occurs when the free base amine amine (əmēn`, ăm`ēn): see under amino group. amine Any of a class of nitrogen-containing organic compounds derived, either in principle or in practice, from ammonia (NH3). is photogenerated. [FIGURE 8 OMITTED] Using this approach led to the recent development of photolatent amines and a photolatent DBN DBN Doing Business - Not DBN De Bonis Non (Legal: appointment of a personal representative to a vacancy) DBN Dial-Back Number DBN Divisible by None (band) DBN Deep Belief Network (1,5-diazabicyclo[4.3.0]non-5-ene). The photolatent amines liberate tertiary amines. One photolatent form of the tertiary amine is an [alpha]-aminoketone (AAK AAK Aleanca për Ardhmërinë e Kosovës (Albanian: Alliance for the Future of Kosovo) AAK Ausschuß Aachener Karneval (Carnival of Aachen Committee, Germany) AAK Asleep At the Keyboard AAK Alive and Kickin' ) derivative. The AAK sterically shields the amine. When the AAK is photolyzed the shielding is lost and it yields a higher pKa tertiary amine that is also more active. It is because of the pKa's of the AAK and the resulting free-amine that makes this photolatent amine interesting. For example, with polyisocyanate/poly thiol-resin coatings, it permits fast curing (< 5 minutes) under direct UV-A light exposure as well as allowing for shadow curing. (14) At the appropriate concentration the AAK cures the resin in the dark (shadow areas) within four to six hours. The cure mechanism is not inhibited by oxygen, so good surface curing results. The overall cure 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. are slower compared to typical free-radical curing of acrylates. This feature can, in principle, lead to better stress relaxation Stress relaxation describes how polymers relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion.[1] in the final polymer network. The photolatent catalyst is now being used in auto refinish re·fin·ish tr.v. re·fin·ished, re·fin·ish·ing, re·fin·ish·es To put a new finish on (furniture). re·fin coating applications, and permits faster, more efficient processing compared to conventional thermal curing. When the photolatent DBN is photoactivated, it undergoes a significant pKa jump ([DELTA]pKa = 4). The DBN that is generated is a strong base and has a pKa = 13. Because of its high pKa value it can efficiently catalyze cat·a·lyze v. To modify, especially to increase, the rate of a chemical reaction by catalysis. catalyze to cause or produce catalysis. reactions such as Michael addition chemistry. The photolatent DBN has a pKa that is well below the threshold pKa to catalyze the Michael addition (Figure 9). Thus, storage stable or one-pack base catalyzed resins result. [FIGURE 9 OMITTED] Some of the current applications where photolatent base catalysts were found to be important are summarized in Table 2. (13) It is expected that more photolatent amines will be developed in the future, which build on this new technology platform.
Table 2--Examples of Where Photolatent Base Catalysts Can Be Used
Typical
Photolatent
Resin Type Base Comments
Carboxy/Epoxy AAK Used in hybrid systems
Photoresists
Generally requires
thermal post-cure step
R-NCO/R'SH Modified AAK Automotive refinish
Shadow cure
Low color
R-NCO/R'OH PL-DBN Adhesives and coatings
Epoxy/Thiol PL-DBN Metal can coatings
Michael Addition PL-DBN Broad utility
(adhesives, coatings, inks)
PHOTOLATENT COLORANTS Photochromic Pho`to`chro´mic a. 1. Of or pertaining to photochromy; produced by photochromy. materials are materials that undergo a reversible color change after exposure to light. A familiar example is materials used in eyeglasses eyeglasses or spectacles, instrument or device for aiding and correcting defective sight. Eyeglasses usually consist of a pair of lenses mounted in a frame to hold them in position before the eyes. that darken dark·en v. dark·ened, dark·en·ing, dark·ens v.tr. 1. a. To make dark or darker. b. To give a darker hue to. 2. To fill with sadness; make gloomy. 3. when in the sunlight, but then reverse to clear in the dark. What is of interest is to use light selectively to create a permanent color. One approach is to photochemically generate a new thermally stable chromophore chromophore /chro·mo·phore/ (kro´mo-for) any chemical group whose presence gives a decided color to a compound and which unites with certain other groups (auxochromes) to form dyes. (such as to produce red, green, or blue colors). (1), (17) This type of an effect could be useful in branding, security, or identification applications. PHOTOLATENT FRAGRANCES Fragrances are important to the coatings industry. Unfortunately, they are fugitive, by virtue of their volatility. One way to overcome this problem is to photogenerate the fragrant aroma in-situ. The fragrance-on-demand concept was recently reduced to practice. (18) The novel photolatent precursor is able to release fragrances of natural or synthetic source. It is thus possible to create scents from floral fruity sweet, fresh citrus and minty, to spicy herbal, or even wine-like or woody aromas. The key benefit of a fragrant product is to provide a pleasant aesthetic perception. It also can serve as a signal, to give a unique branding of the product. [FIGURE 10 OMITTED] Photolatent fragrances are not restricted to simply scening a coated surface; they can also be conveniently used to conceal undesirable odors Odors anosmia Medicine. the absence of the sense of smell; olfactory anesthesia. Also called anosphrasia. — anosmic, adj. halitosis bad breath; an unpleasant odor emanating from the mouth. of newly manufactured products. [FIGURE 11 OMITTED] Some of the odors emitted by new products, even if in low concentrations, have a negative image. Using photolatent fragrance additives in such systems can thus give a pleasant fragrance, which can add value. In this case, irradiation irradiation /ir·ra·di·a·tion/ (i-ra?de-a´shun) 1. radiotherapy. 2. the dispersion of nervous impulse beyond the normal path of conduction. 3. is the last step before packaging or if sufficient light transparency is given, the fragrance release can be triggered at any point via UVA light exposure through the packaging material. Some of the fragrances are known to have a repellent re·pel·lent adj. Capable of driving off or repelling. n. A substance used to drive off or keep away insects. repellent able to repel or drive off; also, an agent that repels. Refers usually to insect repellent. effect on insects, e.g., citronellal cit·ro·nel·lal n. A colorless aromatic liquid, C10H18O, obtained from citronella and certain other essential oils or produced synthetically and used in making perfumes and as a commercial flavoring. , which is often used in candles or insect repellent insect repellent, substance applied to the skin in order to provide protection against biting insects, primarily mosquitoes, ticks, chiggers, fleas, and certain flies. lotions lotions, n.pl nonoily treatments intended to be applied to the skin for a variety of cosmetic or medicinal purposes. . The photolatent products could be used in paint for garden furniture or veranda boards, giving the double benefit of a pleasant scent and repelling insects. SUMMARY Photolatent additives offer useful benefits to the coatings industry. Work in developing this technology is ongoing. Recent developments have yielded important results that were aimed at improving or managing the limitations of light curing. These efforts help to expand horizons into new areas--such as the discovery of novel photo-induced effects that go beyond simple photocuring (color-on-demand and fragrance-on-demand, for example). The future thus holds great promise for even more innovation as these new effects become further developed and are exploited for specific applications. Presented at the Federation of Societies for Coatings Technology's 2006 FutureCoat! Conference, November 1-3, 2006, in New Orleans New Orleans (ôr`lēənz –lənz, ôrlēnz`), city (2006 pop. 187,525), coextensive with Orleans parish, SE La., between the Mississippi River and Lake Pontchartrain, 107 mi (172 km) by water from the river mouth; founded , LA. * Coating Effects Segment, CH-4002 Basel, Switzerland. References (1) Benkhoff, J., Powell, K., Jung, T., Fritzsche, K., Fischer, W., Dietliker, K., and Sitzmann, E., Proc. e|5: UV & EB Technology Expo & Conference 2006, Chicago, IL, April 24-26, 2006. (2) Benkhoff, J., Dietliker, K., Jung, T., Powell, K., and Sitzmann, E., Asia Pacific Coat. J., 19(1), 38-40 (2006). (3) Dietliker, K., "Photoinitiators for Free Radical, Cationic cationic having qualities dependent on having free cations available. cationic detergents are wetting agents that disrupt or damage cell membranes, denature proteins and inactivate enzymes. & Anionic an·i·on n. A negatively charged ion, especially the ion that migrates to an anode in electrolysis. [From Greek, neuter present participle of anienai, to go up : ana-, ana- Photopolymerisation," Bradley, G. (Ed.), Chemistry and Technology of UV&EB Formulation for Coatings, Inks & Paints, Vol. 3, John Wiley John Wiley may refer to:
(4) Sitzmann, E., Fuchs, A., Jung, T., and Wostratzky, D., "Photoinitiators: Their Mechanisms, Use, and Applications," Handbook of Coatings, Calbo, L.J. (Ed.), Marcel Dekker Marcel Dekker is a well-known encyclopedia publishing company with editorial boards found in New York, New York. They are part of the Taylor and Francis publishing group. Initially a textbook publisher, they went to encyclopedia publishing in the late 1990's. , Inc., New York New York, state, United States New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of , pp. 61-125 (2004). (5) Birbaum, J.-L., Huesler, R., Wolf, J.-P., Ilg, S., and Villeneuve, S., Proc. e|5: UV & EB Technology Expo & Conference 2006, Chicago, IL, April 24-26, 2006. (6) Bolle, T., Peter, W., Fuchs, A., and Husler, R., "New Low Emission Photoinitiators for Coatings and Inks," Proc. RadTech Asia 2003, Yokohama, Japan, December 9-12, 2003. (7) Baudin, G. and Jung, T., WO 2002048202 (2002). (8) Burglin, M., Kohler, M., Dietliker, K., Wolf, J.-P., Gatlik, I., Gescheidt, G., Neshchadin, D., and Rist, G., Proc. RadTech USA 2000, Baltimore, MD, p. 577 (2000). (9) Peter, W., Megert, S., Rogez, D., and Sitzmann, E., Eur. Coat. J., No. 11, p. 14-21 (2002). (10) Sitzmann, E.V., Wostratzky, D., Jankauskas, J., and Losapio, G., U.S. 2005234145 (2005). (11) Jung, T., Simmendinger, P., Studer, K., and Tobisch, W., Proc. Radtech e|5: UV & EB Technology Conference 2006, Chicago, IL, April 24-26, 2006. (12) Jung, T., Simmendinger, P., and Tobisch, W., European Coat. J., p. 138-143 (2005), Jung, T. "UV-Plasma Curing: Capture the Third Dimension," Proc. RadTech Europe 05, Barcelona, Spain, 2005. (13) Studer, K., Jung, T., Dietliker, K., Benkhoff, J., Sitzmann, E., and Dogan, N., Proc. e|5: UV & EB Technology Expo & Conference 2006, Chicago, IL, April 24-26, 2006. (14) Dogan, N., Klingenberg, H., Reinerie, L., Ruigrok, D., Wijnands, P., Dietliker, K., Misteli, K., Jung, T., Studer, K., Contich, P., Benkhoff, J. and Sitzmann, E., RadTech Report, 20 (2), 43-52 (2006). (15) Dietliker, K., Jung, T., and Benkhoff, J., Proc. e|5: UV & EB Technology Expo & Conference 2004, May 2-5, 2004. (16) Jung, T., Dietliker, K., and Benkhoff, J., Farbe & Lack, 109, p. 34-41 (2003). (17) Powell, K., Lehmann, U., and Benkhoff, J., Eur. Coat. J., (7-8), 34, 36-38 (2006). (18) Powell, K., Lehmann, U., Benkhoff, J., Fischer, W., and Fritzsche, K., "Smart Coatings V," European Coatings Conference, May 15-16, 2006, Berlin, 2006. by J. Benkhoff, K. Dietliker, K. Powell, T. Jung, K. Studer, and E.V. Sitzmann 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 Inc. * |
|
||||||||||||||||||
Printer friendly
Cite/link
Email
Feedback
Reader Opinion