Photoinitiators and Photopolymerization The term photocurable coatings refers to a photoinduced radically or cationically initiated cross-linking of multifunctional polymerizable resins and monomers. They are used mainly in industrial applications where thermal curing is not possible, e.g., curing of coatings on temperature sensitive substrates, and in imaging applications where only selected areas should be polymerized as in polymer printing plates and photoresists. Furthermore, environmental concerns for reducing emissions from volatile organic compounds (VOC) have been a major driving force of this alternative curing technology. Major research activities are still directed to relating application properties of the network formed with the chemical structure of various reactive compounds used and to the efficiency of the curing reaction, which depends to a considerable extent on the photoinitiation reaction. 1. Photo-induced Reactions Of the many well-known reactions in organic photochemistry only a few are used in coating-related
applications. The photodimerization is an example of a direct photoreaction where every step for polymer build-up is initiated by an absorbed photon, thus every single reaction step is dependent on the quantum yield of the photoreaction (generally very much smaller than one). On the contrary, in catalytic photoinitiation only the initiating step is dependent on the photoreaction (} 1). The efficiency of the photoinitiation is a function of different quantum yields, since several side reactions can occur in every step. Thus, the overall yield of initiation is a complex function of different quantum yields, exemplified in Fig. 1. The photopolymerization reaction is then a chain reaction, where the production of one initiator radical can add up to several thousand monomer units, thus the overall quantum yield for the total reaction is much greater than one. The photopolymerization reaction as depicted in Fig. 2 can be divided into the three steps of: initiation, including the addition of the first monomer unit; propagation, reaction to build up the polymer network; and termination, which can occur for example through recombination of two radicals or by disproportionation and is dependent on the type of monomers and polymerization temperature. The photoinitiated polymerization of multifunctional compounds, therefore,
-one photon reactions O
–N hm + HO O OH
-reactive species formation
OO C P
O C• +
Direct cleavage or .. O RH C
H abstraction reaction
Reaction scheme of radical photopolymerization Initiation
Photo-Initiation Chain Start
Propagation Transfer Termination
Figure 1 Photoreactions.
Photoinitiators and Photopolymerization
on cti tra ) H (D
M (D) R-M* Monomer Initiation of attack Polymerizaion
F = fluorescence IC = internal conversion ∅ ini = ∅ ISC *∅ cleavage *∅ monomer addition P = phosphorescence O2 = Oxygen quencing Quantum yield of initiation M = monomer quencing ISC = intersystem crossing
from which the most important are: (i) high absorption at the exposure wavelength and high molar extinction coefficient, (ii) high quantum yield of formation of initiating species, and (iii) high reactivity of the radical towards the monomer. To increase reactivity amine containing coinitiators (synergists) are often used which have two effects. First, the C–H group adjacent to the nitrogen is a good hydrogen atom donor and the radical thus formed can initiate the polymerization and\or second, the radical can scavenge oxygen, which inhibits the polymerization reaction very effectively (Fig. 3).
Figure 2 Photoinitiation.
leads to a polymer network whose structure and performance characteristics are dependent on the curing conditions and type of materials employed.
2. Materials Used in Photocuring Photocurable coatings generally consist of three main components: the photoinitiator molecule or a photoinitiating system which includes a hydrogen donor coinitiator or a sensitizer; a film forming multifunctional resin which mainly determines the application properties of the coating; and a low-viscosity reactie diluent which takes over the function of viscosity adjustment from the solvent in conventional coatings, but is incorporated in the network and thus also contributes to the application properties of the coating. The development of a large variety of components and additives allows a wide range of properties and applications to be realized.
2.1 Photoinitiators Photoinitiators are molecules that absorb photons upon irradiation with light and form reactive species out of the excited state, which initiate consecutive reactions. The initiating species may be radicals, cations, or anions. Radical photoinitiators are commercially available in large number from companies such as Ciba Specialties (trade names Irgacure and Darocure), Lamberti (Esacure), BASF (Lucirin), and many others. Available in smaller numbers are cationic photoinitiators, mainly sulfonium salts, from Union Carbide (Cyracure), Degussa (Degacure) as well as iodonium salts from General Electric and iron complexes from Ciba. Examples of commercially available photoinitiators are listed in reviews published recently by Davidson (1993) and Fouassier (1995). Almost all radical photoinitiators contain the benzoyl (phenyl– CO–) structural element. The two most important classes are the α-cleavable and the noncleavable photoinitiators (Table 1). A photoinitiator should exhibit several properties, 2
2.2 Resins and Reactie Diluents The main resin classes employed in radical-induced photopolymerization systems are unsaturated polyesters, acrylate oligomers divided in the subclasses epoxy acrylates, polyester acrylates, polyether acrylates, urethane acrylates, and acrylated polyacrylates as well as the less important polyene\thiol systems and acrylated unsaturated oils. Unsaturated polyesters derived from condensation of maleic or fumaric acid with various diols and dissolved in styrene were the earliest UV curable resins. Because of the use of styrene these systems were pushed into the background and acrylate resins now dominate the market. Now epoxy acrylates are the biggest class and are prepared by the reaction of epoxides, e.g., bisphenol A diglycidylether, with acrylic acid. Due to their high viscosity they are usually diluted with reactive diluents such as tripropylene glycol diacrylate. Urethane acrylates are simple addition products of multifunctional isocyanates, such as toluene diisocyanate, isophorone diisocyanate, or hexane diisocyanate or their condensation products, isocyanurates, biurets, allophanates, with hydroxyalkyl acrylates, for instance hydroxyethyl acrylate, hydroxybutyl acrylate, or pentaerythrit triacrylate. Polyester and polyether acrylates are synthesized by esterification of polyester\ether polyols with acrylic acid. Examples of polyesterols are the condensation products of adipic acid with diethylene glycol, 1,6-hexane diol, or trimethylol propane. Commonly used polyetherols are ethoxylated or propoxylated glycerol or trimethylol propane. Such polyether acrylates represent a resin class of low viscosity, which can be used as sole resins as well as reactive diluents. Due to their higher molecular weight the skin irritation levels are lowered significantly compared to conventional diluents to Draize values in the range of 1–2 (Draize rating: 0–2, slightly irritant; 2–5, irritant; 6–8, severe irritant). As reactive diluents monomers and oligomeric acrylates or vinyl ethers are used in order to adjust the application viscosity. Despite the fact that monomers such as styrene, N-vinyl pyrrolidone, and monofunctional esters of acrylic acid are excellent diluents, their use is decreasing due to their high volatility,
Photoinitiators and Photopolymerization Table 1 Photoinitiator types.
O2 OO* +
The two primary functions of a coating are to ensure the desired appearance (e.g., color and gloss) and protection (e.g., scratch and chemical stability and abrasion resistance) of the coated substrate. The formulations used for radiation-curable coatings depend, therefore, first on the specific performance requirements of the application and second on the application technique (viscosity needs). Typically, formulations for UV-curable coatings contain 25– 90% oligomeric resins, 15–60% reactive diluents, 0.5–4% photoinitiators, and 1–50% additives such as pigments, fillers, and stabilizers. Examples of formulations are high-gloss topcoats for wood coating where unsaturated polyesters and styrene are still used in some countries. The more versatile and faster curing, but also more expensive, acrylics are becoming dominant. Polyester or epoxy acrylates either alone or in combination in amounts up to 60%, diluted with 35% of monomers are used for open grain effect wood coatings. For parquet and flooring applications tougher and more abrasion-resistant urethane acrylates are used. Overprint varnishes for paper coatings applied in a roller coating process are lower in viscosity and thus contain up to 70% of di- and trifunctional monomers and 25% of an oligomeric binder.
Figure 3 Mechanism of amine synergists.
strong odor, skin irritation, and flammability. Since monofunctionality provides a higher molecular weight between cross-links or a lower network density resulting in better flexibility, monomers with lower volatility and odor like isobornyl acrylate or trimethylol propane formal monoacrylate have been developed. Multifunctional monomers are available in a broad range since they are produced by esterification reactions of acrylic acid with polyols and the main representatives are tripropylene glycol diacrylate (TPGDA), trimethyolpropane triacrylate (TMPTA), and hexane diol diacrylate (HDDA). Most of the others have only a small market share. Higher functionalized acrylates such as pentaerythritol tetraacrylate or dipentaerythritol hexaacrylate are also available, but only used as minor ingredients since they dramatically increase the cross-link density resulting in hard but brittle films. Such resins and diluents are commercially available from UCB, BASF, Akcros, Cray Valley\Sartomer, and Henkel. Resins used in cationic polymerizing systems are mainly epoxides and vinyl ethers.
3. Structure–Property Relationships The mechanical properties of radiation curable materials can be tailored to the application needs. A review of characterization methods and understanding of the relationships between exposure conditions, chemical structure, and network properties is given by Zumbrunn in Fouassier (1995, Vol. III). The influence of monomer type and functionality for instance on mechanical properties and polymerization rate has been investigated in radical and cationic systems. Generally, increasing the monomer functionality leads to higher cure speed, higher hardness but lower flexibility, lower conversion, and thus higher residual monomer groups. Generalized structure–property relationships of the typical resins used in radiation curable acrylate systems are given in Fig. 4. The performance criteria shown in the diagram are better the further to the edge the values are. The aromatic epoxy acrylates provide high hardness, reactivity, and chemical resistance, whereas urethane acrylates are known for their high flexibility, toughness, and abrasion resistance. The polyester acrylates are fairly good in all criteria, typical all rounders, and the polyether acrylates are very good reactive diluents due to their low viscosity but also usable as pure resin materials. The coating performance obtained is due to the network structure formed. Besides the chemical constitution, factors mostly affecting the mechanical 3
Photoinitiators and Photopolymerization
low ° of crosslinked
ELONGATION at BREAK (%)
Figure 5 Influence of cross-linking on modulus.
Figure 4 Structure property relationships of resins.
properties of coatings are the glass transition temperature (Tg) and the elastically effective chain length between cross-links (Mc). Generally, the higher the cross-link density (i.e., the lower the elastically effective chain length) the harder the film. However flexibility is highest in noncross-linked resins. Tg defines a reversible transition of an amorphous polymer from a state where the chains are relatively mobile into an immobile glassy state. In a liquid reactive, radiation-curable coating, the mobility of the acrylates is reduced during polymerization and finally turns into an amorphous glass. Thus, besides the chemical constitution of the backbone, which is reflected by T _ , g the conversion p and the cross-link density X influence the glass transition: Tg l [Tg_kK (1kp)][1jK (X\(1kX ))] " # _ where Tg is the true backbone, Tg without end groups or cross-links, K characterizes the influence of end " groups, thus reflecting the degree of cure, and K is a # constant accounting for the influence of cross-links. Thus, the Tg of the linear polymer is lowered by end groups if the degree of cure is not 100%, and increased due to the cross-linking reaction. The finally obtained Tg distinguishes between hard and soft coatings. If Tg is above room temperature the coating is hard and turns soft at temperatures above Tg, which is reflected in the modulus curves. The flexibility can be determined by elongation measurements. Linear polymers often have brittle-ductile transition temperatures, Tb, below room temperature and they are tough (flexible) at room temperature. With increasing cross-link density the elastically effective chain length between cross-link decreases and the Tb transition increases until at high cross-link densities only one transition, the glass transition temperature, exists (Fig. 5). Since most of the radiation-cured coatings are highly cross-linked only Tg determines the 4
mechanical properties and this is the reason why most of the coatings are either hard\brittle or soft\flexible. Thus, in order to get hard and flexible coatings, the cross-link density should be decreased to allow a Tb transition below room temperature and the glass transition has to be increased above room temperature by means of chemical composition and cross-linking during photopolymerization. Low cross-link densities are difficult to realize in 100% coatings without using large amounts of monofunctional reactive diluents, since viscosity also increases with molecular weight. Therefore, water-based systems where the viscosity is independent of molecular weight of the resin, or systems applicable by melting, such as hot melt or powder coatings, are options towards hard and flexible coatings (Schwalm 1999) (see also Food Gels; Polymer Coatings (Paints): Water-based; Polymer Coatings: Powder-based). 4. Applications 4.1 Nonimaging Applications: Radiation Curing Radiation curing has been used mainly as an alternative curing mechanism to thermal hardening on temperature-sensitive substrates, in wood and paper coatings, printing inks, and adhesives. Specific applications of photocurable coatings are clear coats for parquet, furniture, vinyl flooring, on plastic substrates (e.g., skies and boards), compact discs, headlight lenses, overprint varnishes (e.g., posters and highgloss packaging), adhesives, protective coatings for optical fibers, and electronic parts. Applications of photocurable coatings on metals (e.g., automotive and coil coating) and exterior uses are just emerging, reflecting the well-known fact that due to VOC regulations the market share of solvent-based coatings is declining and the share of alternative systems (waterbased, powder, and 100% solids UV) is increasing. Besides the ecological advantages resulting from the use of 100% solids systems, leading to nearly no emissions, photocuring is also a very economical curing method. It is a fast room temperature curing process requiring only a low energy consumption and
Photoinitiators and Photopolymerization little space for the equipment. However, as can be deduced from the relative low market share, the widespread use of this curing method is hampered by several limitations. The reduced curability of pigmented UV coatings (interference of the photoinitiation reaction with pigments or UV absorbers), relatively high polymerization shrinkage due to the rapid curing (inducing internal stress, which negatively affects adhesion and mechanical properties), oxygen inhibition effects, and yellowing tendencies are some of the limitations. Until recently photocuring has been associated with niche applications, mainly the curing of solvent-free, low molecular weight acrylate systems for wood and paper coating. Since photopolymerization is now possible in coatings which contain UV absorbers and HALS stabilizers as described by Valet (1996), the whole area of exterior applications, like automotive and industrial coatings, is accessible. Furthermore it has been accepted that photopolymerization is an alternative curing method to thermal curing. As a consequence UV curable water-based and powder coatings, as well as dual cure systems, are being developed in large numbers. Water-based UV curable systems represent a good supplement to 100% UV coatings, since they improve the matting behavior, enable reactive diluent free systems, and may provide high hardness with good flexibility. UV curable powder coatings cure much faster than thermally hardened powders and open a larger process window, since the processes of film formation and cross-linking are separated. After melting of the powder the low viscosity needed for film formation can only be held for a very short time in the case of thermal crosslinking, since the viscosity increases simultaneously. In UV curing, however, this time can be expanded and the UV exposure can be applied at any time. The purposes and chemistries used in dual cure systems are diverse. However, the reason for using dual cure chemistries is always the same, namely compensation of curing in areas where no or insufficient UV light is applied to fully cure the coating. This is, for instance, the case in highly pigmented systems, in shadow areas, in porous parts, or thick layers. Dual cure systems are defined by the fact that the cross-linking reaction takes place in two separate stages, involving the same or different mechanism, whereas hybrid cure systems involve two mechanisms at the same time. Noteworthy are coatings where the first curing mechanism is UV polymerization and the second curing step proceeds at room temperature in ‘‘dark’’ areas, triggered by oxygen or water, for example, relying on the oxidative drying reaction of alkyd or allyl groups, the water-induced curing of isocyanates or siloxane condensation reactions. Furthermore, thermal reactions taking place at higher temperatures, for instance thermal deblocking of isocyanates and subsequent reaction with polyols, are described as the additional curing step.
4.2 Imaging Applications Photocuring is one possibility out of a wide range of photochemically induced reactions applied in imaging technology, e.g., in typical copying processes (electrophotography and diazotyping), in graphic arts applications (diazo systems, colloid-dichromate emulsions, and photopolymer printing plates) and in photoresists (photosolubilizing diazoquinone systems, photoacidcatalyzed systems, azide cross-linking, photodimerization, and photopolymerization). The process relies on applying a coating to a substrate, exposure through the image mask, and subsequent removal (development) of the unexposed (negative) or exposed (positive) parts. Photocurable materials are only used in negative-acting systems (see also Photoresists; Polyresist: Nonspecialty).
(a) Printing plates. Printing plates are classified in the main categories of lithography (offset), letterpress, gravure, and screen printing. Lithographic printing plates are hydrophilic oxidized aluminum plates containing a 1–5 µm light sensitive layer, defined as presensitized plates. The processing results in an image-wise differentiation of hydrophilic and hydrophobic areas. Negative light sensitive layers use, for instance, photoinitiated polymerization of carboxylic group containing acrylate oligomers, which can be developed with alkaline solutions, cross-linking of unsaturated polyesters, or other chemistries reducing the solubility in the exposed areas. In addition, the light-sensitive layers contain dyes in order to provide a good contrast between exposed and unexposed areas. Negative plates are used mainly for newspapers and positive plates for art prints, brochures, and advertising materials. In letterpress negative photocurable systems dominant due to the significantly higher layer thickness compared to lithographic plates, which requires the high quantum efficiency only available with photopolymerizing chain reactions. The formulations of printing plates contain a polymeric binder as a main component, which determines the important application properties of hardness, flexibility, and resistance to printing inks. Binders used in letterpress plates are poly(vinyl alcohol), polyamides, and polyacrylates and for flexographic plates elastomers such as styrene–butadiene and nitrile rubbers. The photoinitiators and cross-linkable oligomers and monomers are predominantly the same as described for radiation curable coatings. UV curing in the graphics art is described in Fouassier and Rabek (1993, Vol. IV, Chap. 7).
(b) Photoresists. Photoresists are used in the production of printed circuit boards (PCB) and inte5
Photoinitiators and Photopolymerization grated chips to define the circuit elements in a chip or PCB. The term photoresist stems from the two functions it has to fulfill, namely to enable a photoinduced generation of pattern, which is used to mask the underlying areas during subsequent image transfer steps, thus, to resist the attack of chemicals. Thompson et al. (1983) and Steppan et al. (1982) give a comprehensive overview about theory, materials, and processing. The types of photoresists are classified by their physical constitution (liquid, dry film), radiation response (x ray, e-beam, and UV), mode of operation (positive\negative), or number of main components (1C, 2C). Contrary to PCB applications where dry-film resists dominate, in integrated chips manufacturing only liquid resists are used. Until recently, negative, twocomponent resists consisting of a cyclized rubber matrix and a bis-arylazide sensitizer, forming nitrenes upon exposure which cross-link the matrix, had more than 50% of the market share. For several years the workhorse of integrated chips manufacturers have been two component positive resists, based on novolacs and a diazonaphthoquinone, which become alkaline soluble upon photo-induced rearrangement to an indene carboxylic acid. These resists respond to near UV radiation and since the required pattern sizes decrease steadily below 0.5 µm shorter exposure wavelength in the deep UV, which prompted higher sensitivity requirements are needed. Thus, completely new resist materials utilizing a ‘‘chemical amplification reaction,’’ based on photoacid generators, evolved. A review of the newest material generations and future challenges has been published by Reichmanis et al. (1999) (see also Photoresists; Photoresists, Specialty; Polyresist: Nonspecialty; Chemically Amplified Photoresists).
(c) Further applications. The latest developments and trends in terms of applications of photocurable systems are listed in Fouassier (1995). They include processes, materials, and applications such as photopolymerizable adhesives, UV curable siloxane coatings for polycarbonates, UV cross-linkable rubbers, photostereolithography, acid- and weather-resistant compositions, nonacrylate UV curable wood coatings, microgels, sol-gel materials for optics, UV curable polyurethane dispersions, aluminum wheel clearcoats, laser initiated polymerization, UV printing on textiles, photocurable colored compositions, and many others. Bibliography Allen N S 1989 Photopolymerisation and Photoimaging Science and Technology. Elsevier, London, New York Davidson R S 1993 The chemistry of photoinitiators—some recent developments. J. Photochem. Photobiol. A 73, 81–96 Fouassier J P 1995 Photoinitiation, Photopolymerization, and Photocuring. Hanser, Munich, Vol. I–IV Fouassier J P, Rabek J F 1993 Photoinitiating Systems. Elsevier Applied Science, London Reichmanis E, Nalamasu O, Houlihan F M 1999 A perspective on lithographic materials: past, present and future challenges. Polym. Mater. Sci. Eng. 80, 301 Schwalm R 1999 Cross-linking effect on mechanical properties of UV curable coatings. Polym. Paint Colour J. 189, 18–22 Steppan H, Buhr G, Vollmann H 1982 Resisttechnik—ein Beitrag der Chemie zur Elektronik. Angew. Chem. 94, 471–85 Thompson L F, Willson C G, Bowden M J 1983 Introduction to Microlithography. ACS Symposium Series 219. American Chemical Society, Washington, DC Valet A 1996 Lichtschutzmittel fuW r Lacke. Vincentz, Hannover, Germany
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 6946–6951 6