Review pubs.acs.org/CR
Strategies to Reduce Oxygen Inhibition in Photoinduced Polymerization Samuel Clark Ligon,† Branislav Husár,†,‡ Harald Wutzel,† Richard Holman,§ and Robert Liska*,† †
Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163/MC, A-1060 Vienna, Austria Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 845 41 Bratislava 45, Slovakia § The Paint Research Association, 14 Castle Mews, High Street, TW12 2NP Hampton, United Kingdom ‡
S Supporting Information *
5.1. 5.2. 5.3. 5.4.
Coatings Ink and Printing Dental and Medical Lithography and Additive Manufacturing Technologies 6. Electron Beam Curing 7. Conclusions Associated Content Supporting Information Author Information Corresponding Author Notes Biographies Acknowledgments References
CONTENTS 1. Introduction 1.1. Summary of Theoretical Background 1.2. Analytical Techniques 2. Physical Strategies for Eliminating Oxygen Inhibition 2.1. Curing in Oxygen-Free Atmosphere 2.2. Physical Barriers 2.3. Permeability and Solubility of Oxygen 2.4. Light Intensity and Wavelength 3. Chemical Strategies for Eliminating Oxygen Inhibition 3.1. Photoinitiators 3.1.1. Type I Photoinitiators 3.1.2. Type II Photoinitiators 3.2. Singlet Oxygen Generators and Scavengers 3.3. Molecular Inerting 3.4. Hydrogen Donors 3.4.1. Amines 3.4.2. Thiols 3.4.3. Silanes 3.4.4. Other Hydrogen Donors 3.5. Other Reducing Agents 3.5.1. Boranes 3.5.2. Phosphines and Phosphites 3.6. Peroxide Decomposition 3.7. Modification to Formulations 3.7.1. Reactive (Meth)acrylates 3.7.2. Multifunctional Acrylates 3.7.3. Hyperbranched Polymers and Dendrimers 3.7.4. N-Vinyl Amides 3.7.5. Alternative Vinyl Monomers 3.7.6. Donor/Acceptor-type Monomers 3.7.7. Hybrid Radical/Cationic Systems 4. Concerns and Associated Problems 5. Applications © XXXX American Chemical Society
A B E F F G G H
X Y Y Y Z Z Z Z Z Z Z AA AB AB
1. INTRODUCTION Photoinduced curing, where curing refers to the irreversible transformation of a liquid polymerizable formulation into a stable solid, has advanced greatly over the last half century.1−3 Photocuring is amenable to low-solvent and solvent-free resinous formulations and is typically faster and less energyintensive than thermally induced processes. It is utilized for decorative and protective coatings of finished and semifinished products such as paper, cardboard, wood panels, textiles, metal and glass automotive parts, and plastic of all types.4−6 Photocuring is applied extensively in the printing industry, for example, in the patterning of flexographic plates or in curing high-quality lithographic inks.7,8 Protective coatings of optical fibers and modern printed circuit boards are other important applications for UV curing. Photocurable dental fillings are quite common and represent only one of a growing number of biomedical applications.9,10 While broad in scope, the expectation of most of these applications is the same: a fluid prepolymer should, in a relatively short time period, harden under the presence of UV or visible light to provide a robust solid polymer. The principal advantage of photocuring is the ability to cure solvent-free formulations within a fraction of a second with high temporal and spatial control, while the primary disadvantage is the relatively high price of the required resins and monomers.
H I I J K L M N N P P P P Q R R R T T T U V V W X
Received: January 9, 2013
A
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propagating radicals (P-M•) to form peroxyl radicals (POO•), which are not energetically favorable toward initiating acrylate polymerization (step b). These peroxyl radicals tend instead to terminate polymerization through radical−radical recombination (step c) (forming peroxide bridges, POOP) or by abstracting hydrogen from an adjacent molecule (step d) (POO• + RH → POOH + R•), where often the newly formed radical (R•) has insufficient reactivity toward the acrylate double bond to reinstate the initiation process. As Scheme 1 illustrates, even after polymerization has been initiated, there is still a possibility for oxygen to react with the propagating polymer chain and hinder further growth. Strategies that reduce oxygen inhibition during the initiation step (step e) are abundant. These include physical approaches such as nitrogen inerting, lamination, and use of higher light intensity in addition to chemical strategies based on, for example, improving the efficiency and effectiveness of light absorption and subsequent radical generation by the photoinitiator. A second class of strategies (steps f and g) includes employment of hydrogen donors (DH) and reducing agents (RA), which can undergo chain transfer with peroxyl radicals to provide more reactive radicals (D• or PO•). A third general class of strategies (step h) tries to reinitiate polymerization by decomposing the hydroperoxides or alkyl peroxides to generate potentially more reactive alkoxy or hydroxyl radicals. Due to the tremendous industrial importance of photocuring, a wealth of scientific publications and patents on strategies to overcome oxygen inhibition have been published,19 including a number of useful short reviews.2,20−28 This paper will extend prior work on oxygen inhibition by providing a compilation of the possible strategies, both in the academic and in the industrial sphere. For further details on a specific strategy, the reader is encouraged to consult associated references. Following an introductory section, which provides the mechanistic and kinetic fundamentals of the role of molecular oxygen on radical photochemistry, solutions are divided into two broad categories. The first category (section 2) includes physical approaches to reduce oxygen inhibition, such as the use of an inerting gas, liquid or wax barriers, and solid cover films, as well as various exposure-related techniques that influence the dose and dose rate of radiation applied to the polymerizable formulation. The second category (section 3) covers chemical strategies that interact with the reaction cycle of molecular oxygen and thus reduce the undesirable effects of oxygen inhibition. Section 4 deals with potential concerns for oxygen inhibition solutions including health and environmental issues, storage stability, and discoloration. Section 5 tries to address some of the special demands imposed by certain popular photocuring applications,29 since the effectiveness of any strategy against oxygen inhibition will be heavily dependent on the prevailing process parameters, such as viscosity, coating thickness,30 light intensity,31,32 and safety regulations33 of the intended application.
These have to be specially functionalized to make them amenable to photochemically initiated curing. The majority of photocurable formulations consist of acrylated or, less commonly, methacrylated resinous materials (multifunctional monomers, prepolymers, and oligomers) and, to adjust viscosity, a portion of reactive diluents. Reactive diluents consist of a wide range of commercially available, relatively low molecular weight acrylated monomers or oligomers and, in some instances, polyester prepolymer resins. Monomers, in addition to modifying viscosity, are used for fine-tuning the mechanical or optical properties of the subsequently cured formulations. While broad in scope of application, all the acrylated materials undergo radical polymerization11 and are thus intrinsically vulnerable to inhibition by molecular oxygen, resulting in an incomplete cure,12 manifested particularly by a tacky upper surface13 or in extreme cases a complete failure to cure.14 Since open-air curing is in most cases the simplest method for industrial processing, the problem of oxygen inhibition is common and is practically universal to free radical photocuring applications. Alternative polymerizable formulation approaches are available that are less sensitive, or insensitive, to oxygen inhibition. These include the thiol−ene systems15 and the cationic photoinitiation of epoxy or vinyl ether resins,16 but these systems do not enjoy the commercial popularity recognized by acrylate-based resins. To understand the problem of oxygen inhibition in photocuring, it is practical to look at an idealized mechanism of photoinduced radical polymerization and the ways in which this can be affected by molecular oxygen (Scheme 1).17,18 First, oxygen may quench the excited state of the photoinitiator (PI) (step a). Second, it may react with primary initiating (R•) or Scheme 1. Mechanistic Pathways of Oxygen Inhibition and Strategies for Mitigationa
1.1. Summary of Theoretical Background
a
Steps: (a) quenching of excited state of photoinitiator, (b) formation of inactive peroxyl radicals from initiating or propagating radical, (c) termination by radical−radical recombination, (d) termination by hydrogen abstraction; (e) initiation stage strategies (i.e., inerting, lamination, light source, photoinitiators, and singlet oxygen scavengers), (f) hydrogen donors, (g) reducing agents, and (h) peroxide decomposition. For added clarity, arrows are color-coded green for polymerization, red for oxygen inhibition, and blue for mitigation strategies.
Photoinduced polymerization begins with a photolabile molecule, the “photoinitiator” (PI), which upon exposure to light of appropriate wavelengths generates a reactive intermediate (for the purposes of this discussion, this intermediate will be a free radical, R•). The photoinitiator should be selected with very good absorption for the wavelength of light used and with high efficiency for B
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Table 1. Rate of Reaction of Selected Radicals with Acrylate Monomer and Molecular Oxygen
a
BA, butyl acrylate; MA, methyl acrylate; MMA, methyl methacrylate; MeCN, acetonitrile.
subsequently forming initiating radicals. The rate of initiation (Ri) will be dependent on both of these factors, as summarized by eq 1: R i = ΦiIabs
The quantum efficiency (Φi) expresses the probability that an absorbed photon will lead to an excited-state species that in turn will result in the generation of an initiating radical. If it is assumed that the initiator molecule is the only species
(1) C
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photoinitiator, ≈10−1 mol·L−1). It should be noted, however, that at the air interface there will be a dynamic process involving absorption and diffusion of oxygen. From a comparison of rates for polymerizations performed under nitrogen and in air, Decker and Jenkins38 calculated the steadystate concentration of oxygen in a model resin system:
absorbing in an incremental thickness (l) of a material undergoing unidirectional irradiation, then the concentration of absorbed photons is proportional to the loss of intensity (Iabs) by the Beer−Lambert equation: Iabs = I0(1 − e−2.3εlc)
(2)
I0 is the intensity of radiation at the upper surface of the absorbing layer, ε is the molar extinction coefficient, l is the optical path length, and c is the concentration of the photoinitiator. For nonmonochromatic light sources, total loss of intensity is determined as the integral of the above equation over the range of utilized wavelengths. These theoretical considerations imply a gradient of decreasing light intensity, and consequently decreasing reactivity, from the surface into the depth of a photopolymerizable coating system. The expectation would be for greatest conversion of potentially polymerizable material to occur at or near the upper surface. However if the upper surface is an air interface, it is here that oxygen effects will be most dominant, and the consequences of this factor will form a significant part of the discussion set out in this paper. Aside from having good absorbance for the utilized light frequencies, a photoinitiator should avoid wasteful excited-state relaxation pathways (thermal relaxation and quenching) that do not provide initiating radicals. Molecular oxygen may react with the initiator by either of two common modes: (1) quenching the excited triplet state [PI]T or (2) scavenging a subsequently generated reactive radical (R•) before it can react with an acrylate functional group from a monomer or resin. Quenching by oxygen is generally a problem only for the so-called type II photoinitiator systems, which generate radicals via a bimolecular reaction (see section 3.1.2 for details). For type II initiators, the triplet state is much longer-lived (ca. 10−6 s) than that of most so-termed type I photoinitiators (ca. 10−9 s), which undergo unimolecular fragmentations.34,35 Hence, excited-state type II initiator molecules have the greater opportunity for a diffusion encounter with dissolved oxygen. Molecular oxygen in its ground state is a triplet biradical, and while it does not readily react with saturated or unsaturated hydrocarbons at room temperature,36 it combines efficiently with either the primary initiating radical R• or with a propagating radical (R-M• or P-M•) and in this respect it is problematic, regardless of the mode of initiation. Oxygen is reported to react very rapidly (kox > 5 × 108 L·mol−1·s−1) with carbon-centered radicals to form peroxyl radicals (R• + O2 → ROO•)37 that do not readily react with the alkenic double bonds of the acrylate monomers; hence, chain polymerization is inhibited. 38 In a typical UV-curable formulation, the reaction of carbon-centered radicals with predissolved oxygen has been shown to be from 2 to several orders of magnitude faster than acrylate initiation (ki ≈ 106− 107 L·mol−1·s−1) or propagation (kp ≈ 103 L·mol−1·s−1). Table 1 provides a list of representative carbon- and phosphoruscentered radicals (stemming from commonly used photoinitiators) and the rates at which they react with acrylate monomer and with oxygen. Concentration of oxygen [O2] (typically on the order of 10−3 mol·L−1 in a well-aerated resin) must be lowered to a level where rate of polymerization becomes competitive (in other words, to kox[O2] ≈ kp[M], where [M] is monomer concentration).38 Reduction in O2 concentration is generally achieved by radicals generated in the early stages of photoinitiation (in a typical formulation with ca. 2 wt %
[O2 ]s =
(R p)O2 ⎤ (R ik t)1/2 ⎡ (R p)N2 ⎥ ⎢ − (R p)N2 ⎥⎦ kox ⎢⎣ (R p)O2
(3)
By application of a termination rate constant (kt) of 3 × 106 L·mol−1·s−1, a concentration [O2]s of 4.2 × 10−6 mol·L−1 was determined for polymerizations performed with a mediumpressure Hg lamp. With a focused argon laser, light intensity was increased by 2 orders of magnitude; however, [O2]s was only doubled. This implies that O2 diffusion plays little to no role during the first 5 ms of a curing reaction (the induction period). Thus, under relatively high intensity irradiation, it is primarily the predissolved oxygen that gives rise to an induction period. Also, a dependence of less than unity of induction time on reciprocal light intensity implies some form of chain peroxidation is occurring. Thus, at short time scales, multiple oxygen molecules are consumed for each initiating radical.38 As the dissolved oxygen concentration is reduced, polymer chains can propagate and diffusion of oxygen from the atmosphere then becomes the problem. These factors lead to uneven curing, where double-bond conversion for the bottom of the resin is generally higher than that near the top surface in direct air contact. Cure on the bottom surface can, of course, be incomplete in pigmented coatings, which are dominated by scattering or absorbance by the pigment, or in thicker samples, when photobleaching of the initiator is insufficient. Such systems often exhibit a differential cure gradient due to the exponential fall-off in light intensity with depth. While the aforementioned mechanisms reasonably describe radical chain growth of linear polymer (Pn) from monomers with one functional group, most photocurable coating and printing ink formulations are based on a blend of multifunctional monomers and resins that rapidly cross-link to form networks.47,48 Cross-linking affects the rates of propagation, transfer, and termination. At low conversion, termination by radical recombination is diffusion-controlled and autoacceleration is observed. Then, as the much more mobile monomer becomes depleted and the remaining available acrylate groups become sterically restricted, autodeceleration is observed. This situation is additionally complicated in the presence of oxygen, since peroxyl radicals do not readily reinitiate polymerization but may terminate by mechanisms as described, for example, by Russell.49 Termination by any of the typical modes of radical recombination is dominant for polymerizations performed at room temperature. At elevated temperature, disproportionation becomes more common and decomposition of any peroxides becomes probable. It should be noted, however, that the stability of simple dialkyl peroxides or hydroperoxides is comparatively high, with 10 h half-life temperatures of approximately 130 and 170 °C, respectively. Carbon-centered radicals that avoid termination, deactivation by peroxide formation, or occlusion in the polymer matrix continue to add to available acrylate groups and so grow the network. Ultimately, the reaction ceases as all available polymerizable functional groups become either depleted or occluded. This explains why, even under optimal conditions (inert atmosphere and long exposure time), a residual concentration of unreacted D
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acrylate moieties will inevitably persist in the apparently “fully cured” system. Both dissolved and absorbed oxygen are a serious challenge to curing of ultrathin coatings in air, especially if low-intensity sources of UV are used and if the formulation is low in viscosity. Such circumstances may result in extremely low double-bond conversions. Models for predicting rates of reaction and double-bond conversion in photopolymerizations have been developed and adapted to account for oxygen inhibition. Goodner and Bowman50 treat the curing monomer 2-hydroxyethyl methacrylate (HEMA) in thin slices and describe heat and mass transfer through the system. Diffusion of mobile species, including monomer and oxygen, is described by eq 4: Di = Di 0e−A i / f
limited. At approximately 30% conversion, propagation becomes diffusion-controlled and both kt and kp decrease dramatically. While these approaches are quite powerful for modeling polymerizations in inert atmosphere, the rate equation must be modified for polymerizations conducted in air, where transfer to oxygen becomes the primary mode of termination:53 ⎛ Ri ⎞ R p(air) = k p[DB]⎜ ⎟ ⎝ kox[O2 ] ⎠
Thus rate of polymerization is dependent on rate of initiation, that is, Rp ∝ Riα; and while α is equal to or less than 0.5 for polymerizations in inert environment, rate is directly proportional to initiation for radical polymerizations in air (α = 1). The aforementioned models were applied to thin films applied on both thermally insulating and thermally conductive surfaces with oxygen flux from the top surface, as should be expected in most coating applications. To model the effects of o x y g e n o n p o ly m e r iz a t io n s p e r f o r m ed i n p o ly (dimethylsiloxane) (PDMS) microchannels, where inhibition occurs symmetrically at both the top and bottom walls, Dendukuri et al.54 relate time (t) to oxygen diffusivity to provide the nondimensional term τ. Oxygen concentration relative to equilibrium provides nondimensional concentration Θ:
(4) −2
2 −1
Diffusion (Di) (e.g., 5 × 10 cm ·s for O2 in ethyl methacrylate) is defined by the pre-exponential factor (Di0) with dependence on fractional free volume ( f) by the speciesspecific rate parameter for initiation Ai. Free volume affects propagation and termination in isothermal systems48 by eqs 5 and 6: kp =
kt =
k p0 1+e
A p( 1f − f1 ) cp
(5)
k t0 1+
τ=
1 R dk p[M] k t0
+e
A t( 1 − 1 ) f fct
(6)
Θ=
The critical fractional free volume at which the kinetic constant for reaction (kp or kt) is equal to that for diffusion (kd) is defined for propagation by fcp and for termination by fct. In environments where fractional free volume is less than fc, the system will be under diffusion control, while if free volume is greater than fc, chemical reaction (radical addition) will be the rate-limiting step. The critical fractional free volume value for monomer reacting with the small molecule oxygen is approximately a fifth of the corresponding free volume requirements for monomer initiation, propagation, and termination. The relative importance that free volume has on a reaction is defined by the rate parameter A [values are given for transfer to oxygen (0.29), HEMA propagation (Ap = 0.66), and termination (At = 1.2)]. The model allows cross-linking so that, even at low conversion, polymer mobility is essentially zero. To allow for measurement of propagation, initial oxygen concentration was set to the steady-state value of 10−6 mol·L−1 (3 orders of magnitude lower than ambient). This model was extended to take advantage of chain-length-dependent termination (CLDT),51,52 which predicts the rate of polymerization from double-bond concentration [DB] with a scaling component α less than 0.5 (the value predicted for polymerizations dominated by termination by radical recombination): ⎛ R ⎞α R p(inert) = k p[DB]⎜ i ⎟ ⎝ 2k t ⎠
(8)
tDox H2
(9)
[O2 ] [O2,eq ]
(10)
Here Dox is diffusivity of oxygen in the oligomer (2.84 × 10−11 m2·s−1), H is height of the channel (10−60 μm), and [O2,eq] is the equilibrium concentration of oxygen in the oligomer (1.5 × 10−3 mol·L−1). Since diffusivity of oxygen in PDMS is 2 orders of magnitude greater than in the monomer, oxygen is continuously replenished at the walls and Θ = 1. In the models, Θ decreases with time, with a minimum in the center of the channel, until a critical value (Θc ≈ 0.001) is reached and gelation can commence. This occurs at τ = 0.045 in the center of the channel and at progressively greater τ values farther from the center. The model accurately predicts complete lack of polymerization at the channel walls and indicates that resolution of objects cured in microfluidic devices can be improved with higher photoinitiator concentration and higher light intensity but not with longer exposure time. 1.2. Analytical Techniques
The light-absorbing characteristics of photoinitiators are determined by UV−vis spectroscopy. Flash photolysis and fast detectors have allowed monitoring of the decay from the excited state to determine important parameters such as quantum efficiency, rate of quenching, and mechanism for generating the initiating species.44,55 Short-lived radical intermediates have been monitored by time-resolved EPR (electron paramagnetic resonance) and CIDNP (chemically induced dynamic nuclear polarization) experiments to provide information on lifetime and molecular geometry.39,56 Direct spectroscopic monitoring of carbon- and oxygen-centered radical intermediates can be difficult, as they tend to have spectroscopic absorbance bands close to those of the parent photoinitiator. An important exception is the peroxyl radical
(7)
The CLDT model predicts diffusion control of the termination kinetic constant so that, at 1% conversion, kt for propagating monomer radicals (R-M•) is 30 times greater than kt for two macroradical chains with more than 10 000 units. The model works well at low conversions and explains autoacceleration, since propagation remains non-diffusionE
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generated by reaction of molecular oxygen with an αaminomethylene radical, which itself has resulted from hydrogen abstraction of a tertiary amine (see Scheme 12 in section 3.4.1). Lalevée et al.57 investigated a series of such alkylaminoperoxyl radicals with absorbance generally in the region of 400 nm. Solvent effects on excited-state energy transfer can be probed by the Stern−Volmer relationship in order to quantify quenching efficiency.58 Kinetic information pertinent to photoinitiated polymerization has been acquired experimentally via a range of techniques. However, in radiation curing technology, two methods predominate: photo differential scanning calorimetry (photo-DSC) and real-time infrared spectroscopy (RTIR). In the first method, a small sample of a curable formulation (e.g., ≈10 mg) is exposed to UV−vis radiation and the heat of reaction is measured with respect to time. It should be kept in mind that this weight, in a typical sample chamber, corresponds to a film thickness of between 100 and 200 μm, far thicker than the film thickness usually found for an industrial curable coating, although still relevant to other applications (dental fillings or potting compounds). Important parameters such as induction periods (due to inhibition by resin stabilizers as well as by dissolved oxygen), rate of reaction, and extent of conversion may all be determined. For an enthalpy−time profile, the exotherm value (milliwatts per gram) is proportional to the instantaneous rate of reaction, where peak height correlates with maximum rate. A comparatively accurate method for determination of the theoretical heat of polymerization was developed by Hoyle et al.,59 and results were verified with ATR-IR (attenuated total reflectance infrared spectroscopy).60 If it is assumed that the total heat evolved predominantly arises from the heat of polymerization of acrylate groups (80.6 kJ·mol−1), then it is possible to estimate the extent of double-bond conversion. As an advantage, DSC instruments normally provide gas inlets, allowing reactions to be monitored in defined atmospheres. For instance, heat generated from photoinduced polymerization of 1,6-hexanediol diacrylate (HDDA) has been measured under nitrogen (378 J·g−1), in a 1:1 mixture of nitrogen and oxygen (268 J·g−1), and in oxygen alone (170 J·g−1).61 A number of difficulties have been encountered with photoDSC. In contrast to thermally induced reactions, the initiating radiation is typically applied directionally to the top of the sample. This can lead to errors, particularly if the sample viscosity is low and a significant meniscus effect has occurred. Furthermore, the radiation dose, dose rate, and dominant wavelengths are often distinct from those used in industrial UV curing. For example, the light intensity applied in the calorimeter is usually relatively low and lacks IR radiation, factors both known to have an important impact in industrial curing lines. To account for the important parameter of sample thickness, Roper et al.62 constructed a calorimetry device to analyze films with thicknesses as low as 5 μm. As expected, for experiments carried out in air, both the rate of reaction and the reported double-bond conversions were much lower in the thinner samples, which was assumed to be due to oxygen inhibition effects. The precision of photo-DSC is, however, somewhat limited by the slow response of most detector systems. Reproducibility may be poor, particularly if insufficient attention is given to sample preparation. By use of IR spectroscopy, double-bond conversion (DBC) can be determined (as 1 − [DB]t/[DB]0) by monitoring the
disappearance of one or a combination of acrylate-specific absorption peaks (i.e., 810, 1410, 1620, and 1640 cm−1). The peak at 1410 cm−1 is less frequently used due to overlap, and the peak at 1640 cm−1 may be inaccurate in formulations with initiators whose carbonyl absorption lies in this region (e.g., thioxanthones). For quantitative IR experiments, peak area rather than height is generally considered to be more representative of the concentration of the absorbing entity. For improved accuracy, especially with overlapping signals, deconvolution software is recommended.60 Transmission-mode IR may be used with transparent samples and ATR-IR can be used to analyze the surface increment (first 1−2 μm) of a sample. The transmission and ATR techniques can be complementary since the former measures the total, throughcure conversion while the latter is confined to the near-surface conversion. For samples that can be easily removed from their substrate, ATR can also be used to measure DBC at both the upper and lower surfaces. Fast-responding MCT (mercury−cadmium−telluride) detectors allow reliable collection of multiple scans per second, permitting real-time determination of DBC.63 RTIR spectroscopy is generally performed in transmission mode, although it may also be coupled with ATR.64 IR methods tend to be preferred to photo-DSC because they more realistically simulate practical conditions of photocuring and tend to have better reproducibility. Other analytical techniques that have been used to study the extent of cure include spectroscopic methodsRaman,65,66 near-IR,67 photoacoustic spectroscopy, and fluorescence68,69 and confocal microscopy, which has been used to nondestructively determine DBC as a function of layer thickness.70 While they are related to DBC, the surface and bulk mechanical properties of the cured formulation are difficult to quantify by the above-mentioned spectroscopic methods. For this reason, and particularly for industrial applications, certain physical methods tend to be more relevant: pendulum hardness, scratch resistance,71 rheology,72 shrinkage measurement,73 and resistance to stains and solvents.74 Pendulum hardness, based on the method of König, is in fact very commonly used in industry, as it is robust and simple and commercial devices are relatively inexpensive. In the method, surface hardness is expressed as the time required to dampen the swing angle of an oscillating pendulum supported on stainless steel bearings resting atop the coating surface.75 Samples more affected by oxygen inhibition will be softer and tackier at their surfaces and will retard the rotational motion of the ball bearings more effectively, thus exhibiting lower hardness values.
2. PHYSICAL STRATEGIES FOR ELIMINATING OXYGEN INHIBITION 2.1. Curing in Oxygen-Free Atmosphere
One of the oldest and most widespread strategies to prevent diffusion of oxygen from the atmosphere into the polymerizing resin is blanketing with an inert gas,76−78 which is typically nitrogen. Curing under inert gas can improve performance while allowing a reduction in the photoinitiator concentration by an order of magnitude.79,80 Inerting is an easily feasible method under laboratory conditions; however, it becomes more difficult and expensive on an industrial scale. UV units require an appropriate design that include a nitrogen distribution system, quartz windows to retain the inerted F
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Table 2. Gas Permeability of Some Common Polymers permeability coefficient (× 10−10 cm·s−1) polymer
O2
N2
CO2
ref
poly(dimethylsiloxane), PDMS poly(tetrafluoroethylene), PTFE polyethylene, LDPE poly(methyl methacrylate), PMMA poly(ethylene terephthalate), PETE poly(vinyl alcohol), PVA
605 4.9 2.89 0.154 0.03 0.00052
300 1.33 0.97 8.18 0.001 0.00045
3240 12.7 12.6 3.10 0.029 0.0005
103 103 103 104 103 103
Photopolymerization carried out on resin submerged beneath a clear liquid has been suggested.92 This method may be inappropriate for an industrial-scale continuous coating line but has been used for the production of thick individually cured items, for example, flexographic printing plates. Another possibility to prevent oxygen diffusion is the addition of a wax (up to 10 wt % in a formulation) that floats on the surface of the coating prior to curing.93,94 Wax barriers are limited by a ceiling temperature, above which they fail to eliminate oxygen ingress. The ceiling temperature may be increased by choosing a wax additive with higher melting point, but this is often at the expense of compatibility with the rest of the formulation. Combination of these strategies has also been tested and was shown not to bring any significant improvement, as no quantitative difference in either final conversion or rate of polymerization was observed between resins cured under inert atmosphere and those cured with a wax barrier coating.93 Difficulties in postmodification of the less-adhesive surface by sanding or polishing may make the wax approach unattractive, although better interfacial adhesion properties have been achieved by using fatty alcohols or fatty acids instead of wax.95 Inerting by the wax method has found application mainly in UV-curable printing inks.96−99 Packaging ink formulations in particular have traditionally contained waxes to prevent interfacial scuffing, and as an added benefit they are less prone to oxygen inhibition. Inerting can also be achieved simply by covering the surface with a UV-transparent film that prevents oxygen ingression and can be peeled off after curing.100−102 This method is obviously not practical for all applications. The permeability of the chosen polymer to molecular oxygen (Table 2) may also be of relevance, particularly in thin films.
area, cooling systems, sealing to minimize gas consumption, residual oxygen analyzers, and an exhaust for gases.81,82 The increased costs for storing the nitrogen supply, continuous nitrogen consumption, and equipment costs per UV lamp unit may be compensated in part by faster throughput, reduction in photoinitiator, and improvement of the coating’s quality. Significant savings in curing of 3D objects can be achieved by a blanketing approach instead of curing in an inert room.80 UVinerted production lines exist in nearly all fields of application, including furniture,83 poly(vinyl chloride) (PVC) flooring,70 silicone release coatings,84 and others.85,86 Indeed, because oxygen absorption and permeation is greater for resins and oligomers with silicone acrylates (see Table 2), inerting is essential for obtaining consistent product performance for such items as UV-cured silicone release papers.87 In order to remove the oxygen from the system more effectively, a prepurging stage may be necessary. Prepurging is especially relevant to very thin (
