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Journal of Coatings Technology and Research

, Volume 16, Issue 6, pp 1527–1541 | Cite as

NIR LEDs and NIR lasers as feasible alternatives to replace oven processes for treatment of thermal-responsive coatings

  • Christian SchmitzEmail author
  • Bernd Strehmel
Article

Abstract

Near-infrared (NIR) laser sources (800–1000 nm) can potentially reduce the processing time for curing by a fast heating and incorporation of NIR absorbers into the coating. The latter converts NIR laser light absorbed into thermal energy. This curing technique was successfully applied to one-component thermoset coatings based on blocked polyisocyanate/hydroxy-polyester and melamine formaldehyde/hydroxy-acrylate resins with heptamethine cyanines as near-infrared absorbing material. The laser curing was additionally compared with LED sources. In general, the curing time significantly decreases in comparison with traditional heat sources. Furthermore, the photopolymerization of acrylates or epoxides can be induced simultaneously by adding suitable initiators due to photochemical generation of radicals and cations. Curing of the thermoset resin system and the photopolymerization process created interpenetrating networks. Principally, the techniques reported based on photonic NIR sources may help to substitute oven techniques where thermal activation of curing reactions is typically induced by oven or heating with infrared radiators for coating applications.

Keywords

Laser curing Near-infrared light Thermoset coating Light-emitting diode Thermal radiation Radiation curing 

Introduction

The thermal curing of coatings for many industrial applications uses ovens as heating source requesting temperatures above 120°C. The benefit of these coating systems in comparison with others, which cure under ambient conditions, can be seen in their higher crosslinking density due to the higher mobility within the coating matrix at these temperatures. This may improve mechanical properties and chemical resistance. A higher glass transition temperature (Tg) would be an additional benefit.1,2 In contrast to two-component coating materials, the blocked crosslinking reaction at room temperature conditions ensures a certain shelf life. Though thermosetting reactions are already common in coating processes applying several resin combinations,1 the reduction of curing time and energy waste has aroused many interests in different application fields. Particularly, coated work pieces of bigger size extend the curing time due to a slower heating of the surrounding depending on the heat capacity of the substrate.3,4 Furthermore, the thermal stability of the substrate limits the application temperature for the baking process. This might be important for temperature-sensitive substrates.

In some applications, infrared (IR) radiators successfully replaced ovens applying a source providing for exposure a wavelength range from red light up to several thousand nanometers. This manufacturing method achieves a higher throughput5 and milder conditions with no thermal damage of the work piece.3 The absorbed IR radiation transfers the energy into heat, thereby inducing the thermal crosslinking of the coating. This occurs upon activation, which is the excitation of vibrations, and deactivation, resulting in the generation of thermal energy.6,7 Nevertheless, the substrate usually also absorbs in the IR region and warms up during exposure. The IR radiator usually emits continuously over its entire available wavelength range.6,8 However, only the region with an overlapping region of the absorbance of the coating material and the emission contributes to the transformation of energy, that is, the conversion of radiation into heat. This example shows the wasting of energy because only a small fraction would be converted. However, such IR radiators can be seen as more efficient compared to ovens.

IR light sources exhibiting a narrow emission range and absorbers embedded in the coating can be seen to work with more selectivity to absorb IR excitation energy. This provides an element to control the curing reaction with higher precision as the aforementioned IR sources. The absorbers embedded into the coating system provide the possibility for a local response of the exposed region if absorbance of the substrate and heat dissipation into the surrounding can be minimized.9 Semiconductor materials allow the fabrication of LED and diode laser devices with half widths of 40 nm10 or monochromatic light, respectively. Near-infrared (NIR) LEDs can be used as array architectures delivering intensities about 1.5 W cm−2 or more. Nevertheless, their emission occurs in a wide divergence angle. This affords small distances of the LED light source to the work piece for an efficient heat transfer.

NIR diode lasers can be seen as a more powerful source for near-infrared light, while their divergence of the beam appears as low.11, 12, 13, 14 Optical optimization of the beam improves the homogeneity of the intensity distribution and the resolution of the laser spot, making this laser technology interesting for industrial thermal applications15, 16, 17, 18, 19, 20, 21, 22, 23 and structuring of lithographic printing plates by photochemical reactions induced with NIR initiators.9,24,25 Two-dimensional surfaces can be exposed by modulation of the light source into nearly linear top-hat distribution of the intensity profile.26, 27, 28, 29 This line-focused laser has already been introduced for the thermal drying of waterborne printing inks.30,31 Thus, this laser light technology depicts an alternative for ovens or IR radiators applied for curing of thermoset coatings.32

This contribution shows the application of either NIR LEDs or diode lasers for the thermal curing of thermoset resins approving the feasibility of this technique. Since the chosen absorbers can also work as a photoinitiator for polymerization, the latter complements dual combination of thermal and radiation curing.

Background

The heating process with NIR radiation occurs as a result of interaction between radiation and the exposed material. As opposed to heating a material in an oven, convection plays no role in the energy transfer between the heat source and the work piece. Thus, heating rates on the surface with NIR exposure can be much higher compared to oven processes. The curing reaction starts with absorption of radiation (equation 1) of an absorber (Abs) followed by deactivation processes such as fluorescence (equation 2) and internal conversion (equation 3) of the excited state (Abs*).6 The latter converts the excitation energy to heat available for curing. Fluorescence occurs with lower efficiency.33
$${\text{Abs}}\mathop{\longrightarrow}^{hv}{\text{Abs}}^{*}$$
(1)
$${\text{Abs}}^{*} \to {\text{Abs}} + hv^{\prime }$$
(2)
$${\text{Abs}}^{*} \to {\text{Abs}} + \Delta$$
(3)
The output of blackbody IR emitters shows a relatively broad wavelength range between the visible red light up to several thousand nanometers (Fig. 1), which facilitates their use for thermal applications without integration of tailor-made absorbers.6 Narrow-band light sources such as LEDs or lasers emitting in the NIR require introduction of absorber components into the coating. The benefit can be seen in a selective absorption of the released radiation within the coating material and high yields of heat transfer.7
Fig. 1

Emission spectrum of the IR radiator compared with the spectra of the high-power LED with the emission maximum at 805 nm and the laser light sources at 808 and 980 nm

Heptamethine cyanine absorbers (e.g., S1, S2, S3, S4 in Scheme 1) can absorb radiation depending on the structural pattern between 700 and 1200 nm. Particular structural variations possess a deep impact on sensitizer properties such as absorption range, fluorescence quantum yield, and solubility in the coating. The latter determines their practical use since the polarity of coatings varies in a wide range, and therefore phenomena such as aggregation of absorber molecules substantially change the photophysical properties.34, 35, 36 This might lead to the case that an absorber could be impractical. Furthermore, S2, S3, and S4 exhibit a fluorescence quantum yield between 5% and 15%,37 which means that between 95% and 85% of the absorbed radiation would be released as heat in the deactivation process of the absorber. Thus, these materials are a good choice regarding heat transfer from light sources emitting around 700–850 nm. S1 exhibits almost complete radiationless deactivation. From our best knowledge, photophysical processes based on triplet states have not been reported for S1S4 yet.38
Scheme 1

Molecular pattern of the NIR absorber (S1S4), the initiators for radical and cationic polymerization (IS-NTf2, IS-PF6), and the monomers (TMPTMA, TPGDA)

Another aspect is the addition of initiator (In) forming together with the absorber of the NIR photoinitiator system. The initiator can be selected from acceptor-type molecules, namely iodonium salts or triazines. This gives the possibility for its use in radical and cationic photopolymerization. The photoexcited absorber and the initiator undergo an electron transfer. In this case, the absorber works as a sensitizer (Sens) for the reduction of iodonium salts or triazine resulting in bond cleavage and therefore initiating radicals. It is well known that this cleavage can be induced with UV light39,40 or shifted to longer wavelengths in the visible light41, 42, 43, 44, 45 and NIR46, 47, 48, 49 with the absorber acting as a photosensitizer after excitation (equation 4). In the sensitized reaction, an electron is transferred (equation 7) from the excited absorber (Sens*) to the initiator and the decomposition (equation 8) of the reduced initiator (In−·) results in radicals. Additionally, the sensitized photoreaction generates acidic cations by the decomposition of Sens (equation 9). Deactivation processes (equations 5 and 6) compete with the electron transfer reaction.
$${\text{Sens}}\mathop{\longrightarrow}^{hv}{\text{Sens}}^{*}$$
(4)
$${\text{Sens}}^{*} \to {\text{Sens}} + hv^{\prime }$$
(5)
$${\text{Sens}}^{*} \to {\text{Sens}} + \Delta$$
(6)
$${\text{Sens}}^{*} + {\text{In}} \to {\text{Sens}}^{ + \cdot } + {\text{In}}^{ - \cdot }$$
(7)
$${\text{In}}^{ - \cdot } \to {\text{radicals}} + {\text{products}}$$
(8)
$${\text{Sens}}^{ + \cdot } \to {\text{cations}} + {\text{products}} .$$
(9)

This makes the initiation of (meth-)acrylate or epoxide monomers responsive to NIR radiation provided by lasers33,50, 51, 52 or LEDs.37,50 A combination of the photophysical transfer into heat and a photochemical-induced polymerization raises the attraction of NIR application for coating industry. We already reported a NIR powder coating, which was cured by a NIR diode laser with line-shaped focus, showing that this might be an alternative to replace ovens.51,52 Other reports are known for a similar thermal initiated radical polymerization using liquid monomers.53 In this contribution, we describe the formation of interpenetrating networks by thermal curing of the oligomeric resin and photopolymerization of monomers acting as reactive diluents substituting volatile organic compounds.

Experimental

Materials and preparation of coatings

The blocked polyisocyanates with caprolactam, Desmodur BL1265/1MPA/X (CSC Jäcklechemie), and with diethyl malonate, Desmodur BL3475 BA/SN (CSC Jäcklechemie) as the blocking agents, the polyester resin Desmophen 651 MPA (CSC Jäcklechemie), the acrylate resin Macrynal SM500/60X (Allnex Group) and the melamine formaldehyde resin Luwipal 044 (BASF) and the epoxy resin Epikote 357 (BASF) were provided as a gift and used without further treatment. Tripropyleneglycol diacrylate (TPGDA/Alfa Aesar) and trimethylolpropane trimethacrylate (TMPTMA/ALFA Aesar) were purchased and separated from stabilizers by column chromatography with basic aluminum oxide (Carl Roth). The near-infrared absorber S1 (S0991), S2 (S0507), S3 (S2265), and S4 (S2025) as well as the iodonium salt IS-NTf2 (S2430) were received from FEW Chemicals. IS-PF6 was purchased from Aldrich. Ethanol (VWR Chemicals) was used for reducing the processing viscosity of the coatings and preparing a proper dissolution of the NIR absorber/initiator system. Para-toluenesulfonic acid (98%, Alfa Aesar) was used as an acid catalyst.

The mixtures of the baking varnishes based on the blocked polyisocyanate–polyester resin in combination with TMPTMA or TPGDA were exposed with near-infrared light in the absence of any organic solvents. The remaining solvents from the basic resins (methoxypropyl acetate, xylene, solvent naphtha, and butyl acetate) were removed as far as possible by vacuum distillation at 80°C and 10 mbar.

LED exposure

The coating samples were applied by drawdown with a thickness of 200 µm on a glass or transparent and colorless polystyrene and dried under room temperature conditions for 10 min. In the case of the solvent-free mixtures for the IPNs, a thickness of 80 µm was chosen. The substrate was fixed with clamps not contacting the laboratory bench, which could result in an exchange of heat between the substrate and the surrounding. Over a distance of 30 mm, the LED (prototype from Phoseon) emitted an intensity of 1.2 W cm−2 with a wavelength maximum at 805 nm as a line-shaped focus with the width of 12 mm and a length of 80 mm.

Laser exposure

The samples were applied by drawdown in the same manner as described in LED exposure. The coatings were exposed with laser light at 808 nm and 980 nm using the diode laser modules CNI-808-10000-5-FDA (Laser 2000) and CNI-980-25W-5-FC-A1-TTL1-MM400SMA (Laser 2000), respectively. The laser light passed a diffuser (ED1-S20-MD from Thorlabs) scattering the light. The resulting beam has an area of 1.8 cm−2 and an intensity of 5.1 W cm−2 for the 808 nm laser and 3.1 cm−2 for the 980 nm laser. The intensity of the 980 nm laser can be tuned within the range of 0.1–8.4 W cm−2 controlled by a power sensor (S350C + PM400 from Thorlabs). The sample was moved in one direction with a linear motor with a speed of 2–27 mm s−2. As a result, the sample was cured within a line with a width in relation to the diameter of the focus and an exposure time in correlation with line speed and focus size. Some samples were also irradiated with a laser with 200 mm line focus and a focus width between 2 mm and 8 mm showing the scalability of the laser heating process.

Bleaching

The absorber and initiator were dissolved in a mixture of Desmophen 651 and ethanol. The dried film of 30 µm was irradiated with LED or laser light. The polyester resin does not crosslink and is still soluble in organic solvents after irradiation. A sufficient amount of the sample was dissolved in 5 mL ethanol, and the absorbance was recorded with a Cary 5000. The absorbance was given in relation to the starting mixture, and kinetic parameters were calculated by the estimated concentration of the absorber using a first-order kinetic plot.

Mechanical properties

The pendulum damping test shows the combined properties of the tackiness onto the surface and the hardness of the coating. A more adequate curing of the coating results in a high number of oscillation. The test was executed as described in DIN EN ISO 1522:2007.58

Stress relaxation tests were performed on free films of the cured coating detached from the polystyrene substrate. The stress was monitored at constant strain (1.0%) of a length between 8 and 10 mm, thickness of 50 µm, and a width between 3.5 and 5.0 mm. The stress after 10 min of elongation at 35°C was used for comparing the mechanical durability.

The glass transition temperature of the cured films was determined by dynamic mechanical analysis of free films with a dimension as described at the stress relaxation tests. The frequency was 1 Hz using a heat rate of 3 K min−1 and amplitude of 15 µm.

The Scratch hardness was tested with equipment from Erichsen (Model 318). In contrast to the usual procedure, the coating was scratched down to the substrate and the required force was recorded.

Gel content

Soxhlet extraction with acetone (VWR Chemicals) as the solvent was used, determining the gel content of the cured polymer films. The extraction was conducted over a period of 6 h, and the solid gel and sol content was gravimetrically determined after drying under vacuum conditions (40°C/10 mbar).

Heating curve

The temperature of the exposed films was monitored with a resistance thermometer (88306 K-IEC from Omega Engineering) comparing the heating at different absorber concentrations. The tip of the thermometer was immersed into a 200-µm-thick coating of Desmophen 651.

LED light-induced thermal curing

The tested baking varnishes comprise either a blocked isocyanate/hydroxy-functional polyester resin blend or a hydroxy-functional polyacrylate/melamine formaldehyde resin blend. Organic solvents were added to adjust the viscosity for application on the substrate. The reaction of the isocyanates occurs at elevated temperatures by deblocking the blocking agent followed by the polyaddition of the free isocyanate group with the hydroxy-polyester (Desmophen 651 MPA) used in a 1:1 ratio of hydroxy-functional and blocked isocyanate groups. The curing temperature relates to the stability of blocked isocyanate. Resin types with diethyl malonate (Desmodur BL3475 BS/SN, NCO-Mal) and caprolactam (Desmodur BL1265/1 MPA/X, NCO-Cap) were chosen for the tests requiring a curing period of 15 min with an oven temperature around 140 and 175°C, respectively, confirmed by testing the relaxation modulus in tensile mode (Fig. 2).
Fig. 2

Relaxation modulus plotted against time of the coating systems after oven and LED (1.2 W cm−2) curing using S2 (15 mg g−1) as the absorber. left: NCO-Cap middle: NCO-Mal right: Acryl-Mel

According to the deblocking temperature given in the literature,54,55 an oven heating of 140°C needed longer time (> 45 min) for vitrification of the resin blend NCO-Cap. Therefore, it was not displayed in Fig. 2. On the other side, increasing the temperature to 175°C improves the mechanical resistance of the NCO-Mal film and accelerates the reaction rate.

The acrylate/melamine formaldehyde-based coating (Acryl-Mel) reacts upon (trans-)etherification resulting in either ether or ethylene links resulting in release of the etherifying alcohol or formaldehyde. Primarily, the acid catalyst influences the curing temperature of such a system, which was para-toluenesulfonic acid. Addition of 5.0 wt.% acid catalyst to the nonvolatile content reduces the process conditions to 120–140°C within 15 min, which can be considered as sufficient curing (Fig. 3). Hence, the sulfonic acid catalyst is not blocked with an amine, the conversion of the coating already occurs under lower temperature conditions, and the storability at room temperature is only possible for several days.
Fig. 3

Heating curve of the LED exposure (I = 1.2 W cm−2) from coatings containing S2 as the absorber and oven heating under circulating air condition at 140°C

Alternatively, the curing can be induced by heat by the release of NIR radiation provided by NIR LEDs in combination with respective absorbers. Temperature can rise up to 150°C after adding S2 with a concentration of 15 mg g−1 (Fig. 2). In the absence of the absorber, the average temperature ended at 62°C. Thus, interaction of the matrix also causes radiationless deactivation processes, which would be mainly vibrations. They are necessary to transfer energy. As the heat transfer with radiation is more direct compared to oven heating at 140°C, the heating rate of the coating material is faster as can be seen from the slopes of the curves depicted in Fig. 2. By noting that the heating rate is reduced as object and oven temperature converge, the oven temperature needs to be set higher than the desired object temperature for sufficient curing. The maximum temperature generated by exposure to LED radiation is limited by the equilibrium between energy input and cooling by the heat exchange with the surrounding. In the case of the absorber S2, a decrease in the temperature after 200 s was noticed as the absorbance decreases. Bleaching of S2 explains these observations, which will be described in more detail later.

Furthermore, the acceleration of the curing process with LEDs exceeds the difference of the heating rate. As Fig. 2 shows, the curing of the coating systems NCO-Mal, NCO-Cap, and Acryl-Mel is at least three times faster compared to the oven process, resulting in solidified films with equal (ERelax) relaxation results. Mainly, this is due to a longer induction or warming-up period under oven conditions, which can be seen by the data below 50 MPa within the first 200 s of the oven time. At the beginning of the LED curing, the mechanical resistance increases immediately without any induction period correlating with the fast heating rate. The maximum ERelax is equal to the oven curing at 140°C, but it does not exceed values achieved at 175°C. This originates from the lower maximum temperature limiting the progress of the curing reaction. An increase in absorber concentration improves the conversion and network density of the polymer even outperforming the final properties of the oven-cured films at 175°C (Fig. 4). Addition of 45 mg g−1 of S2 to NCO-Mal makes this result accessible after 3-min LED exposure. This result clearly demonstrates the feasibility to replace oven processes by such photonic driven systems.
Fig. 4

Relaxation modulus of NCO-Mal after LED exposure with LED at 805 nm (I = 1.2 W cm−2) with S2 as absorber varying the concentration (30, 35, 45 mg g−1) and the reference sample being cured at 175°C in a convection oven

The pendulum damping (Fig. 5a) relates to the hardness and the tackiness of the film. So, it shows that beneath similar mechanical properties of the LED cured films, there is no difference in the curing of the surface without tackiness either. Additionally, the gel content in Fig. 5b does not differ between a sample cured within 3–10 min and the oven-cured reference. Thus, the insoluble network of about 80% was already formed within the first 3 min, but the ongoing reaction increased the network density. Data of the Tg were also taken by DMA measurements at the minimum of energy density when the film conditions after curing were comparable to the oven heating (Tables 1 and 2). The Tg of Acryl-Mel is in the same magnitude as the LED- and oven-cured films. As the other properties indicate, the crosslinking reaches its maximum after 300 s of LED exposure because temperature conditions are equal to the oven baking. The induction temperature of NCO-Cap is higher compared to the Acryl-Mel system. Therefore, Tg is slightly lower in the case of LED curing versus oven treatment at 175°C, but can compete with results of curing temperatures at 140°C.
Fig. 5

Pendulum hardness and gel content of NCO-Mal after LED exposure with LED at 805 nm (I = 1.2 W cm−2) with S2 (15 mg g−1) as absorber and the reference sample being cured at 175°C in a convection oven

Table 1

Glass transition temperatures of NCO-Cap after oven and IR radiator. LED or laser curing using S2 (15 mg g−1) for the LED emission at 805 nm and laser emission at 808 nm and S1 (15 mg g−1) for laser curing

Curing conditions

Energy density (J cm−2)

Tg (E′) (°C)

Tg (E″) (°C)

Tg (tan δ) (°C)

Oven 140°C/15 min

57

57

85

Oven 175°C/15 min

86

81

108

LED 805 nm/1.2 W cm−2/300 s

360

77

69

97

Lasera 808 nm/5.1 W cm−2/16 s

82

69

67

94

Lasera 980 nm/5.5 W cm−2/16 s

86

100

91

115

Laserb 980/12.4 W cm−2/6 s

75

77

71

90

IR radiator/300 s

71

54

90

aSpot-focused laser was used, bline-focused laser was used

Table 2

Glass transition temperatures of Acryl-Mel after oven curing, LED or laser exposure using S2 (15 mg g−1) for the LED emission at 805 nm, and laser emission at 808 nm and S1 (15 mg g−1) for laser curing

Curing conditions

Energy density (J cm−2)

Tg (E′) (°C)

Tg (E″) (°C)

Tg (tan δ) (°C)

Oven 140°C/15 min

71

75

82

LED 805 nm/1.2 W cm−2/300 s

360

71

79

90

Lasera 808 nm/5.1 W cm−2/16 s

82

58

64

78

Lasera 980 nm/5.5 W cm−2/4 s

86

57

62

72

Laserb 980/9.4 W cm−2/4 s

75

60

60

67

aSpot-focused laser was used, bline-focused laser was used

Laser light-induced thermal curing

Laser light sources facilitate higher energy input due to higher available intensity. The low divergence of the radiation can be seen as one benefit for exposure of local spots with high spatial resolution as demonstrated to process printing plates.9,24 Nevertheless, it restricts the application for homogeneous irradiation of two-dimensional surfaces. Modulation of the laser spot into a line focus with microoptics enhances applicability for use in industrial coating processes.26 Consequently, NIR diode lasers are an adequate choice for the NIR baking technology.

Compared to LED exposure experiments, laser irradiation was carried out in a much shorter time frame, that is, 16 s or less using a 980 and 808 nm emitting laser with a spot focus combined with S1- and S2-doped baking varnishes, respectively. The temperature curve monitored on the laser exposure of dried Desmophen 651 films (Fig. 6) using the aforementioned emission wavelengths and absorbers demonstrates the fast heating within several seconds above 100°C. At similar intensities of the lasers and nearly the same extinction level of the absorbers, the radiation with 980 nm has a higher heating rate. This might be explained by a higher quantum yield of internal conversion of S1, which possesses the ability to convert the radiation into heat with more efficiency.
Fig. 6

Heating of a coating under laser exposure using S1 in combination with the laser emission of 980 nm (I = 5.5 W cm−2) and S2 in combination with 808 nm (I = 5.1 W cm−2) using 15 mg g−1 of the absorbers

Since the irradiation curing time with lasers is much shorter, monitoring of ERelax is plotted against energy density (Fig. 7), that is, the product of intensity and exposure time. Choosing the maximum availability from the light source, the intensity was constant at 5.5 and 12.5 W cm−2 for the 808 and the 980 nm laser, respectively. The slope of ERelax is in every case higher compared to LED curing, which indicates a more efficient energy input by the laser light source. The faster heating at these higher intensities overcomes the cooling by convection of the room temperature atmosphere. The crosslinking occurs at least 30 times faster in comparison with the NIR LED. In addition, the toughness already increases after two seconds.
Fig. 7

Relaxation modulus plotted against energy density of the laser and LED irradiation of the coating systems after curing using S2 (15 mg g−1) as the absorber. left: NCO-Cap right: Acryl-Mel

An important drawback is the appearance of blistering on the film surface caused by degassing from the residual solvents and the deblocking agent or butanol as the transetherification product. Vitrification occurs too fast, giving the material no possibility to reflow after formation of the gas bubbles. To prevent the blistering, the exposure should not exceed 16 s. Especially in the case of the Acryl-Mel system, blistering interferes with film formation, which prevents curing at an extended process window resulting in a higher ERelax. Tests on scratch hardness of the laser-cured coatings were also carried out under different energy doses (Fig. 8) using S1. Therefore, hardness improves as the level of curing increases at a higher energy density.
Fig. 8

Scratch hardness of NCO-Cap (left) and Acryl-Mel (right) after exposure with spot-focused and line-focused laser using S1 as an absorber (15 mg g−1) and a laser emission at 980 nm. The tests were performed under variation of both parameters time and intensity of the laser exposure with a detailed description tabled in the supporting information

A design of experiment (DoE) was performed regarding the conditions of the exposure (time, intensity, and absorber concentration) to examine the impact of each factor. All three factors affect the heating of the coating because of the relation to the sum of induced excitation–deactivation processes by radiation. The range of these parameters was chosen to the limits in which neither insufficient curing, blistering nor burning appears. A linear univariate ANOVA was used testing the significance of the factors in the chosen limits. The tests were carried out using S1 and exposing to an excitation wavelength of the laser at 980 nm applying a coating system comprising NCO-Cap or Acryl-Mel.

Supplementary information gives a detailed description of the experimental results. Processing time is the dominating factor with the highest impact on the curing efficiency of both coating systems (Figs. 9 and 10, Tables 3 and 4). This can be observed from the slope in this direction of the 3D surface and the significance, which is far below the chosen level of 5% for the F test. Obviously, the chemical reaction in the bulk needs several minutes to occur, even though a faster heating can be achieved under high-intensity conditions with the laser light source. The influence of absorber concentration and intensity differs between the two systems. A higher absorber concentration significantly enhances ERelax of Acryl-Mel films more extremely in the case of exposure times above 8 s. On the other side, the cS1 level does not interfere with curing of NCO-Cap shown in Table 4 and the surfaces shown in Fig. 10 merging into each other.
Fig. 9

3D map of DoE results Acryl-Mel cured by spot-focused laser at 980 nm with S1 as the absorber varying exposure time and intensity. left: 10 mg g−1 of S1, right: 15 mg g−1 of S1

Fig. 10

3D map of DoE results NCO-Cap cured by spot-focused laser at 980 nm with S1 as the absorber varying exposure time and intensity. left: 10 mg g−1 of S1, right: 15 mg g−1 of S1

Table 3

ANOVA of Acryl-MelR2 0.84

Factor

Degrees of freedom

Mean square

F value

Significance

c S1

1

5.2 × 104

8.6

8 × 10−3

I

2

1.5 × 104

2.4

0.114

t

3

1.8 × 105

30.1

< 10−4

Error

20

6.0 × 103

  
Table 4

ANOVA of NCO-CapR2 0.75

Factor

Degrees of freedom

Mean square

F value

Significance

c S1

1

3.0 × 103

0.2

0.696

I

2

2.3 × 105

12.2

3 × 10−4

t

3

2.1 × 105

11.3

2 × 10−4

Error

20

1.9 × 104

  

The relation of intensity to the response also differs between these two coatings; that is, the curing of NCO-Cap is significantly influenced by the intensity, while Acryl-Mel showed no big response. The reason for the discrepancy of those factors was found in uptake of heat, the loss by cooling of the surrounding, and the different onset temperatures for the curing of those coating systems. Thus, NCO-Cap requires a much higher curing temperature compared to Acryl-Mel and a threshold of the intensity located above 2.5 W cm−2 for the curing within several seconds. The limits of this DoE were chosen in a narrow range, and of course the effect of both factors still could be shown for both systems under longer time exposures choosing a wider range of the factors.

The validity is 77% and 84%, and the linear model explains the impact of the factors on the respond well. Nevertheless, detecting other effects requires further investigation on the laser curing experiments. First of all, a nonlinear relation between time and ERelax has to be considered, since this was already observed in the pretests with the laser exposure (Figs. 7 and 8). Furthermore, interactions between the factors were not considered in the model. Supplementary information gives more details of this analysis, indicating similar significance of the three main effects as shown in Table 1. This has also an effect on the interactions of time/intensity and time/cS1, respectively. The graph in Fig. 10 (DoE) shows a strong increase in the response at the higher levels. However, inclusion of interactions results in a decrease in the degree of freedom of the error and leads also to an increase in the uncertainty based on data information.

Laser with line focus for curing

The laser used exhibits a top hat and a Gaussian distribution in the two dimensions of laser beam intensity. The use of such equipment facilitates curing of larger areas by linear motion of the line focus across the coating surface. Therefore, the feasibility of this laser tool for thermal curing opens technical applications at an industrial scale. This was tested by experiments offering a laser line exposing a length of 200 mm and a width of 2 mm or rather up to 8 mm if the distance between laser beam and substrate was out of the focal length.

With this setup, NCO-Cap and Acryl-Mel had varying exposure times, controlled by the scanning speed of the beam, intensity, and width of the line, which changes the energy input. Additionally, the number of scans was varied with a cooling period of several seconds between the scans to simulate a process with more than one laser line resulting in a reduction of the blistering. The graph in Fig. 8 compares the scratch hardness of the films after exposure to the spot focus and the line focus represented without consideration of the impact of each single factor. There is no discrepancy between these values showing a successful scaling up of the laser curing process (see also Tables 1 and 2).

It is also possible to cure these bakeable coatings on heat-sensitive substrates unless the material absorbs in the NIR region. In the case of white paper and slightly brown cotton, the curing of the coatings was satisfying and no burning or deformation of the substrate occurred. This allows the application of coatings with improved mechanical resistance and high Tg on such substrates. Because the heat dissipation differs between metals, plastics, textiles, wood, etc., the parameters of the exposure need to be arranged. Also, blistering addresses demands on the material and the formulation of the coating for exposure at very high intensities of the line-focused laser beam, that is, 6.3–100 W cm−2.

Interpenetrating networks by baking of coatings and polymers

Exchange of volatile organic solvents with reactive diluents can be seen as another possibility to reduce blistering. S2, S3, and S4 photochemically generate together with In active species for radical or cationic polymerization (equations 49) by absorption of NIR radiation. The thermal curing of the baking resin and the photopolymerization of monomer occur simultaneously upon NIR irradiation, forming two separated networks, which interpenetrate each other. Further benefits of such a process are the reduction of volatile compounds, improvement and tuning of mechanical properties by the ratio of the two reactive systems, and a reduction of inhibition of the polymerization by either oxygen or humidity. The sealing of the surface by the crosslinking of the thermoset causes the latter by preventing diffusion of air into the top layer.

Tripropyleneglycol diacrylate (TPGDA) and trimethylolpropane triacrylate (TMPTMA) (Scheme 1) were chosen as monomers for radical photopolymerization. The influence on mechanical properties of the films differs between these types due to different number and concentration of reactive groups resulting in different network densities. For cationic polymerization, a bisphenol A diglycidyl ether (BDGE) monomer was chosen, which reacts upon ring-opening polymerization. The choice of the initiator depends on the kind of photopolymerization. IS-NTf2 possesses a high solubility and, moreover, a better possibility to dissociate into single solvated ions.38 On the other hand, formation of ion pairs comprising the iodonium cation and NTf2 anion in the monomers results in less reactivity. Thus, both the excellent solubility and the capability to dissociate into single solvated ions result in a high reactivity between the absorbers and the iodonium cation generating high amounts of initiating species.38 However, the NTf2 anion inhibits the cationic polymerization due to its nucleophilic character. As an alternative, the use of IS-PF6 exhibiting weak coordinating properties results in better efficiency for cationic polymerization. S2, S3, and S4 have been approved as suitable operating sensitizers inducing radical polymerization applying high-power LED exposure. Nevertheless, only S4 can be seen as a reliable choice for the cationic polymerization process since it fulfills requirements of high generation of acidic cationic species, while formation of less nucleophilic compounds occurs with minor efficiency. The addition of monomer was carried out with a constant viscosity of 38–40 Pa·s between each sample.

The conversion of the monomers was tested with FTIR after exposure. S3 is feasible for the radical photopolymerization applying the 805 nm LED device and the 808 nm laser resulting in interpenetrating networks even under air (Fig. 11). This sensitizer was already shown to be efficient for curing of TPGDA and TMPTMA, but diffusion of oxygen inhibited the reaction of the monomer not blended with the baking varnish.37 Thus, this approach achieves nearly complete conversion of TPGDA and about 50% conversion of TMPTMA, which is high considering the trifunctionality of the monomer leading to gelation at relatively low conversion of methacrylates. In comparison with the IPN with NCO-Mal, the conversion of TPGDA in NCO-Cap is low. An explanation is the faster curing of NCO-Mal induced at lower temperatures compared to NCO-Cap and a faster sealing of the surface prevented the oxygen inhibition more effectively.
Fig. 11

Monomer conversion after laser (λem = 808 nm, I = 5.1 W cm−2, t = 16 s) and LED (I = 1.2 W cm−2/t = 5 min) under air atmosphere controlled by FTIR measurements

Laser curing leads to similar results as the LED curing. This reduces the curing time by a factor of 18. In addition, the energy input of 92 J cm−2 is also lower compared to the threshold of LED exposure (360 J cm−2) for sufficient crosslinking. The Tg of the films given in Table 3 is around 60–90°C depending on the method of determination. This value is close to the same magnitude as the crosslinked neat NCO-Cap (Table 5). The appearance of only one Tg shows that the monomers were crosslinked within the thermosetting resin showing no phase separation.
Table 5

Glass transition temperature of the IPNs cured with laser (λem = 808 nm, I = 5.1 W cm−2, t = 16 s) and LED light sources (I = 1.2 W cm−2, t = 5 min) under air atmosphere; NCO-Cap/TPGDA no film, but gelation

Coating

NIR source

Tg (E′) (°C)

Tg (E″) (°C)

Tg (tan δ) (°C)

NCO-Cap/TMPTMA

Laser

71

64

90

NCO-Cap/TMPTMA

LED

66

63

85

NCO-Mal/TPGDA

Laser

64

63

80

NCO-Mal/TPGDA

LED

67

62

86

S2 and S4 did not initiate the curing of the (meth-)acrylates under air and also a reference test with S1 as an absorber using the 980 nm laser failed for curing. Earlier experiments showed the ineffectiveness of these absorbers under low-intensity NIR light.37 The combination of S1 and IS-NTf2 can only be seen as a thermal radical initiator system inducing the polymerization above 160°C.37 Therefore, radical generation of S1, S2, and S4 does not overcome the oxygen inhibition by the diffusion processes, which still takes place in the resin–monomer blend.

The cationic photopolymerization of Ep with S4 and IS-PF6 achieves a conversion of around 30% after 5-min LED or 16-s laser exposure (Table 6). Unfortunately, this reaction does not occur under humid atmospheric conditions addressing the demand to prevent inhibition by lamination between two glass slides. The polymerization of epoxides fails in the case of the other absorbers, though generation of cationic species was already proven with S3 and IS-PF6 as an initiator system. As mentioned before, the nucleophilic photoproducts of the absorber inhibit polymerization of the epoxide.
Table 6

Conversion of Ep and NCO-Cap/Ep with S4 (15 mg g−1) and IS-PF6 (20 mg g−1) laminated between microslides (d = 100 µm) with LED (I = 1.2 W cm−2, t = 5 min) and laser (808 nm, I = 5.1 W cm−2, t = 16 s) detected by FTIR measurements; conductivity of IS-PF6 (3.2 × 10−5 M) in Ep and NCO-Cap/Ep

Coating

Molar conductivity (S cm2 mol−1)

LED (%)

Laser 808 (%)

NCO-Cap/Ep

16.6

0

0

Ep

21.3

33

29

The photopolymerization of the blend NCO-Cap/Ep is not successful even in the case of lamination of the samples. Two effects by change of the media could be responsible for this failure. First, the hydroxyl groups of the polyester react with the growing chain of the epoxy polymer, which finally results into a chain transfer and a reduction of the polymerization rate. Also, a higher probability for formation of ion pairs of the IS-PF6 in the resin–monomer as surrounding environment reduces the efficiency of the electron transfer reaction between the initiator and absorber. The decrease in the conductivity of Ep containing IS-PF6 indicates the higher probability of ion pair formation. Conductivity measurements confirm this hypothesis. Finally, a lower concentration of initiating species is insufficient for successful photopolymerization. This was already indicated by other investigations testing different epoxy monomers for NIR-induced cationic photopolymerization and conductivity of monomers containing IS-PF6. We will report more details about the NIR-induced cationic photopolymerization soon.56

Bleaching of the absorbers

S1S4 also absorb in the visible range resulting in a green- or blue-colored coating. This limits the application of NIR photopolymers or thermosets to coatings without the desire to fulfill aspects of appearance. NIR light induces bleaching of the green color, but forms red-, brown-, or yellow-colored photoproducts, which can be seen in some cases as an exclusion criterion for some coating applications. It was already shown that bleaching of the band appearing at around 800 or 1000 nm indicates cleavage within the conjugated system. The products of decomposition absorb at lower wavelengths depending on their photophysical properties. After longtime exposure, the main photoproducts depict structural motifs of either indolenine or benzindolenine absorbed in the UV and visible blue part.

Figures 12, 13, and 14 depict bleaching of S1, S2, and S3 and the calculated concentration of the absorbers in relation to the exposure time. This occurred either at LED or at laser exposure, but the rate constants of the first-order plot depended on the intensity of the light source (Table 7). Operation at intensities > 1.2 W cm−2 also requires the inclusion of thermal effects to consider the decomposition mechanism since the absorbers were found to be unstable after 5 min applying oven temperatures above 150°C. Especially in the case of the laser exposure of S1 in Fig. 14, the period until 4 s might be interpreted as a heating phase with no bleaching.
Fig. 12

Bleaching of S2 under LED exposure (805 nm; 1.2 W cm−2) of Desmophen 651 films containing 2.3 × 10−6 mol g−1 of the absorber

Fig. 13

Bleaching of S3 under LED exposure (805 nm; 1.2 W cm−2) of Desmophen 651 films containing 2.3 × 10−6 mol g−1 of the absorber

Fig. 14

Bleaching of S1 under laser exposure (980 nm; 7.7 W cm−2) of Desmophen 651 films containing 2.3 × 10−6 mol g−1 of the absorber

Table 7

Bleaching constants (kbleach) of S1, S2, and S3 under LED 805 nm (1.2 W cm−2) or laser 980 nm (7.7 W cm−2) exposure using a first-order plot; the concentration of the absorber in the film was 2.3 × 10−6 mol g−1; optionally, the solution contained 4.5 × 10−4 mol g−1 of IS-NTf2

NIR source

Intensity (W cm−2)

Initiator

Absorber

kbleach (s−1)

LED

1.2

S2

1.70 × 10−2

LED

1.2

S3

1.04 × 10−2

LED

0.1

S3

2.0 × 10−4

LED

0.1

IS-NTf2

S3

8.40 × 10−3

Laser

7.7

S1

1.51 × 10−1

Bleaching of S3 under low radiation intensities of 0.1 W cm−2 indicates photochemical decomposition of the absorber because the temperature does not exceed 40°C in this experiment. The fast bleaching after the photochemical reaction between IS-NTf2 and S3 was already shown57 and is proven by a kinetic constant, which is a magnitude of ten higher than bleaching without the initiator. This was associated with a very instable oxidized state (Sens) of S3. The photodecomposition mechanism of S4 is unique compared to the other heptamethine cyanine absorbers.56 Thus, cationic polymerization is very efficient due to the molecular pattern of the photoproducts.

A fast bleaching supports a good appearance if a slightly yellow or colorless film will be obtained. This disagrees with the demand on fully curing of the coating, which requires high maximum temperatures since the drop of absorbance reduces the heat transfer. A good agreement would be a reduction of the absorber concentration finally extending the curing time or adjustments of the radiation intensity. Exposure parameters give the opportunities for tuning the system as required.

Conclusion

The thermal curing of coatings by laser light could become a useful tool in the future. This technique accelerates the curing process of a coating and decreases the consumption of energy compared to oven-driven processes. It might also open the opportunity to replace oven processes by this energy-saving photonic method that can generate heat on demand in small spaces. However, the high-intensity input by the laser requires a certain set of process parameters depending on the coating material and heat dissipation; hence, surface defects can appear. One approach reducing the peaks of the intensity can be a wider distribution of the laser light on a rectangular area or operating with multiple laser scans in a row. Blistering or cavitation bubbles as a result of overheating can be eliminated by keeping up a homogeneous level of energy input over time. Additionally, the exchange of volatile organic solvents by monomers for NIR light-induced polymerization in such thermo-induced coating systems reduces these surface defects and offers an environmentally friendly solution.

Beneath the saving of time and energy needed for processing, the high local resolution of the heat uptake opens this process for special applications. This can be heat-sensitive substrates, which are not burned upon laser heating, facilitating new applications for bakeable coatings resulting in improvement in the mechanical properties of the coated material. Furthermore, the localized exposure with a laser spot enables manufacturing of structural patterns comparable to the use of any kind of thermosetting coating material.

Notes

Acknowledgments

The authors thank the county of North Rhine-Westphalia for funding the project REFUBELAS (Grant 005-1703-0006) and FEW Chemicals GmbH for the NIR sensitizers.

Supplementary material

11998_2019_197_MOESM1_ESM.pdf (99 kb)
Supplementary material 1 (PDF 98 kb)

References

  1. 1.
    Tillet, G, Boutevin, B, Ameduri, B, “Chemical Reactions of Polymer Crosslinking and Post-crosslinking at Room and Medium Temperature.” Prog. Polym. Sci., 36 (2) 191–217 (2011)Google Scholar
  2. 2.
    Enns, JB, Gillham, JK, “Time-Temperature-Transformation (TTT) Cure Diagram-Modeling the Cure Behavior of Thermosets.” J. Appl. Polym. Sci., 28 (8) 2567–2591 (1983)Google Scholar
  3. 3.
    Hampshire, RJ, “The Use of Radiant Heat Transfer in the Curing of Coatings on Complex Geometries and Problematic Substrates.” Pigment Resin Technol., 26 (4) 225–228 (1997)Google Scholar
  4. 4.
    Dickie, RA, Bauer, DR, Ward, SM, Wagner, DA, “Modeling Paint and Adhesive Cure in Automotive Applications.” Prog. Organ. Coat., 31 (3) 209–216 (1997)Google Scholar
  5. 5.
    Sawyer, M, “Infrared Curing Systems Offer Alternative to Tried-and-True Convection Heat Sources.” Met. Finish., 104 (11) 9–11 (2006)Google Scholar
  6. 6.
    Abliz, D, Duan, Y, Steuernagel, L, Xie, L, Li, D, Ziegmann, G, “Curing Methods for Advanced Polymer Composites—A Review.” Polym. Polym. Compos., 21 (6) 341–348 (2013)Google Scholar
  7. 7.
    Howell, JR, Siegel, R, Mengüc, MR, Thermal Radiation Heat Transfer. CRC Press, New York, 2010Google Scholar
  8. 8.
    Kane, R, Sirek, S, “The T3 Quartz Infrared Lamps.” In: Kane, R, Sell, H (eds.) Revolution in Lamps: A Chronicle of 50 Years of Progress, pp. 65–74. Fairmont Press, Lilburn, 2001Google Scholar
  9. 9.
    Baumann, H, Hoffmann-Walbeck, T, Wenning, W, Lehmann, H-J, Simpson, CD, Mustroph, H, Stebani, U, Telser, T, Weichmann, A, Studenroth, R, Imaging Technology, 3. Imaging in Graphic Arts. Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. KGaA, New York, 2015Google Scholar
  10. 10.
    Schubert, EF, Light Emitting Diodes-Schubert. Cambridge University Press, Cambridge, 2006Google Scholar
  11. 11.
    Bachmann, F, Takahashi, R, “Chances and Limitations of High-Power Diode Lasers.” Rev. Laser Eng., 31 (5) 313–317 (2003)Google Scholar
  12. 12.
    Bao, L, Bai, J, Price, K, DeVito, M, Grimshaw, M, Dong, W, Guan, X, Zhang, S, Zhou, H, Bruce, K, Dawson, D, Kanskar, M, Martinsen, R, Haden, J, “Reliability of High Power/Brightness Diode Lasers Emitting from 790 to 980 nm.” Proc. SPIE, pp. 8605 (2013)Google Scholar
  13. 13.
    Kanskar, M, Bao, L, Bai, J, Chen, Z, Dahlen, D, DeVito, M, Dong, W, Grimshaw, M, Haden, J, Guan, X, Hemenway, M, Kennedy, K, Martinsen, R, Tibbals, J, Urbanek, W, Zhang, S, “High Reliability of High Hower and High Brightness Diode Lasers.” Proc. of SPIE, 8965 (2014)Google Scholar
  14. 14.
    Abhinandan, L, Chari, R, Nath, AK, Trivedi, MK, “Laser Curing of Thermosetting Powder Coatings: A Detailed Investigation.” J. Laser Appl., 11 (6) 248–257 (1999)Google Scholar
  15. 15.
    Blais, C, Chalco, PA, “Semiconductor Chip Packaging Method which Heat Cures an Encapsulant Deposited on a Chip Using a Laser Beam to Heat the Back Side of the Chip.” US Patent 5,457,299, 1995Google Scholar
  16. 16.
    Chala, TF, Wu, CM, Chou, MH, Gebeyehu, MB, Cheng, KB, “Highly Efficient Near Infrared Photothermal Conversion Properties of Reduced Tungsten Oxide/Polyurethane Nanocomposites.” Nanomaterials (Basel), 7 (7) 191 (2017)Google Scholar
  17. 17.
    Chua, CT, Lee, YP, Zhou, MS and Chan, L, “Laser Curing of Spin-On Dielectric Thin Films.” US Patent 6,121,130, 2000Google Scholar
  18. 18.
    Hirshey, JA, Busiello, M, “Manufacturing System Implementing Laser-Curing of Epoxied Joints.” US Patent 0305070 A1, 2017Google Scholar
  19. 19.
    Hong, Z, Liang, R, “IR-Laser Assisted Additive Freeform Optics Manufacturing.” Sci. Rep., 7 (1) 7145 (2017)Google Scholar
  20. 20.
    Hoult, AP, Crane, SJ, “Diode-Laser Curing of Liquid Epoxide Encapsulants.” US Patent 613794 B2, 2005Google Scholar
  21. 21.
    Mackwood, AP, Crafer, RC, “Thermal Modelling of Laser Welding and Related Processes: A Literature Review.” Opt. Laser Technol., 37 (2) 99–115 (2005)Google Scholar
  22. 22.
    Simone, G, “An Experimental Investigation on the Laser Cure of Thermosetting Powder: An Empirical Model for the Local Coating.” Prog. Org. Coat., 68 (4) 340–346 (2010)Google Scholar
  23. 23.
    Suzuki, A, Mochizuki, N, “PET Microfiber Prepared by Carbon Dioxide Laser Heating.” J. Appl. Polym. Sci., 88 (14) 3279–3283 (2003)Google Scholar
  24. 24.
    Baumann, H, “Lithographische Druckplatten für Laserbelichtung.” Chemie in unserer Zeit, 48 (1) 14–29 (2015)Google Scholar
  25. 25.
    Kunita, K, Oohashi, H, Ooshima, Y, “Novel Trialkoxy-Substituted Onium Salts as Highly Sensitive and Stable Photoinitiators Reactive to IR Laser.” J. Photopolym. Sci. Technol., 27 (6) 695–702 (2014)Google Scholar
  26. 26.
    Forbes, A, Bayer, A, Meinschien, J, Mitra, T, Brodner, M, Lizotte, TE, “Beam Shaping of Line Generators Based on High Power Diode Lasers to Achieve High Intensity and Uniformity Levels.” Proc. of SPIE, 7062 70620X-70620X-7 (2008)Google Scholar
  27. 27.
    Beier, B, “Arraytechnologie Statt Einzelner Laserdiode.” Laser Tech. J., 8 (2) 34–36 (2011)Google Scholar
  28. 28.
    Wood, GL, Homburg, O, Hauschild, D, Kubacki, F, Lissotschenko, V, Dubinskii, MA, “Efficient Beam Shaping for High-Power Laser Applications.” Proc. of SPIE 6216 621608-1-621608-8 (2006)Google Scholar
  29. 29.
    Neukum, J, “Diodenlaserbarren in der Druckindustrie.” Laser Tech. J., 8 (4) 22–23 (2011)Google Scholar
  30. 30.
    Hoynant, P, Pitz, H, “Verfahren zum Trocknen von Druckfarbe und Druckfarbe.” German Patent 102008056237, 2009Google Scholar
  31. 31.
    Pitz, H, Hauck, A, Anweiler, W, Hachmann, P, “Method for Drying an Ink on a Printed Material in a Printing Press and Printing Press.” German Patent 10316472, 2004Google Scholar
  32. 32.
    Stollenwerk, J, Weigt, W, Zschuppe, M, Meixner, M, “Sol-Gel-Lacke: Laser statt Trockenöfen.” Farbe und Lack, 121 (3) 88–92 (2013)Google Scholar
  33. 33.
    Brömme, T, Schmitz, C, Oprych, D, Wenda, A, Strehmel, V, Grabolle, M, Resch-Genger, U, Ernst, S, Reiner, K, Keil, D, Lüs, P, Baumann, H, Strehmel, B, “Digital Imaging of Lithographic Materials by Radical Photopolymerization and Photonic Baking with NIR Diode Lasers.” Chem. Eng. Technol., 39 (1) 13–25 (2016)Google Scholar
  34. 34.
    Kasha, M, Rawls, HR, Ashraf El-Bayoumi, M, “The Exciton Model in Molecular Spectroscopy.” Pure Appl. Chem., 11 (3–4) 371–392 (1965)Google Scholar
  35. 35.
    West, W, Pearce, S, “The Dimeric State of Cyanine Dyes.” J. Phys. Chem., 69 (6) 1894–1903 (1965)Google Scholar
  36. 36.
    Emerson, ES, Conlin, MA, Rosenoff, AE, Norland, KS, Rodriguez, H, Chin, D, Bird, GR, “The Geometrical Structure and Absorption Spectrum of a Cyanine Dye Aggregate.” J. Phys. Chem., 71 (8) 2396–2403 (1967)Google Scholar
  37. 37.
    Schmitz, C, Halbhuber, A, Keil, D, Strehmel, B, “NIR-Sensitized Photoinitiated Radical Polymerization and Proton Generation with Cyanines and LED Arrays.” Prog. Org. Coat., 100 32–46 (2016)Google Scholar
  38. 38.
    Brömme, T, Oprych, D, Horst, J, Pinto, PS, Strehmel, B, “New Iodonium Salts in NIR Sensitized Radical Photopolymerization of Multifunctional Monomers.” RSC Adv., 5 (86) 69915–69924 (2015)Google Scholar
  39. 39.
    Crivello, JV, Lam, JHW, “Diaryliodonium Salts. A New Class of Photoinitiators for Cationic Polymerization.” Macromolecules, 10 (6) 1307–1315 (1977)Google Scholar
  40. 40.
    Pohlers, G, Scaiano, JC, Sinta, R, “A Novel Photometric Method for the Determination of Photoacid Generation Efficiencies Using Benzothiazole and Xanthene Dyes as Acid Sensors.” Chem. Mater., 9 (12) 3222–3230 (1997)Google Scholar
  41. 41.
    Crivello, JV, Lam, JHW, “Dye-Sensitized Photoinitiated Cationic Polymerization.” J. Polym. Sci. Polym. Chem. Ed., 16 (10) 2441–2451 (1978)Google Scholar
  42. 42.
    Crivello, JV, “A New Visible Light Sensitive Photoinitiator System for the Cationic Polymerization of Epoxides.” J. Polym. Sci. Part A Polym. Chem., 47 (3) 866–875 (2009)Google Scholar
  43. 43.
    Xiao, P, Zhang, J, Dumur, F, Tehfe, MA, Morlet-Savary, F, Graff, B, Gigmes, D, Fouassier, JP, Lalevée, J, “Visible Light Sensitive Photoinitiating Systems: Recent Progress in Cationic and Radical Photopolymerization Reactions Under Soft Conditions.” Prog. Polym. Sci., 41 32–66 (2015)Google Scholar
  44. 44.
    Yagci, Y, Jockusch, S, Turro, NJ, “Photoinitiated Polymerization: Advances, Challenges, and Opportunities.” Macromolecules, 43 (15) 6245–6260 (2010)Google Scholar
  45. 45.
    Fouassier, JP, Morlet-Savary, F, Lalevée, J, Allonas, X, Ley, C, “Dyes as Photoinitiators or Photosensitizers of Polymerization Reactions.” Materials, 3 (12) 5130–5142 (2010)Google Scholar
  46. 46.
    Hatano, T, Fukui, K, Karatsu, T, Kitamura, A, Urano, T, “Sensitization Mechanisms of Photopolymer Coating Layer using Infrared Dye.” J. Photopolym. Sci. Technol., 13 (5) 697–701 (2000)Google Scholar
  47. 47.
    Karatsu, T, Yanai, M, Yagai, S, Mizukami, J, Urano, T, Kitamura, A, “Evaluation of Sensitizing Ability of Barbiturate-Functionalized Non-Ionic Cyanine Dyes; Application for Photoinduced Radical Generation System Initiated by Near IR Light.” J. Photochem. Photobiol. A, 170 (2) 123–129 (2005)Google Scholar
  48. 48.
    Zhang, S, Li, B, Tang, L, Wang, X, Liu, D, Zhou, Q, “Studies on the Near Infrared Laser Induced Photopolymerization Employing a Cyanine Dye-Borate Complex as the Photoinitiator.” Polymer, 42 (18) 7575–7582 (2001)Google Scholar
  49. 49.
    Urano, T, Ishikawa, M, Sato, Y, Itoh, H, “Sensitizer Dyes and Sensitization Mechanisms in Photopolymer Coating Layer II.” J. Photopolym. Sci. Technol., 12 (5) 711–716 (1999)Google Scholar
  50. 50.
    Bonardi, AH, Dumur, F, Grant, TM, Noirbent, G, Gigmes, D, Lessard, BH, Fouassier, JP, Lalevée, J, “High Performance Near-Infrared (NIR) Photoinitiating Systems Operating under Low Light Intensity and in the Presence of Oxygen.” Macromolecules, 51 (4) 1314–1324 (2018)Google Scholar
  51. 51.
    Schmitz, C, Strehmel, B, “Photochemical Treatment of Powder Coatings and VOC-Free Coatings with NIR Lasers Exhibiting Line-Shaped Focus: Physical and Chemical Solidification.” ChemPhotoChem, 1 (1) 26–34 (2017)Google Scholar
  52. 52.
    Schmitz, C, Strehmel, B, “Laser Focus on Curing.” Eur. Coat. J., 4 40–44 (2018)Google Scholar
  53. 53.
    Bonardi, AH, Bonardi, F, Morlet-Savary, F, Dietlin, C, Noirbent, G, Grant, TM, Fouassier, JP, Dumur, F, Lessard, BH, Gigmes, D, Lalevée, J, “Photoinduced Thermal Polymerization Reactions.” Macromolecules, 51 (21) 8808–8820 (2018)Google Scholar
  54. 54.
    Lee, JM, Subramani, S, Lee, YS, Kim, JH, “Thermal Decomposition Behavior of Blocked Diisocyanates Derived from Mixture of Blocking Agents.” Macromol. Res., 13 (5) 427–434 (2005)Google Scholar
  55. 55.
    Griffin, GR, Willwerth, LJ, “The Thermal Dissociation of Blocked Toluene Diisocyanates.” Ind. Eng. Chem. Product Res. Dev., 1 (4) 265–268 (1962)Google Scholar
  56. 56.
    Schmitz, C, Pang, Y, Gläser, M, Gülz, A, Horst, J, Jäger, M, Strehmel, B, “New High-Power LED Opens Photochemistry for NIR-Sensitized Radical and Cationic Photopolymerization.” Angewandte Chemie, submitted for publication (2018)Google Scholar
  57. 57.
    Brömme, T, Schmitz, C, Moszner, N, Burtscher, P, Strehmel, N, Strehmel, B, “Photochemical Oxidation of NIR Photosensitizers in the Presence of Radical Initiators and Their Prospective Use in Dental Applications.” Chem. Sel., 1 (3) 524–532 (2016)Google Scholar
  58. 58.
    Paints and Varnishes—Pendelum Damping Test.” DIN EN ISO 1522:2007-04 (2007)Google Scholar

Copyright information

© American Coatings Association 2019

Authors and Affiliations

  1. 1.Department of Chemistry, Institute of Coatings and Surface ChemistryNiederrhein University of Applied SciencesKrefeldGermany

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