Introduction

Owing to their unique characteristics and advantages with respect to thermal- and solvent-based coatings, UV-curable coatings have diverse industrial applications. Among the main types of polymers available, polyester acrylated resins are the most widely used. These resins exhibit excellent characteristics, such as high reactivity, versatility, and excellent adhesion properties. They also possess high gloss and adequate hardness, mechanical properties, and resistance to chemicals and water.

In general, resins must have a desirable finish, coloring, and surface finish, among other physical and mechanical properties. Due to recent changes in regulations, coatings are diverging from formulations with volatile organic compounds that pose significant health and environmental risks. Since the 1980s, resins have been cured by ultraviolet (UV) exposure for various applications, such as coatings, varnishes, lithographic and flexographic inks, and printing varnishes.1 UV-cured coatings have several advantages, such as rapid curing at room temperature, long shelf life, early gelation,2, 3 low energy consumption, and low solvent formulation,4 which impart these resins with easy handling and low material loss. Varnishes do not evaporate during curing, which renders them an excellent choice that complies with the most recent VOC regulations.5

Resins systems for UV curing usually comprise a base resin, a monomer, photoinitiators, and a synergistic component. Acrylate monomers, owing to their rapid reaction rates, form polymer resins and are used as diluents to control the viscosity and rheology of a system.4 Photoinitiators (PIs), which are key components in the formulation of UV-curing resins, absorb UV energy, form free radicals, and thereby initiate polymerization. These PIs can be categorized as Type I or II. Type-I PIs create two radicals that may both initiate polymerization, while Type-II PIs undergo photoreduction by a hydrogen or an electron donor. Therefore, they require a synergistic component such as a tertiary amine, an ether, or an ester to form initiating or reactive ketyl radicals.4 The photoinitiated polymerization of acrylates is one of the most efficient processes for producing highly crosslinked materials.6 A wide variety of PIs are commercially available, each with unique physical and chemical properties that can considerably affect the performance and appearance of the final product. In food products, for example, the migration of PIs can cause odor.7, 8

Photopolymerization occurs in technologies such as composites, dental resins, and industrial coatings. For Type-I PIs, systems with a visible spectrum ranging from 400 to 700 nm were initially introduced for dental applications. UV-Hg lamps are extensively used as a radiation source owing to their wide wavelength spectra distribution, which easily accommodates the wide variety of commercially available PIs. However, the risk of contamination by Hg along with the large amount of heat generated causes these lamps to be potentially harmful.9 A recent development called light emitting diode UV curing (UV-LED) addresses the limitations of old and more harmful technologies such as UV-Hg lamps. These lamps possess adequate efficiency and do not emit radiation below 240 nm, which could be harmful to the ozone layer. UV-LED lamps are monochromatic, with the majority of energy concentrated in a narrow wavelength region ranging from 365 to 405 nm. As such, an additional challenge of UV-LED curing is finding an appropriate PI in such a narrow wavelength range.9,10,11 In addition, an uncured coating must be attachable; therefore, it may become susceptible to particles that can diminish the finish of the product.12

This study aimed to evaluate six different types of commercial photoinitiators and their influence on the physical, optical, mechanical, and dynamic mechanical properties of coatings. To the best of the authors’ knowledge, many studies on the influence of the type of PI on the kinetic, chemical, mechanical, and corrosion properties have been reported; however, a knowledge gap in research still exists regarding the evaluation of the final optical and morphological properties of some commercial formulations.

Experimental methods and materials

Materials

Tetra-acrylate polyester resin, with four acrylate functional groups and a viscosity of 15000 cP, and triacrylate monomer propoxylated glyceryl triacrylate (GPTA), with three reactive functional groups and a viscosity of 500 cP, were both procured from Qualipoly Chemical Corp. (Kaohsiung, Taiwan) and used as received. Synergistic amine grade Ebecryl LED 03, with three acrylate functional groups and a viscosity of 600 cP, was procured from Allnex (Frankfurt am Main, Germany).

The photoinitiators used to prepare the varnish coating and their properties are summarized in Table 1. Omnirad LED 1711, Omnirad LED 2805, Omnirad LED 1710, Esacure 3644, TPO, and BAPO were procured from IGM Resins (Valinhos, SP, Brazil). Information regarding some formulations was only partly disclosed by the manufacturers in the technical datasheet; some of these formulations are a mixture of two photoinitiators at an unknown ratio and/or diluted with other unknown components. The chemical structure of each photoinitiator is shown in Fig. 1. The coating samples were named PI-X, where X is the number of the corresponding photoinitiator in Table 1; for example, the sample with LED 1711 was named PI-1. The acrylic polyurethane sealant and aliphatic hardener were procured from Qualipoly Chemical Corp. (Kaohsiung, Taiwan), and solvent ethylacetate was procured from Brenntag (Essen, Germany). According to the manufacturers, all photoinitiators are recommended for curing at 365–395 nm.

Table 1 Photoinitiators (PIs) and their respective supplier, viscosity, visual aspect, components, and attributed sample name
Fig. 1
figure 1

Chemical structures of the photoinitiator components

Varnish preparation

The varnish components were mixed in a Cowles mixer with a volume of 0.225 L and a rotor speed of 1750 rpm for 20 min for each sample. The weight proportion of the components is listed in Table 2.

Table 2 Varnish formulation with the respective components and weight percentages (%wt)

Varnish application and curing

The varnishes were applied on stainless steel AISI 1020 with dimensions of 80 × 100 × 1 mm3. For colorimetry analysis, paper cards with dimensions of 19.4 × 28.9 cm2 were used. These paper cards did not have optical whiteners in their formulation. The stainless steel plates were cleaned first with a xylene solvent and then with isopropyl alcohol. After cleaning, the plates were dried in an oven at 60°C for 5 min.

Because the varnish does not directly adhere to the steel substrate, before its application, the application of a sealant layer is required. A polyurethane sealant was applied to the steel substrate and reticulated using an aliphatic hardener. The sealant was cured for 72 h prior to varnish application.

The varnish was first diluted with ethyl acetate for reduced viscosity and then applied using a spray gun (brand DeVilbiss, Scottsdale, AZ, USA) with a nozzle of 1.8 mm and a pressure of 200 kPa. Three layers of varnish were applied to each substrate. The substrates were then left in flash-off for 20 min prior to UV-LED exposure to facilitate the release of the remaining air bubbles from mixing.

The substrates were exposed to UV-LED light for approximately 5 s, and radiation was measured at 395 nm with a power of 2 W·cm−2 at a distance of 2 cm.

Characterization

The UV-vis spectra of the photoinitiators were obtained using a Beckman DU 530 Life Science UV/Vis spectrophotometer. The powder samples were diluted in acetone, and scanning was performed in a wavelength range of 200–500 nm.

The 13C nuclear magnetic resonance (NMR) spectra of the photoinitiator were recorded with a Bruker Fourier 300 FT-NMR spectrometer at 75.48 MHz for the 13C nucleus. The samples were diluted with chloroform.

The varnish in its liquid (prior to curing) and film (after curing) forms was examined using Fourier transform infrared spectroscopy in the attenuated reflectance (ATR) mode with Shimadzu IR-Prestige 21 equipment in a wavenumber range of 4000–500 cm−1.

The thermogravimetric analysis of the dried varnishes was performed with Shimadzu TGA-50 equipment at a heating rate of 10°C·min−1 from 25 to 1000°C under a nitrogen flux of 50 mL·min−1. Here, 10 mg of each sample was used for analysis.

The DSC analysis of the dried varnishes was performed with Shimadzu DSC-60 equipment at a heating rate of 10°C·min−1 from −50 to 200°C. Only one heating scan was performed.

Dynamic mechanical analysis was performed with DMA Q800 AT equipment (TA Instruments) in a film tension clamp. The film dimensions were 35 × 5.3 × 1 mm3. Testing was performed from −50 to 125°C at a heating rate of 3°C·min−1, frequency of 1 Hz, and amplitude of 0.1%.

The tensile strength of the varnishes was measured using DMA Q800 AT equipment (TA Instruments) using a film tension clamp. Testing was performed in the controlled force stress/strain mode. Test specimens with dimensions of 35 mm × 5.3 mm × 1 mm were used. The analyses were performed at 30°C and a force rate of 3 N·mm−1 up to 18 N, which is the limit of the load cell of the equipment.

Color analysis was performed using a bench spectrophotometer, X-rite model i-5, and Color iMatch Professional software with SLIT aper (version 9.3.20). An illuminant, D65, which corresponds to the daylight emitted by the sun at noon, was used. A system with coordinates L*, a*, and b was used, where L* is the main axis with L = 0 and 100 representing black and white, respectively; a* is the amount of green with (−) and (+) representing green and red, respectively; and b* is the amount of blue with (−) and (+) representing blue and yellow, respectively. Here, the (+) of b* is indicative of sample yellowing.13

The surface morphology of the varnishes was evaluated using atomic force microscopy (AFM) with Shimadzu SPM-9700. The tests were performed in the dynamic tapping mode with a scan rate in the range of 0.5–1 Hz.

Results and discussion

Characterization of the photoinitiators

The chemical composition of the photoinitiators was confirmed using 13C NMR. Because all photoinitiators used in this study are industrial secrets, and most of the formulations comprise mixtures, cross-checking the data available in the datasheet with the NMR spectra of all photoinitiator samples was necessary. For this reason, in the spectra, some peaks are unknown, and some cannot be assigned. The 13C NMR spectra of all photoinitiators are shown in the Electronic Supplementary Material.

Peaks in the range of 19–22 ppm are present in all samples and are attributable to the carbon of the methyl groups bonded to the aromatic rings. The 128–139 ppm region is ascribed to the carbon in the aromatic rings present in all structures. For PI-1, the peak at 16 ppm represents the carbon from methyl groups bonded to CH2O-. The 24–35 ppm region is ascribed to the carbon in cyclohexane, and the peak at 62 ppm is associated with the carbon bonded to oxygen (C–O). The peaks at 205 ppm and within the range of 214–216 ppm are attributable to the carbon in carbonyl groups (C=O). PI-2 exhibits the same peaks at 16 and 204 ppm, whereas PI-3 exhibits different peaks at 16, 62, and 214–216 ppm. The observed similarities are attributable to these three photoinitiators being formulated with the same components, albeit in different proportions, which are an industrial secret. PI-4 exhibits peaks in the ranges of 19–21, 126–139, and 171–180 ppm, which are ascribed to the carbon atoms in the carbonyl groups. PI-5 and PI-6 exhibit peaks at 19–21, 126–129, 207, 219, and 220 ppm, with the last three attributable to the carbonyl groups in TPO and BAPO.14, 15

The UV-vis spectra of all photoinitiators were recorded and are shown in Fig. 2. All samples show a broad absorbance band in the range of 300–400 nm, in accordance with the data supplied by the manufacturer as well as the wavelength of the UV-LED lamp used in this study.

Fig. 2
figure 2

UV-vis spectra of the photoinitiators

Curing and thermal analysis

Figure 3 shows the FTIR spectra of the varnishes before and after UV-LED curing. No major differences are observed in the wavenumber range of ~4000–2000 cm−1.

Fig. 3
figure 3

FTIR spectra of the different varnishes with their respective photoinitiators (a) before and (b) after curing

In the 1700–700 cm−1 region (magnified in Fig. 4), the band attributed to CH groups at 810 cm−1 significantly decreases for all formulations (Fig. 4a). Moreover, the bands at 984 cm−1 (Fig. 4b), corresponding to vinyl groups (CH2=CH–), 1410 cm−1 (CH2=CH), and 1635 cm−1 (Fig. 4c) (C=C from acrylic groups) exhibit decreased intensity after curing. These results are in accordance with those reported by Kim and Seo.16 where a curing mechanism was developed for tetra-acrylate polyester resin cured by using conventional UV lamps. According to the authors, this is indicative of the efficiency of curing, considering the photoinitiator and lamp system. Cho et al.17 also highlighted that the band at 812 cm−1 is indicative of complete curing in acrylate-polyester-type varnishes. Furthermore, Kunwong et al.18 referred to the bands at 810 and 1635 cm−1 for evaluating the efficiency of resin curing.

Fig. 4
figure 4

Magnified FTIR spectra of the varnishes in the wavenumber ranges of (a) 900–730, (b) 1100–870, and (c) 1440–1380 cm−1

The results of DSC analysis are shown in Fig. 5. In all thermograms, a discontinuity in the range of 40–50°C can be observed, which is related to the glass transition (Tg) of the material; this will be discussed later in the section on dynamic mechanical analysis. Major exothermal peaks cannot be observed, implying that the varnishes were completely cured and corroborating with the results of FTIR analysis, where C=C was absent.

Fig. 5
figure 5

DSC thermograms of the varnishes post curing

The results of TGA analysis are shown in Fig. 6. All curves show similar behaviors. From Fig. 4a, considering that thermal degradation occurred at <300°C, a slight variation in thermal stability can be observed. This indicates the presence of oligomers and some remaining components that did not completely react during curing. Kunwong et al.18 also ascribed this loss to volatile compounds. Degradation at >450°C originates from the degradation of aromatic compounds, as photoinitiators possess aromatic rings. By differentiating the mass loss thermograms, three main thermal events in the degradation of the varnishes can be observed (Fig. 6b).

Fig. 6
figure 6

Thermogravimetric analyses of the varnishes: (a) mass loss, (b) first derivative of the mass loss

The first event at approximately 290°C is attributed to the evaporation of volatile compounds that did not react during curing. The second event at approximately 399°C is attributed to the degradation of the main polyester chains. The third event at 460°C is attributed to the degradation of aromatic compounds, present in the photoinitiators, which have a relatively high degradation temperature. Table 3 lists all thermal degradation events and temperatures for the varnish samples.

Table 3 Mass loss events obtained from the first derivative curves of the TGA analysis

Dynamic mechanical analysis

Figure 7 shows the results of the dynamic mechanical testing of the samples, where the storage modulus, tan delta peak height, and dislocation were evaluated. Figure 7a displays the storage modulus and elastic component of the deformation cycle, providing information about the stiffness of the sample.19 The storage modulus reaches a glassy plateau in the temperature range of −50–0°C. In this temperature range, the sample is in its glassy state, where the polymer chains are closely packed together with negligible free volume. PI-2 and PI-6 exhibit the highest storage moduli compared to those exhibited by the other varnishes, indicating that these samples are more rigid. With an increase in temperature from 30°C, the samples begin exhibiting a rubbery plateau—from 75°C onwards—where the free volume and, therefore, the molecular motions increase. This is correlated with the glass transition temperature (Tg). In the rubbery plateau, PI-2 exhibits a higher modulus than those exhibited by the remaining samples, caused by the restriction of the free movement of polymer chains.20

Fig. 7
figure 7

Results of the dynamic mechanical analysis of the varnish films: (a) storage modulus and (b) damping (tan delta)

Figure 7b shows the damping (tan delta) of the samples. The tan delta peak is also used as an indicator of Tg in thermosetting resins,21 where the peak center indicates the glass transition. The tan delta peak also provides information about the degree of crosslinking of resins in two ways: the peak dislocates to higher temperatures, and the peak height decreases.6 Among all samples, PI-4 displays the lowest peak height and the highest Tg. This behavior could be attributed to the increased distribution of the molecular weights of the reactive groups participating in crosslinking formation or the increased heterogeneity of the crosslinked structure.6 Lorandi et al.22 also used the center of the tan delta peak height to estimate the Tg of resins. The highest value was found for PI-4. Asif et al.6 studied polyurethane resins cured by UV-light and attributed the increase in Tg to a higher crosslink density.

The behavior observed in the results of dynamic mechanical analysis could be attributed to sample curing, as indicated by the differences in the tan delta peak height and X-axis position (temperature). This behavior appears to be governed more by the inhibited free radical polymerization induced by the presence of oxygen. Karasu et al. reported that a drawback of UV-LED curing systems is the low emission intensity, which can increase the oxygen-induced inhibition of polymerization. Molecular oxygen deactivates the excited states of the photoinitiator and scavenges the propagating radicals, resulting in peroxy radicals that cannot continue polymerization and causing an increase in the heterogeneity of the crosslinked structure.23 In this study, although synergistic amines were used to mitigate the inhibition of polymerization due to the presence of oxygen.1 the ratio might need adjustment.

Tensile testing

The tensile strengths of the varnishes were evaluated, and the stress, strain, and modulus are summarized in Table 4. PI-3 exhibits the lowest strength, and its elongation is comparable to those of the other samples. Among all samples, no significant difference was observed in the tensile strength within standard deviation. PI-6 exhibited the highest elongation at break, which could be attributed to the type of photoinitiator used or to its reactivity, forming polymers with different molecular structures.

Table 4 Tensile strength results of the varnish films: tensile strength (MPa), elongation at break (%), and elastic modulus (MPa)

The highest elastic modulus is observed for PI-4. These results are in accordance with those of dynamic mechanical analysis, specifically the tan delta peak height, where a peak reduction was observed, possibly indicating that PI-4 is highly crosslinked and, therefore, more rigid at 30°C.

Colorimetry

The colorimetric results are summarized in Table 5. The readings compare the cured and wet films. The variable that represents film yellowing is the variation in b* values. Among the wet and cured films, all varnish formulations presented variations in b* values. The higher the b* value, the higher the yellow saturation, which is an undesirable characteristic in terms of optical properties. Three photoinitiators—PI-1, PI-5, and PI-6—presented negative variations in the b* values, implying that the starting resin already had a degree of yellowing and lost some of this characteristic during curing. For the PIs that had a positive variation in b* values, yellowing occurred after curing.

Table 5 Colorimetry results of the varnishes

As the PI cannot dye the varnish, another important parameter is the b* value of the wet film. Evaluating the degree of post-curing yellowing is important because it can interfere with the final color of the product and lead to undesirable results. The presence of a synergistic amine is known to cause an increase in the yellowing of the varnish1; however, this effect can be attenuated using a PI, especially if this material does not completely react during UV-LED curing. The absolute values of b* are also relevant because the yellowing of the films could be caused by the photoinitiator. In this case, the final varnish must be as transparent as possible to not interfere with the color.

Morphology of the varnishes

Figure 8 presents the scanning probe micrographs of the varnishes. The varnish with the highest surface roughness is PI-2, whereas those with the least roughness are PI-4 and PI-5. In the case of PI-3, irregularities are observed. While this could be the result of the remaining air bubbles trapped in the samples, some of the surface and thickness irregularities could also be ascribed to the processing of the films by spray coating. According to Tedde et al. (2009), spray coating is considered the most flexible processing technique; however, one of its disadvantages is the high surface roughness of the resulting films.24 This could partly explain the low tensile strength of this varnish, as defects in the film could lead to points of failure, yielding samples with reduced mechanical performance. It should be noted that this study was performed with relatively thick films (1 mm) where oxygen inhibition is more pronounced; consequently, the surface can be relatively more compliant than the inner layer, which can increase the roughness of the sample surface.23

Fig. 8
figure 8

AFM profiles of the cured varnishes

The lower the surface tension of a polymer, the lower its wettability. This is measured when the polymer is in its wet state. Internal tensions caused by shrinkage after curing could also affect the behavior of the polymer films, as they are the cause of such points of tension, which could extend to the surface, inside the molecular structure.

Conclusions

In this study, varnishes with six different types of photoinitiators were obtained and characterized. Using FTIR, the complete curing of the films was observed in all samples. PI-4 exhibited the best dynamic mechanical properties with sufficiently high glass transition temperature and the highest modulus, indicating that it was a rigid film with a sufficiently high crosslinked network. In contrast, PI-1 and PI-5 exhibited the lowest photoyellowing. PI-1 exhibited satisfactory mechanical properties and promise as an alternative for a varnish with adequate surface finish. Due to the large thickness of the films produced, a high roughness as well as a large standard deviation in tensile strength were observed in all samples.