A thiol-functional amine synergist as a co-initiator for DLP 3D printing applications

Magdalena Jankowska^a, Ozge Ozukanar^b, Emrah Çakmakçi*^c and Joanna Ortyl*^def
aCracow University of Technology, Faculty of Chemical Engineering and Technology, Department of Biotechnology and Physical Chemistry, CUT Doctoral School, Cracow, Poland
^bDepartment of Chemistry, Istanbul Technical University, 34469 Istanbul, Türkiye
^cDepartment of Chemistry, Marmara University, 34722 Istanbul, Türkiye. E-mail: emrah.cakmakci@marmara.edu.tr
^dCracow University of Technology, Faculty of Chemical Engineering and Technology, Department of Biotechnology and Physical Chemistry, Cracow, Poland. E-mail: jortyl@pk.edu.pl
^ePhoto HiTech Ltd, Bobrzynskiego 14, 30-348 Cracow, Poland
^fPhoto4Chem Lea 114, 30-133 Cracow, Poland

First published on 19th August 2025

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1. Introduction

Currently, photopolymerization processes, and consequently, 3D printing in light-initiated technologies, are used in biomedical engineering,^1–6 medicine,^7–12 and dentistry.^13–16 Additive manufacturing utilizing DLP (digital light processing) technology allows three-dimensional structures with excellent precision to be obtained.^17–19 DLP 3D printing has become very popular owing to its ability to produce high-resolution materials and perfect surface finishes, which are extremely important in applications that require very high-quality materials.^20–22 DLP 3D printing, and consequently photopolymerization processes, are becoming increasingly popular owing to the high availability of monomers that polymerize according to the radical^23–25 and cationic^26–31 mechanism.

The crucial factor determining the effective photopolymerization process during DLP printing is the proper selection of initiating system components.³² The initiator should completely or partially absorb the radiation emitted by the 3D printer projector (typically, λ = 405 nm).³³ The appropriate photoinitiator employed in the composition undergoing polymerization absorbs the radiation emitted by the light source and, under its influence, is broken down into radicals or ions (cations or anions), which in turn initiate the polymerization process. Initiation systems often include various amines^34,35 such as N,N-dimethyl-p-toluidine, 2-ethyl-dimethylbenzoate, and N-phenylglycine. The amines used in the initiator systems significantly accelerate the photopolymerization process, which is very important from an application point of view because it allows a significant reduction in the exposure time of individual layers during the 3D printing process. Shortening the exposure time reduces the duration of the printing process, resulting in lower costs. The selection of suitable monomers for photocurable resins is also significant as it determines the properties of the resulting material.^36–40 The monomers employed affected the mechanical parameters. The appropriate selection of input materials for the 3D printing process allows the fabrication of materials with specific properties tailored to unique applications. Therefore, materials with low polymerization shrinkage, high strength, and low cytotoxicity are obtained.

There is a growing trend towards the use of bio-based monomers and polymers in several applications, including 3D printing and photocuring, owing to the environmental issues associated with petroleum-derived polymers.^41–44 To efficiently design and synthesize bio-based monomers, one should consider the principles of green chemistry (GC), which was put forward by Paul Anastas in 1998 as an outcome of the approaches to resolve the problems of the depletion of petroleum resources and increasing environmental pollution.⁴⁵ According to the GC principles, the utilization of renewable feedstocks is of great importance. GC also emphasizes that materials should be designed for degradation. Therefore, several monomers have been prepared in recent years for photocuring and 3D-printing by taking these principles into account.^46–52 Herein, a monomer with a high bio-based content was synthesized using vanillin and the diamine dimer. The new monomer prepared in this study contains imine linkages that can be easily degraded under acidic conditions.

The search, design, and synthesis of new materials dedicated to DLP 3D printing is a major challenge and a focus area for many researchers involved in photopolymerization processes. This article presents newly synthesized materials dedicated to photopolymerization and their application in DLP 3D printing. The subject of this article is a newly synthesized diamine–vanillin dimethacrylate monomer and amine synergist AS. The effect of the addition of the AS compound on the radical photopolymerization kinetics was investigated. The effect of AS on the oxygen inhibition characteristics of photopolymerization processes that proceed according to the radical mechanism was also analyzed. The final stage of this research was an application study related to the utilization of the newly synthesized compounds in additive manufacturing using the DLP technique, which represents a promising approach in terms of suitability for further applications related to biomedical engineering.

2. Experimental

2.1. Materials

The newly synthesized amine synergist AS was employed as an amine co-initiator in the radical photopolymerization process; the synthesis description and scheme are presented in the supplement in the chapter titled “Amine synergist”, and spectra confirming the structure of the amine are presented in Fig. S1–S4. Ethyl 4-dimethylaminobenzoate (EDB; Alfa Aesar) was used as the amine reference. The following compounds were used as commercial initiators: bis-(4-t-butylphenyl)iodonium hexafluorophosphate (IOD; Lambson), diphenyliodonium hexafluorophosphate (HIP; Alfa Aesar), (2,4,6-trimethyl benzoyl) phosphine oxide (TPO; Allnex), and (7-methoxy-4-methylcoumarin-3-yl)phenyliodonium hexafluorophosphate (7MP). The structures of all the photoinitiating system components are shown in Fig. 1.

The monomers chosen for the kinetics of the photopolymerization process and 3D printing experiments were trimethylolpropane triacrylate (TMPTA, Sigma Aldrich), 1,6-hexanediol diacrylate (HDDA, Sigma Aldrich), diurethane dimethacrylate, a mixture of isomers (UDMA, Sigma Aldrich), and the newly synthesized biobased monomer diamine–vanillin dimethacrylate. The synthesis description of the diamine–vanillin dimethacrylate monomer and the synthesis scheme are presented in the Supplement in the chapter titled “Biobased monomer”, and the spectra confirming the structure of the obtained monomer are presented in Fig. S5–S8. The structure of the monomer was validated using NMR and FT-IR. The monomer structures employed to investigate radical photopolymerization kinetics are shown in Fig. 2.

Silicon dioxide was utilized to obtain photocurable polymer nanocomposites (nanoparticle size 10–20 nm; Sigma Aldrich).

2.2. Characterization methods

3. Results and discussion

3.1. Spectroscopic and electrochemical properties of the amine synergist and other components of initiating systems

Absorbance analyses were conducted to understand the spectroscopic characteristics of the initiating system components. Absorbance measurements of the commercial cationic photoinitiators based on iodonium salts: IOD (bis-(4-t-butylphenyl)iodonium hexafluorophosphate), HIP (diphenyliodonium hexafluorophosphate), and 7MP ((7-methoxy-4-methylcoumarin-3-yl)phenyliodonium hexafluorophosphate) and amine compounds as co-initiators (commercially available amine EDB (ethyl 4-dimethylaminobenzoate) and the newly synthesized co-initiator AS for initiating radical photopolymerization) were performed. The measurements demonstrated that the amine synergist AS absorbs radiation in the ultraviolet range, reaching a wavelength of λ = 370 nm, whereas the absorption spectrum of the commercial co-initiator EDB reaches λ = 350 nm, which already suggests an advantage of AS over EDB, as it allows an increase in the range of light sources used for the photopolymerization process. Moreover, the absorption characteristics of the initiators, which were subsequently applied to initiate the radical photopolymerization process in combination with an amine co-initiator, were also compared. As shown in Fig. 4a, commercially available cationic photoinitiators based on the iodonium salt IOD and HIP absorb irradiation up to approximately 320 nm. However, the absorption of the cationic initiator, 7MP, extended to the visible range. The absorbance spectra of all initiators/co-initiators employed to initiate the radical photopolymerization process are shown in Fig. 4a and b.

The molar extinction coefficients and absorption maxima are summarized in Table 1.

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In addition, the photostability of the amine synergist AS in acetonitrile was measured to check the stability of the newly synthesized compound under different ultraviolet light sources (λ = 300 nm, λ = 340 nm, and 365 nm). Research using light sources with λ = 340 nm and λ = 365 nm confirmed that the compound was stable. The photolysis characteristics of the AS compound are presented in the SI (Fig. S9a–S9c). Moreover, to confirm the feasibility of employing the amine synergist AS as a co-initiator in two-component initiator systems for initiating radical photopolymerization, its oxidation potential was compared to that of the commonly used EDB co-initiator. Electrochemical tests were performed on the amine synergists using cyclic voltammetry. Studies have shown that the oxidation potential of amine synergists is 792 mV, whereas literature data show that the oxidation potential of EDB is 1058 mV (vs. Ag/AgCl).⁶² The cyclic voltammogram curves of the oxidation of the amine synergist AS in acetonitrile are shown in the SI (Fig. S10).

3.2. Effect of amine synergist addition on radical photopolymerization kinetics under both aerobic and oxygen-limited conditions

The first stage of kinetic studies involved checking the effect of the amine synergist additive on radical photopolymerization kinetics under different conditions, that is, under aerobic and atmospheric oxygen-limited conditions. The composition of the monomer mixture was optimized for further kinetic experiments and for light-initiated 3D printing. For these experiments, the synthesized biobased monomer containing imine linkages (DVDMA), displaying degradability under acidic conditions or leading to dynamic covalent networks, was employed. Various amounts of this monomer (0, 10, 20, 30, and 40% by weight) were added to the UDMA/HDDA (60/40 w/w) formulation, replacing UDMA. The commercially available TPO radical initiator (1% w/w) was used as the photoinitiator. The initiator was weighed in relation to the monomer matrix. The precise compositions of the resins investigated are listed in Table 2. Measurements were performed under oxygen-limited conditions. For the measurements, a Vis-LED emitting radiation with a wavelength of λ = 405 nm was applied, where the intensity of the light incident on the examined sample was 5.58 mW cm⁻².

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Investigations of the effect of biobased monomer addition on kinetic parameters showed that functional group conversion first increased with 10% and 20% biobased monomer addition and then started to decrease (Fig. 5a). The functional group conversion was calculated based on the disappearance of the band characteristic of acrylate groups – υ = 1635 cm⁻¹ (Fig. 5b). When the concentration of the newly synthesized monomer was 40%, a dramatic decrease in conversion was observed compared to a smaller amount of biobased monomer-containing formulations (≥30%). This was attributed to the increased viscosity of the formulation. The formulation with the composition UDMA:HDDA:biobased monomer 40:40:20 was selected for subsequent experiments related to the effect of atmospheric oxygen on the photopolymerizing formulation and 3D printing experiments because it had the highest conversion of acrylate groups (C = 88%), the highest value of the dC/dt kinetic curve slope (dC/dt = 4.37 s⁻¹, Fig. 5c), and a short induction time (2.51 s, Fig. 5c) (Table 3).

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Table 3 demonstrates the effect of biobased monomer addition on the conversion of functional groups, slope of the kinetic curve, and induction time during the measurement of the radical photopolymerization process kinetics.

The primary kinetic research stage allowed optimization of the monomer composition of the photocurable resin. Subsequently, the effects of the amine synergist additives (0, 1, 2, and 5% w/w) on the kinetics of the radical photopolymerization process were examined (Table 4). The amine synergist, like the initiator, was weighed relative to the monomer matrix. Initially, the tests were conducted under anaerobic conditions, where a drop of the photopolymerizing composition was applied between two pieces of polypropylene film. The measurements involved a Vis-LED emitting radiation with a wavelength of λ = 405 nm, where the intensity of the light incident on the studied sample was 5.58 mW cm⁻². The exact formulations of the resins are listed in Table 4.

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Based on infrared spectroscopy measurements, it was found that addition of small amounts of amine synergists (1% or 2% by weight) did not affect the kinetics of the process, as the conversion of CC bonds remained almost constant (88–89%) for both compositions with and without amine synergists. Addition of 5% amine synergist caused a significant deterioration of the kinetic parameters; it can be seen that the conversion of functional groups decreased (68%), and the value of the slope of the kinetic curve also declined, indicating that the process occurs more slowly (Fig. 6a and c). The functional group conversion was calculated based on the disappearance of the band characteristic of CC bonds (Fig. 6b).

Table 5 summarizes the results obtained from radical photopolymerization carried out under atmospheric oxygen-limited conditions for resins with varying amounts of the amine synergist AS.

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For comparison, analogous measurements were performed under aerobic conditions. We also investigated the effect of AS on this selected formulation under an oxygen atmosphere (Fig. 7a–c). A resin drop was placed on a pastille fabricated from barium fluoride. These results clearly demonstrate that the addition of the newly synthesized AS was effective under aerobic conditions. Although the conversion was 0% in the absence of AS, it increased to 19% when the AS concentration was 5%, as shown in Table 6. With an increase in the AS content in the investigated composition, a decrease in the oxygen inhibition effect was observed, which is a characteristic of radical photopolymerization. The higher the amount of the amine synergist AS in the photocurable resin, the higher the acrylate monomer conversion. For Resin-20% BIO-TPO, the degree of double-bond over-reactivity was 0%, confirming the absence of radical photopolymerization in the presence of atmospheric oxygen. With the increase in the AS compound in the photocurable compositions, the occurrence of radical photopolymerization and the reduction of the oxygen inhibition phenomenon can be observed. In the case of Resin-20% BIO-TPO-5%AS, in which the addition of the AS compound was 5% by weight, the conversion of CC bonds was 19%. Reduced oxygen inhibition was caused by the presence of a tertiary amine and free thiol groups in the structure of the newly synthesized amine synergist.

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Table 6 summarizes the results obtained from radical photopolymerization carried out under atmospheric oxygen conditions for resins with varying amounts of the amine synergist AS.

Moreover, for the AS compound and all the photocurable resins with the AS additive, thermal stability tests were carried out to verify how the AS additive affects the stability of the compositions and whether the established storage conditions of the photocurable resins are necessary. The same temperature range was established for both the AS compound and AS-added resins. The test samples were heated in the range from 20 °C to 200 °C. The temperature measurement of the amine synergist carried out proved that the tested compound is stable over the entire range of temperatures investigated. Furthermore, at temperatures up to 200 °C, no significant thermal effects were observed for the photocurable resins without the AS additive (Resin-20% BIO-TPO) and with 1% (Resin-20% BIO-TPO-1%AS) and 2% AS (Resin-20% BIO-TPO-2%AS) additive. Therefore, at increased temperatures, the addition of AS and, consequently, thiol groups does not affect the thermal stability of the compositions. In the case of the addition of 5% AS to the photocurable resin (Resin-20% BIO-TPO-5%AS), thermal effects can be observed on the DSC plot, which appear at a temperature of about 140 °C, which, however, does not cause any problems related to the storage of resins at room temperature. No signs of instability of the examined formulations were observed during the tests. Time and temperature dependence graphs obtained by the DSC method for the AS compound and the resins in Table 4 are included in the SI (Fig. S11–S15).

3.3. 3D printing experiments with photocurable radical resins based on monomers: UDMA/HDDA/biobased monomer

Subsequently, based on kinetic studies, resins were selected for projector light-initiated 3D printing experiments. Three photocurable resins with a TPO photoinitiator were selected for 3D printing experiments. The first and most fundamental component was a resin containing a UDMA/HDDA/biobased monomer (Resin-20% BIO-TPO). Subsequently, the basic resin was modified to obtain further photocurable resins for visible-light-initiated additive processes. The first modification was the addition of 5% by weight of the amine synergist to Resin-20% BIO-TPO, resulting in Resin-20% BIO-TPO-5%AS. Next, the resin with 5% AS was further modified with 2% nanosilica addition (Resin-20% BIO-TPO-5%AS-2% SiO₂). The nanosilica was weighed in relation to the monomer matrix. The compositions were selected for the formation of spatial structures using light, as shown in Table 7.

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Viscosity measurements of the monomer matrices were performed before the DLP 3D printing experiments were conducted. Viscosity is an extremely important parameter that determines the suitability of photocurable resins for light-initiated 3D printing. Low-viscosity resins are generally advantageous for the printing process; however, parts produced from such resins tend to exhibit high brittleness. Conversely, higher viscosity photocurable resins facilitate the fabrication of objects with improved mechanical properties. The viscosity value provides essential information about the 3D printing process, including considerations such as the need for support structures Table 8 presents the matrices subjected to viscosity measurements.

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Viscosity measurements showed that the viscosity of Matrix-20% BIO containing UDMA/HDDA/biobased monomer was 728 mPa s. Addition of 5% AS to Matrix-20% BIO caused a slight increase in viscosity, and the viscosity of Matrix-20% BIO-5%AS became 740 mPa s. The viscosity of the matrix containing Matrix-20% BIO-5%AS-2% SiO₂ nanosilica is 850 mPa s. The viscosity experiments demonstrated that the investigated resins are high viscosity resins and show suitability for DLP 3D printing experiments. Fig. 8 presents the viscosity values of the monomer matrices utilized for projector-light-initiated 3D printing.

Jacob's working curve test, an essential tool for determining the basic parameters of 3D printing, was performed. The test allowed the determination of the critical energy E_c and depth of light penetration D_p. The intersection of the obtained curves with the X-axis indicates the values of the critical energy, which indicates the minimum exposure required to cure the solid layer, whereas the slope of the curves indicates the depth of penetration (Fig. 9a). The test was based on printing five slices (1 × 1 cm) of each resin without a printing platform (Fig. 9b) using different resin exposure times (5, 10, 15, 20, and 25 s – Fig. 9c). Based on the thickness of the slices measured with a micrometer screw, exposure time of a slice, and intensity of light incident on the printing object during printing, the basic printing parameters defining the effectiveness of the photopolymerizing resin were determined. Based on the quotient of the curing time of each slice and the intensity of the light incident on the printed slice (measured with a ThorLabs PM 160 Optical Power Meter), E₀ (light energy at the surface [mJ cm⁻²]) was calculated.

When compared, it can be seen that the addition of AS decreased the E_c value to 2.04 mJ cm⁻² (Resin-20% BIO-TPO-5%AS) from 3.84 mJ cm⁻² (Resin-20% BIO-TPO). The decrease in critical energy implies that less energy is required for curing. Moreover, it can be seen that the penetration depth also decreased when AS was incorporated. The decreased light penetration depth indicates greater control over the resolution and quality of the printing. Thus, it can be said that the synthesized AS is an intriguing additive for 3D printing applications. When fillers are used in 3D printing resins, more energy is required for curing and E_c increases. We added 2% SiO₂ nanoparticles to 5% AS-containing formulations and found that the E_c was almost the same as that of the filler-free formulation. This situation was attributed to the oxygen inhibition function of AS, which increased the curing speed and decreased E_c. Table 9 summarizes the 3D printing parameters obtained during Jacob's working curve test.

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The very promising results obtained during Jacob's working curve test allowed the resins to be applied in the printing of three-dimensional structures. The same printer settings were used for all the resins. The power of the printing device was 20%, which provides a light intensity incident on the printing object of 1.99 mW cm⁻². The exposure time for the initial printed layers was 30 s, while that for the remaining layers was 8 s. The thickness of each layer is 100 μm. The obtained structures were visualized on an optical microscope, as well as on a scanning electron microscope. The following graph illustrates the object obtained from Resin-20% BIO-TPO-5%AS-2% SiO₂ (Fig. 10). Graphics of the prints obtained with the other resins (Resin-20% BIO-TPO and Resin-20% BIO-TPO-5%AS) are shown in the SI (Fig. S16–S17a).

In addition, SEM imaging was performed on Resin-20% BIO-TPO-5%AS and Resin-20% BIO-TPO-5%AS-2% SiO₂ prints to study the resolution of the obtained 3D structures. Visualization using scanning electron microscopy (SEM) enabled the acquisition of precise images of the surface, which provided information about the high quality of the layers of the resulting print. Fig. 11 shows the graph obtained for the structure printed with Resin-20% BIO-TPO-5%AS-2% SiO₂. A graphic of a 3D object printed with Resin-20% BIO-TPO-5%AS is shown in the SI (Fig. S17b).

SEM analysis of the print obtained from the photocurable resin Resin-20% BIO-TPO-5%AS-2% SiO₂ showed that the resulting print has a high resolution, as evidenced by the clearly visible layers. Additionally, based on the above-mentioned SEM analysis, it can also be deduced that Resin-20% BIO-TPO-5%AS-2% SiO₂ is highly suitable for DLP 3D printing, as the thickness of the obtained layers is very similar to the model layer thickness of 100 μm, confirming that the investigated resin is characterized by very low polymerization shrinkage.

3.4. The application of AS as a co-initiator for cationic photoinitiators to initiate radical photopolymerization

The first stage of this research investigated the effects of different AS contents on the kinetics of the photopolymerization process and DLP 3D printing. The second stage of the research is related to the application of AS in two-component initiator systems, where commercial cationic photoinitiators based on iodonium salts (IOD and HIP), characterized by an absorption spectrum reaching up to approximately 320 nm, were applied as photoinitiators. The use of amines in two-component initiator systems allows for the generation of reactive radicals that can initiate the radical photopolymerization process using a light source with a wavelength that commercial iodonium salts cannot absorb. Infrared spectroscopy was used to compare the amine synergist AS with EDB as co-initiators for initiating radical photopolymerization. For this purpose, four resins differentiated by the initiator system were prepared. The resin compositions are listed in Table 10. The compositions were prepared to contain twice the molar excess of amine relative to the iodonium salt.

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Radical photopolymerization was performed under conditions of limited access to atmospheric oxygen, and a UV-LED diode emitting radiation with a wavelength of λ = 340 nm was applied as the light source. The light source was turned on 10 s after the measurement. The light intensity incident on the photopolymerizing radical resin is 6.99 mW cm⁻². Fig. 12 illustrates the results obtained during the radical photopolymerization of the TMPTA monomer and the spectra before and after photopolymerization, indicating the band characteristics of acrylates.

As shown in Fig. 12, two-component initiator systems with AS were more effective than those containing EDB. The TMPTA acrylate monomer conversion was approximately 50% for resins containing AS + IOD (Resin-AS+IOD_TMPTA) and AS+HIP (Resin-AS+HIP_TMPTA), whereas analogous resins with EDB only reacted at 18% (Resin-EDB+HIP_TMPTA) and 27% (Resin-EDB+IOD_TMPTA), as shown in Table 11. Based on preliminary application studies, the newly synthesized amine synergist AS has a significant advantage over commercially available EDB. The better results obtained for AS than for EDB are due to differences in the molar values of the extinction coefficients, as well as the oxidation potential. The molar extinction coefficient for EDB reached very low values at λ = 340 nm, whereas the oxidation potential of EDB was higher than that of the newly synthesized amine synergist AS.

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The next stage was to prepare a radical resin with a two-component initiator system containing the amine synergist AS and commercial cationic photoinitiators based on the iodonium salt 7MP ((7-methoxy-4-methylcoumarin-3-yl)phenyliodonium hexafluorophosphate). The radical resin formulations used are listed in Table 12. Because the absorption range of the photoinitiator 7MP extends into the visible range, Vis-LED@405 nm was employed. The light intensity incident on the photopolymerizing radical resin is 15.98 mW cm⁻².

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The investigation involved checking the effect of the addition of amine synergists on the initiation of radical photopolymerization, where a typical cationic photoinitiator was applied as the photoinitiator, which is ideal for initiating the cationic photopolymerization of epoxy and vinyl monomers, as shown in the literature.⁶³ As shown in Fig. 13, the cationic initiator 7MP was not suitable for initiating radical photopolymerization of TMPTA, as the resulting conversion of acrylate groups was 0%. As the amount of the co-initiator AS in the composition increases, higher conversions of acrylate groups can be observed, reaching 61% at a fivefold molar excess of AS relative to the photoinitiator 7MP. The combination of AS with a cationic initiator allows the generation of reactive radicals, and consequently, the efficient initiation of radical photopolymerization under visible light. With the increasing amount of AS in the photocurable resins, it is possible to observe not only an increase in the degree of over-reactivity of the functional groups, but also an increase in the speed of the photopolymerization process, as evidenced by the increasing value of the slope of the kinetic curve and the decreasing value of the induction time (Fig. 13a and c). Fig. 13b presents the disappearance of the band characteristic of the acrylate groups during radical photopolymerization of the TMPTA monomer.

Table 13 summarizes the results obtained from radical photopolymerization carried out under atmospheric oxygen-limited conditions for resins containing the 7MP initiator by varying the amount of the co-initiator AS.

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The final research stage on the application of a two-component initiator system with AS as the co-initiator and 7MP as the photoinitiator was the 3D printing experiment. An initiator system containing twofold molar excess of the amine relative to the 7MP initiator was chosen for the investigation. The two resins were polymerized according to a free radical mechanism. One contained UDMA/HDDA monomers (50/50 w/w) – Resin-7MP:AS (1:2)_UDMA/HDDA, whereas the other contained an additional biobased monomer UDMA/HDDA/biobased monomer (40/40/20 w/w) – Resin-7MP:AS (1:2)_UDMA/HDDA/BIO. First, kinetic studies were performed on the prepared compositions to determine the effect of biobased monomer addition on the kinetics of the radical photopolymerization process. The research was conducted by applying Fourier transform infrared spectroscopy, where an LED emitting radiation with a wavelength of λ = 405 nm was used as the light source. This allowed for the creation of conditions analogous to 3D printing, where the light source was a projector with the same wavelength.

As shown in Fig. 14, addition of 20% biobased monomer to the UDMA/HDDA mixture caused a decrease in the degree of functional group conversion, which was negligible at 15%. Despite obtaining a lower conversion for the resin with the addition of biobased monomers (Resin-7MP:AS (1:2)_UDMA/HDDA/BIO), the other kinetic parameters improved, the induction time decreased, and the value of the slope of the kinetic curve increased (Table 14).

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The promising results obtained for radical resins with the AS + 7MP two-component initiation system allow them to be further developed and explored for potential applications in 3D printing. First, the viscosity properties of the resins dedicated to 3D printing were measured. Therefore, two formulations were prepared: radical monomers and amine synergists. The precise formulation for viscosity research is presented in Table 15.

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Viscosity measurements showed that Matrix-UDMA/HDDA has a viscosity of 540 mPa s. Modification of the above-mentioned matrix by addition of 20% biobased monomer (Matrix-UDMA/HDDA/BIO) results in a significant increase in the viscosity of the resin formulation (738 mPa s), suggesting that the structures obtained from the photocurable resin with the addition of a biobased monomer will have better mechanical properties compared to the resin without the addition of a biobased monomer. Fig. 15 illustrates the viscosity values for the different monomer matrices.

For photocurable resins: Resin-7MP:AS (1:2)_UDMA/HDDA and Resin-7MP:AS (1:2)_UDMA/HDDA/BIO, Jacob's working curve test was carried out using the same procedure as for resins containing the TPO initiator (Table 16).

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Jacob's working curve test showed that addition of 20% biobased monomer to the UDMA/HDDA mixture resulted in an almost nine-fold increase in the critical energy value while reducing the depth of light penetration, which is not beneficial for 3D printing. The exposure time for the initial layers of both Resin-7MP:AS (1:2)_UDMA/HDDA and Resin-7MP:AS (1:2)_UDMA/HDDA/BIO was 100 s, while that for the remaining layers was 30 s. The difference is in the intensity of the light incident on the printed object. For Resin-7MP:AS (1:2)_UDMA/HDDA, the light intensity during printing was 2.99 mW cm⁻², whereas that of Resin-7MP:AS (1:2)_UDMA/HDDA/BIO was 3.98 mW cm⁻². The thickness of the printed layer in both cases was 100 μm. The plots of the dependence of light penetration depth D_p on the critical energy E_c for each of the studied resins with the 7MP initiator are presented in the SI (Fig. S18). Fig. 16a presents images of Resin-7MP:AS (1:2)_UDMA/HDDA printed on an optical microscope, whereas Fig. 16b illustrates images of this print taken on a scanning electron microscope. The SEM image confirmed the high quality of the obtained three-dimensional object. Single layers can be seen, as well as pixels, which indicates that the 7MP + AS two-component initiator system is highly suitable for initiating radical photopolymerization and can successfully find applications in light-initiated 3D printing. In addition, as shown in Fig. 16b, the pixels of the printed object are approximately 50 μm in size, which is equal to the resolution of the printer employed, indicating very low polymerization shrinkage.

Moreover, microscopic analysis of the object printed from Resin-7MP:AS (1:2)_UDMA/HDDA/BIO was performed. According to Fig. 17, the printed shapes are visible, but defects between the printed layers can also be seen. Based on the obtained prints, it can be concluded that the addition of a biomonomer results in the deterioration of print quality, but is a very promising approach for biomedical applications. The SEM analysis of the print obtained from Resin-7MP:AS (1:2)_UDMA/HDDA/BIO is presented in the SI (Fig. S19).

In addition, hardness tests were conducted on the prints received from Resin-7MP:AS (1:2)_UDMA/HDDA and Resin-7MP:AS (1:2)_UDMA/HDDA/BIO using the Shore scale. The printout obtained from Resin-7MP:AS (1:2)_UDMA/HDDA was characterized by a hardness of 71 ± 1, whereas the printout obtained from Resin-7MP:AS (1:2)_UDMA/HDDA/BIO with the addition of biobased monomers was characterized by a hardness of 83 ± 2. The high viscosity of the biobased monomer increased the viscosity of the UDMA/HDDA monomer matrix, resulting in improved mechanical properties. Summarizing the validity of the application of the biobased monomer to the photopolymerization process, it can be concluded that the addition of the biobased monomer reduces the induction time during the radical photopolymerization process, which is suitable for projector light-initiated 3D printing, and improves the mechanical properties of the printed three-dimensional structures.

3. Conclusion

This paper describes novel synthesized amine compounds and their potential applications in photochemistry. It has been demonstrated that it can be successfully employed in two-component initiator systems in conjunction with an iodonium salt that absorbs up to approximately λ = 320 nm (IOD or HIP) to initiate radical photopolymerization under light-emitting radiation with a wavelength of λ = 340 nm. In addition, it has also been proven that the newly synthesized amine synergist, owing to the presence of a tertiary amine and free thiol groups in its structure, can reduce oxygen inhibition, which is an unfavorable phenomenon characteristic of the radical photopolymerization process. The application of the amine synergist creates new opportunities to replace the current two-component solution. The used of the amine synergist allows the replacement of two independent ingredients, namely an amine (source of additional radicals) and a thiol (reduction of oxygen inhibition), with a single ingredient containing both an amine and a thiol group. In addition, due to the absorption spectrum reaching the UV-A range, the use of the amine synergist allows the expansion of the capabilities of the light sources employed, allowing for greater application possibilities. Most commercially available amines absorb radiation in the ultraviolet range, reaching up to about 350 nm, which does not allow the application of a λ = 365 nm light source. The presence of both groups in a single molecule enables synergistic action, which translates into increased reactivity, selectivity and stability of the system. The employment of a single compound containing an amine group and a thiol group is a more efficient and economical solution than the use of two separate components, which may have important implications for the further development of modern methods of synthesis and chemical applications. Furthermore, it has been proven that the AS compound can be employed in a two-component initiator system in combination with a standard 7MP cationic initiator using light in the visible range. The 7MP initiator, as a one-component initiator system, was insufficient to efficiently initiate radical photopolymerization; when combined with AS, there was a high initiation efficiency. In addition, amine synergists can be utilized in DLP 3D printing, as confirmed by the obtained prints characterized by very high resolution, precision, and surface quality. Another aspect discussed in this article is the synthesis and application of a novel monomer – diamine–vanillin dimethacrylate. The effects of the newly synthesized monomer on the radical photopolymerization kinetics and the DLP 3D printing process were investigated. It has been proven that the addition of a biobased monomer to photocurable resins reduces the induction time. This also proved the suitability of the biobased monomer for obtaining three-dimensional structures with improved mechanical properties. DVDMA bio-based monomers contain imine linkages that are easily cleaved under acidic conditions.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data supporting this article have been included as part of the SI.

Supplementary Information includes the following details: (1) description of the synthesis of the amine synergist (AS) compound and its spectroscopic characterization, including analysis by NMR and FT-IR spectroscopy; (2) description of the synthesis and spectroscopic characterization (including NMR and FT-IR spectroscopy) of the bio-based monomer, dimmer diamine-vanillin dimethacrylate (DVDMA); (3) photolysis measurements of the AS compound in acetonitrile upon irradiation with LEDs at 300 nm, 340 nm, and 365 nm; (4) cyclic voltammetry curves of the amine synergist (AS) in acetonitrile; (5) measurements of the thermal stability of the amine synergist (AS); (6) differential thermal analysis of resin compositions, including Resin-20% BIO-TPO and formulations containing the amine synergist at 1% and 2% concentrations (Resin-20% BIO-TPO-1% AS and Resin-20% BIO-TPO-2% AS); (7) experimental results related to 3D printing of various compositions, including graphics showing 3D models, 2D images, and 3D images of prints obtained with Resin-20% BIO-TPO and Resin-20% BIO-TPO-5% AS; (8) Jacob’s working curves for a photocurable resin based on UDMA/HDDA with varying amounts of bio-based monomer and 7 MP initiator; (9) results of 3D printing experiments with compositions based on monomers UDMA/HDDA/BIO, including SEM images of a print obtained with Resin-7MP:AS (1 : 2)_UDMA/HDDA/BIO using DLP technology; (10) results of the recycling process of the dimer diamine from photocured films. See DOI: https://doi.org/10.1039/d5py00603a.

Acknowledgements

This research was financed by the Medical Research Agency under the project 'Onco-Lumi-3D,' No. KPOD.07.07-IW.07-0125/24, from the National Recovery and Resilience Plan (NRRP) program, Component D: Efficiency, Accessibility, and Quality of the Healthcare System, Investment D3.1.1: Comprehensive Development of Research in Medical and Health Sciences. The NRRP program was funded by the European Union Recovery Instrument (NextGenerationEU) through the Recovery and Resilience Facility (RRF) for the period 2021–2026.

References

1. C. Greant, B. Van Durme, J. Van Hoorick and S. Van Vlierberghe, Adv. Funct. Mater., 2023, 33 Search PubMed .
2. A. N. Generalova, P. A. Demina, R. A. Akasov and E. V. Khaydukov, Russ. Chem. Rev., 2023, 92, 2 CrossRef .
3. A. Randhawa, S. D. Dutta, K. Ganguly, D. K. Patel, T. V. Patil and K. T. Lim, Macromol. Biosci., 2023, 23 Search PubMed .
4. N. Vidakis, M. Petousis, N. Michailidis, C. David, V. Saltas, D. Sagris, M. Spiridaki, A. Argyros, N. Mountakis and V. Papadakis, Ceram. Int., 2024, 50, 14919–14935 CrossRef CAS .
5. Y. Qi, H. Lv, Q. Huang and G. Pan, Eur. Polym. J., 2024, 213, 113133 CrossRef CAS .
6. I. Chiulan, E. B. Heggset, Ş. I. Voicu and G. Chinga-Carrasco, Biomacromolecules, 2021, 22, 1795–1814 CrossRef CAS .
7. S. Enbergs, J. Spinnen, T. Dehne and M. Sittinger, J. Tissue Eng. Regener. Med., 2023, 2023 Search PubMed .
8. D. Bahati, M. Bricha and K. El Mabrouk, Adv. Eng. Mater., 2023, 25 Search PubMed .
9. I. Roohani, E. Newsom and H. Zreiqat, Int. Mater. Rev., 2023, 68, 1075–1097 CrossRef .
10. X. Xu, A. Awad, P. Robles-Martinez, S. Gaisford, A. Goyanes and A. W. Basit, J. Controlled Release, 2021, 329, 743–757 CrossRef PubMed .
11. I. Seoane-Viaño, S. J. Trenfield, A. W. Basit and A. Goyanes, Adv. Drug Delivery Rev., 2021, 174, 553–575 CrossRef .
12. Y. Bao, N. Paunović and J. Leroux, Adv. Funct. Mater., 2022, 32, 2109864 CrossRef .
13. A. Bagheri and J. Jin, ACS Appl. Polym. Mater., 2019, 1, 593–611 CrossRef .
14. Z. Hu, J. Xu, Y. Wang, L. Xu, Y. Yi, B. Liu and Z. Wang, ACS Omega, 2023, 8, 32396–32403 CrossRef .
15. J. Cao, X. Liu, A. Cameron, J. Aarts and J. J. E. Choi, J. Mech. Behav. Biomed. Mater., 2024, 150 Search PubMed .
16. M. Topa-Skwarczyńska, M. Jankowska, A. Gruchała-Hałat, F. Petko, M. Galek and J. Ortyl, Dent. Mater., 2023, 39, 729–742 CrossRef PubMed .
17. Z. Lu, W. Gao, F. Liu, J. Cui, S. Feng, C. Liang, Y. Guo, Z. Wang, Z. Mao and B. Zhang, Addit. Manuf., 2024, 94, 104443 Search PubMed .
18. H. Ding, M. Dong, Q. Zheng and Z. L. Wu, Mol. Syst. Des. Eng., 2022, 7, 1017–1029 RSC .
19. E. Hola and J. Ortyl, Eur. Polym. J., 2021, 150 Search PubMed .
20. L. Yao, P. Hu, Z. Wu, W. Liu, Q. Lv, Z. Nie and H. Zhengdi, in Journal of Physics: Conference Series, Institute of Physics Publishing, 2020, vol. 1549 Search PubMed .
21. X. Wang, J. Liu, Y. Zhang, P. M. Kristiansen, A. Islam, M. Gilchrist and N. Zhang, Virtual Phys. Prototyp., 2023, 18 Search PubMed .
22. F. Li, X. Ji, Z. Wu, C. Qi, J. Lai, Q. Xian and B. Sun, Mater. Lett., 2020, 276, 128037 Search PubMed .
23. I. Kamińska, J. Ortyl and R. Popielarz, Polym. Test., 2015, 42, 99–107 Search PubMed .
24. M. Topa, J. Ortyl, A. Chachaj-Brekiesz, I. Kamińska-Borek, M. Pilch and R. Popielarz, Spectrochim. Acta, Part A, 2018, 199, 430–440 Search PubMed .
25. D. Nowak, J. Ortyl, I. Kamińska-Borek, K. Kukuła, M. Topa and R. Popielarz, Polym. Test., 2018, 67, 144–150 Search PubMed .
26. J. Ortyl, M. Galek, P. Milart and R. Popielarz, Polym. Test., 2012, 31, 466–473 Search PubMed .
27. J. Ortyl, P. Milart and R. Popielarz, Polym. Test., 2013, 32, 708–715 CrossRef .
28. J. Ortyl, M. Galica, R. Popielarz and D. Bogdał, Pol. J. Chem. Technol., 2014, 16, 75–80 CrossRef .
29. J. Ortyl, M. Topa, I. Kamińska-Borek and R. Popielarz, Eur. Polym. J., 2019, 116, 45–55 CrossRef .
30. M. Topa, E. Hola, M. Galek, F. Petko, M. Pilch, R. Popielarz, F. Morlet-Savary, B. Graff, J. Laleveé and J. Ortyl, Polym. Chem., 2020, 11, 5261–5278 RSC .
31. J. Ortyl, P. Fiedor, A. Chachaj-Brekiesz, M. Pilch, E. Hola and M. Galek, Sensors, 2019, 19, 1668 CrossRef CAS PubMed .
32. K. Trembecka-Wójciga, M. Jankowska, W. Tomal, A. Jarzębska, Ł. Maj, T. Czeppe, P. Petrzak, A. Chachaj-Brekiesz and J. Ortyl, Eur. Polym. J., 2023, 198, 112403 CrossRef .
33. E. Geisler, M. Lecompère and O. Soppera, Photonics Res., 2022, 10, 1344 CrossRef .
34. J. Jakubiak, X. Allonas, J. P. Fouassier, A. Sionkowska, E. Andrzejewska, L. Å. Linden and J. F. Rabek, Polymer, 2003, 44, 5219–5226 CrossRef CAS .
35. E. Hola, M. Pilch and J. Ortyl, Catalysts, 2020, 10, 1–28 CrossRef .
36. J. W. Park, G. S. Shim, J. H. Back, H. J. Kim, S. Shin and T. S. Hwang, Polym. Test., 2016, 56, 344–353 CrossRef CAS .
37. V. Jašek, V. Melčová, S. Figalla, J. Fučík, P. Menčík and R. Přikryl, ACS Appl. Polym. Mater., 2023, 5, 9909–9917 CrossRef .
38. L. Schittecatte, V. Geertsen, D. Bonamy, T. Nguyen and P. Guenoun, MRS Commun., 2023, 13, 357–377 CrossRef CAS .
39. S. C. Ligon-Auer, M. Schwentenwein, C. Gorsche, J. Stampfl and R. Liska, Polym. Chem., 2016, 7, 257–286 RSC .
40. A. Al Rashid, W. Ahmed, M. Y. Khalid and M. Koç, Addit. Manuf., 2021, 47 Search PubMed .
41. S. Luleburgaz, E. Cakmakci, H. Durmaz and U. Tunca, Eur. Polym. J., 2024, 112897 CrossRef CAS .
42. S. A. Begum, P. S. G. Krishnan and K. Kanny, Polym. Sci., Ser. A, 2023, 65(5), 421–446 CrossRef .
43. K. N. A. Putri, V. Intasanta and V. P. Hoven, Heliyon, 2024, 10(4), e25873 CrossRef CAS PubMed .
44. H. Zeidler, D. Klemm, F. Böttger-Hiller, S. Fritsch, M. J. Le Guen and S. Singamneni, Procedia Manuf., 2018, 21, 117–124 CrossRef .
45. P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York, 1998, p. 30 Search PubMed .
46. M. Bergoglio, E. Rossegger, S. Schlögl, T. Griesser, C. Waly, F. Arbeiter and M. Sangermano, Polymers, 2024, 16(11), 1510 CrossRef CAS .
47. K. Janssen, G. H. Schnelting, M. Waterink, J. Guit, J. Hul, C. Ye and V. S. Voet, Macromol. Mater. Eng., 2024, 2400036 CrossRef CAS .
48. A. Fantoni, J. Ecker, M. Ahmadi, T. Koch, J. Stampfl, R. Liska and S. Baudis, ACS Sustainable Chem. Eng., 2023, 11(32), 12004–12013 CrossRef CAS .
49. M. A. El-Tayeb, T. M. Dawoud, K. S. Almaary, F. Ameen and H. A. Khonakdar, J. Polym. Environ., 2025, 33(1), 358–373 CrossRef CAS .
50. E. Skliutas, M. Lebedevaite, S. Kasetaite, S. Rekštytė, S. Lileikis, J. Ostrauskaite and M. Malinauskas, Sci. Rep., 2020, 10(1), 9758 CrossRef CAS PubMed .
51. V. Schimpf, A. Asmacher, A. Fuchs, K. Stoll, B. Bruchmann and R. Mülhaupt, Macromol. Mater. Eng., 2020, 305(8), 2000210 CrossRef CAS .
52. X. Chu, J. Tu, H. R. Berensmann, J. J. La Scala and G. R. Palmese, Polymers, 2023, 15(9), 2007 CrossRef CAS .
53. W. Tomal, T. Świergosz, M. Pilch, W. Kasprzyk and J. Ortyl, Polym. Chem., 2021, 12, 3661–3676 RSC .
54. E. Hola, M. Topa, A. Chachaj-Brekiesz, M. Pilch, P. Fiedor, M. Galek and J. Ortyl, RSC Adv., 2020, 10, 7509–7522 RSC .
55. W. Tomal, A. Chachaj-Brekiesz, R. Popielarz and J. Ortyl, RSC Adv., 2020, 10, 32162–32182 RSC .
56. F. Petko, M. Galek, E. Hola, M. Topa-Skwarczyńska, W. Tomal, M. Jankowska, M. Pilch, R. Popielarz, B. Graff, F. Morlet-Savary, J. Lalevee and J. Ortyl, Chem. Mater., 2022, 34, 10077–10092 CrossRef CAS .
57. M. Jankowska, A. Chachaj-Brekiesz, K. Trembecka-Wójciga, A. Jarzębska, M. Topa-Skwarczyńska, M. Pilch and J. Ortyl, Polym. Chem., 2023, 14, 2088–2106 RSC .
58. F. Petko, E. Hola, M. Jankowska, A. Gruchała-Hałat and J. Ortyl, Virtual Phys. Prototyp., 2023, 18, 1 Search PubMed .
59. W. Tomal, D. Krok, A. Chachaj-Brekiesz, P. Lepcio and J. Ortyl, Addit. Manuf., 2021, 48, 102447 CAS .
60. Z. Altıntaş, E. Çakmakçı, M. V. Kahraman, N. K. Apohan and A. Güngör, J. Solgel. Sci. Technol., 2011, 58, 612–618 CrossRef .
61. O. Ozukanar, E. Çakmakçi, O. Daglar, H. Durmaz and V. Kumbaraci, Eur. Polym. J., 2023, 195, 112203 CrossRef CAS .
62. P. Fiedor, M. Pilch, P. Szymaszek, A. Chachaj-Brekiesz, M. Galek and J. Ortyl, Catalysts, 2020, 10, 284 CrossRef CAS .
63. F. Petko, A. Świeży, M. Jankowska, P. Stalmach and J. Ortyl, Polym. Chem., 2023, 14, 3018–3034 RSC .
64. R. Mo, L. Song, J. Hu, X. Sheng and X. Zhang, Polym. Chem., 2020, 11, 974–981 RSC .
65. H. Tran, S. Nikzad, J. A. Chiong, N. J. Schuster, A. E. Peña-Alcántara, V. R. Feig, Y. Q. Zheng and Z. Bao, Chem. Mater., 2021, 33, 7465–7474 CrossRef .
66. K. Fukuda, M. Shimoda, M. Sukegawa, T. Nobori and J. M. Lehn, Green Chem., 2012, 14, 2907–2911 Search PubMed .
67. J. Stouten, G. H. Schnelting, J. Hul, N. Sijstermans, K. Janssen, T. Darikwa and K. V. Bernaerts, ACS Appl. Mater. Interfaces, 2023, 15(22), 27110–27119 CrossRef .
68. O. Ozukanar, E. Çakmakçi and V. Kumbaraci, Macromol. Mater. Eng., 2024, 2400322 Search PubMed .
69. T. Türel and Ž. Tomović, ACS Sustainable Chem. Eng., 2023, 11, 8308–8316 Search PubMed .

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