1. Introduction
Development of functional and sustainable high-performance coatings based on UV-curable fluorinated resins captured attention of scientific community over the past few years, as communication technologies and related devices and/or components are pushing forward increasingly the requirements on their optical properties. Owing to their unique properties ranging from high thermal and chemical resistance [
1,
2,
3], low surface tension [
4], low refractive index and transmittance losses [
5,
6], large thermo-optic coefficients [
7], color stability [
8] UV-curable fluorinated resins are the preferred selection in several industrial and commercial applications. Besides integrated optical devices, cladding for optical fibers, photovoltaics and microelectromechanical system (MEMS) devices, other applications such as marine antifouling coatings [
9,
10], automotive clear coat formulations [
11,
12] and batteries [
13,
14], coatings of biomedical devices (
e.g., artificial hearts, vascular prostheses or catheters) [
15], microfluidic devices [
16,
17], textile industry, food packaging [
18],
etc. were also suggested.
One of the most widely deployed and researched UV-curable perfluoropolyether (PFPE)-urethane methacrylate resin, referred as Fluorolink®, developed by Solvay SA under its top tier portfolio of specialty polymers, can be readily blended with conventional acrylate monomers. This finds an application niche in the aforementioned areas. In relation with this, Ursino
et. al. (2019) reported on the use of Fluorolink® MD700 as surface modifier for polyamide (PA) and polyether sulfone (PES) to deliver hydrophilic/hydrophobic membranes for water treatment showing increased stability and resistance to chemical cleaning agents and salty solutions [
19]. Gibbons
et. al. (2021) co-authored an extended database containing experimentally measured refractive indices, optical transmission and optical absorption spectra of the same oligomer prepared as thin film by variable spectroscopic ellipsometry, as an open access content to be used for various photo-curable synthesis processes [
20]. Bongiovanni
et. al. reported on Fluorolink® D10H [
21,
22] and Fluorolink® MD700 [
23] considering various synthesis routes, wide range of commercial photo-initiators (
e.g., Darocur® and Irgacure® representatives), identical UV exposure conditions. The resulting materials were characterized by mechanical, thermal, or surface properties and are relatively high referred in literature. Thus, in addition to reporting enhanced thermal resistance, reasonable mechanical and chemical resistance, very low refractive index and surface tension, the authors debated on extending the application range as adhesives. Recently, Goralczyk
et. al. (2022) underlined Fluorolink® MD700 versatility, both from application and manufacturing technology perspectives, by 3D printing a transparent microfluidic device using stereolithography (SLA) [
24]. Their results exhibited enhanced mechanical, thermal, and optical properties owing to the inherent features of PFPE polymer.
Radical type photo-initiators are equally important in carrying out the photopolymerization processes, particularly in industrial environments, since their addition to the chemical solution leads to higher production rates, diminished energy consumption and environmental pollution while their amount together with curing conditions significantly influence obtained material properties [
25]. The selection of photo- initiators depends mainly on its compatibility with the resin. Omnirad free radical photo-initiators of type I from IGM Resins®, former known as the Irgacure series, are the most common photo-initiators, followed by Darocur® TPO and Lucirin® TPO from BASF [
26].
To further underscore, crosslinking strategies and curing process play a crucial role both for coating morphology, crosslinking degree, and material properties. In their state-of-the-art contribution on various curing technologies, Javadi
et. al., (2016) revised,
inter-alia, the UV curing line equipment, from lamps and reflectors to LED based light sources, applications related configuration adaptation, environmental conditions, a range of polymers selection and photoinitiated mechanism of their polymerization from chosen references [
27]. Kredel
et. al. (2021) reported different crosslinking strategies for fluorine-containing polymer coatings cured by UV mercury lamps [
28]. Reported results demonstrated enhanced water and oil-repellency along with durability of developed coatings. In their contribution, Ghazali
et. al. (2021) favored on UV-LED technology deployment for polymer-based coatings curing. They proved energy efficiency, reduced heat generation, easiness during operating, environmental safety compared to UV mercury lamp, respectively [
29]. Further, they emphasized on the limited number of publications on polyurethane coatings using UV-LED and demonstrated the enhanced properties of their formulation, from the high degree of C==C conversion (96-98%) resulted in relatively high transparency and development of green side aspect of photopolymerization. Similar statements can be found in the recent review of Dall Agnol
et. al. (2021) on UV-curable waterborne polyurethane coatings [
30]. The authors pointed out the main factors that determine the UV curing behavior of UV-curable coatings, including the wavelength and intensity of radiation, reaction temperature and reactivity of functional groups. In relation with above mentioned, increased radiation exposure time of formulations lead to a decrease of the crosslinking rate of the C=C bond and further degradation of other material properties, including tensile strength, hardness, or solvent resistance.
Owing to their low refractive index values, fluorinated polymers are sought the best candidates for developing optical structures as antireflecting, protective or barrier coatings [
31]. Furthermore, thermo-optic coefficients (TOC) are of critical importance as small changes may affect device performances [
32,
33]. Razali
et. al. reported on thermo-optical coefficients of two different acrylate, cyclomer and fluorinated polymers, respectively. Their findings revealed a linear dependency of refractive index as a function of temperature, with slopes of -10
-4, the lowest being found for fluorinated polymers.
The main objective of this study was to provide a new synthesis route for a selection of fluoroacrylate oligomer and monomer for fabrication of transparent coatings for photonic applications of enhanced performance, by aid of an
in-house developed UV-LED computer assisted curing system. The UV-LED technology was formerly used within the research group on a selection of fluorinated resins owing to their application potential for integrated optical devices [
34]. In summary, it was concluded that exposure time strongly influences the conversion rate and hence the network density, which is significant in the coating properties.
The main contributions underpinning this study are summarized as follows:
A novel synthesis route for transparent coatings preparation based on selection (e.g., different crosslinking agents), curing conditions (e.g., exposure time) with high conversion rates over extended UV-LED exposure time range.
A selection of fluorinated resins-based transparent coatings deliverables under different curing exposure times.
Data processing and analysis on the comprehensive database containing viscoelastic and optical properties (i.e., refractive indices, thermo-optical coefficients, and optical losses).
2. Materials and Methods
2.1. Materials
A combination of a bifunctional perfluoropolyether (PFPE)-urethane methacrylate oligomer (Fluorolink® MD700) with a high fluorine content (62 wt.%) and a tetra-functional PFPE-urethane acrylate (Fluorolink® AD1700, both Solvay Specialty Polymers) with a ratio of 80:20 wt.%, was stirred at 100 °C for 4 h to act as reference polymer blend. The mixture was left to cool down to room temperature prior addition of supplementary components. The oligomer in the above combination was chosen for its high reactivity and low viscosity (430 cP at 25 °C) as well as water/oil repellence properties whilst the second one was added to the acrylic mixture owing to its effectiveness on fingerprints and stains removal.
The photo-initiator used in all formulations is a phenyl bis(2,4,6-trimethylbenzoyl)-phosphine oxide (Omnirad® 2100, IGM Resins B.V., Netherlands) due to its efficiency to initiate radical polymerization of acrylates after UV exposure. This was added to the resin formulation above (i.e., resulted weight ratio 75:20:5), 5 wt.%, stirred at room temperature for 15 min, and stored at 5 °C in dark conditions for further usage.
Two crosslinking agents, namely, dimethyl(divinyl)silane (H
2C=CH)
2Si(CH3)
2, 97% purity (DM(DV)S) and trimethoxy(vinyl)silane H
2C=CHSi(OCH
3)
3, 97% purity (TM(V)S) (both Sigma-Aldrich) was added separately to the basic formulation above, in a weight ratio of 70:15:5:10.
Figure 1 depicts the chemical structures of aforementioned chemicals.
2.2. Coating preparation and UV-LED assisted curing process
The fluorinated acrylate films were prepared on standard BK7 soda-lime glass slides (dimension L 76 x H 26 mm, thickness 1.2-1.5 mm) (from Carl Roth GmbH, Germany) by doctor blading technique at room temperature resulting in 300 µm thick films. These were flushed into a mold for 2 min under nitrogen atmosphere and subjected to UV-LED curing by passing under a conveyor belt type platform at different speeds (5, 50, 150, 250, and 500 mm/s). A UV-cross-linking reaction was carried out under nitrogen atmosphere to avoid oxygen inhibition, at T=23 ± 2 °C. The UV-LED source, disposed as a matrix of 4x3 LEDs emitting at 390 nm a radiation of 705 mW/cm2, was kept at 12 mm from the coating surface.
Samples for measurement of optical properties, such as thermo-optic coefficients (TOC) and optical loss, were spin-coated using a WS 400-6TFM LITE spin-coater (from Laurell Technologies Co., USA). Film thickness resulted in approx. 20 µm through spinning at 1500 rpm for 10 s. Next, these were subjected to UV-curing for 15 min in a UVH-254 lamp (from Panacol-Elosol GmbH, Germany) keeping the same values for radiation and distance from their surface as above mentioned. No additional baking was necessary to complete the crosslinking reaction. Thin films were being subjected to experimental measurements promptly after their preparation to avoid post-curing effects.
2.3. Curing behaviour and characterization
The chemical components of the UV-LED cured fluorinated acrylate film samples were confirmed by real-time Fourier-transform infrared (FT-IR) spectroscopy using a Nicolet 5700 spectrometer (Thermo-Fisher Scientific Inc., Waltham, MA, USA), equipped with a diamond probe, in the 4000-600 cm
-1 range. The FTIR spectra of samples, both upper and bottom sides, were recorded in the 3000-150 cm
-1 range, 32 scans, resolution of 4 cm
-1. The degree of curing was evaluated by monitoring the intensity of the peak at 1640 cm
-1 assigned to the C=C bond stretch vibrations in the methacrylate group. Omnic™7.2 software was used for spectra evaluation. Degree of conversion (%
Conversion), demonstrating the curing efficiency, was further evaluated using the following formula:
where
and
are the absorbances at 1640 cm
-1 of fluorinated acrylate coatings before and after the exposure to UV-LED emission at different times, as set by the imposed speed of the carriage return holding device.
2.4. Viscoelastic properties by means of Dynamic Mechanical Analysis (DMA)
Dynamic mechanical measurements were performed using a controlled stress rheometer RSA6 analyzer from Rheometric Scientific (TA Instruments, New Castle, USA) at a frequency of 1Hz, in tensile mode, from -40 °C to 250 °C, using a 5 K/min heating rate in the dynamic step. The experimental runs comply with the ASTM D5026-15 standard procedure or its ISO 6721-4:2019 equivalent. Sample dimensions were set as 55x11.5xh mm2 (length x width x height) and carefully shaped using a sharp cutter. Measurement set-up and data acquisition were handled using a Rheology Advantage Instrument Control AR environment enabling information gathering for the storage (E’) and loss (E’’) moduli as well as loss factor (tanδ) as a function of temperature.
2.5. Optical properties measurements
2.5.1. Refractive index (n)
The refractive index (n) of fluorinated acrylate films was determined using a Metricon 2010/M prism coupling system (Metricon Co, USA) in TE mode, using a wavelength of 632.8 nm under a pressure of 22 psi, at 20 °C. The refractive indices, as obtained at room temperature, accounts for the various degree of curing as the UV-LED head moves under different speeds. Free standing films removed from substrate were used for this measurement. The values of refractive index were obtained from the kick at measurement curve using software in Metricon 2010/M.
2.5.2. Thermo-optic coefficients (TOC)
Thermo-optic coefficients (TOC) were determined from the thermal dependence of the refractive index (n(T)). Samples were prepared in this case on Si wafer substrate and increasing the temperature gradually, within the range of 30 °C-200 °C, TOC values were obtained as slope of n(T) curve.
2.5.3. Optical loss
Optical loss values of spin coated UV-cured films were acquired using the prism coupling system Metricon 2010/M as above. The samples were prepared on long Si substrate (at least 6 cm long). First, the films of Fluorolink® MD700 were prepared and cured, then the films of investigated samples were deposited on the top and cured. Optical losses of the planar waveguides were measured by a technique involving measurement of transmitted and scattered light intensity as a function of propagation distance along the waveguide at 633 nm and 1547 nm [
35]. The values of optical losses were calculated in Metricon software by exponential fitting of the scattered light intensity
vs distance curve.
4. Conclusions
In this contribution, a series of perfluoropolyether (PFPE)-urethane methacrylate transparent coatings has been developed based on an acrylic mixture using a neutral fluorinated polymer network, i.e., Fluorolink® MD 700 and cured, in dynamic mode, by means of a UV-LED in-place developed conveyor belt configuration. Resulted coatings properties are highly influenced both by crosslinking agents and curing times.
The viscoelastic properties of transparent coatings developed, based on different crosslinking agents, showed degradation in their storage modulus and negative shifts in their Tg with respect to the selected reference. The reported experimental data indicate the combination using dimethyl(divinyl)silane as a crosslinking agent shows relatively small differences in viscoelastic properties compared to the reference, and therefore improved performance in different environments and circumstances.
Refractive indices, independent of the crosslinking agent used, showed negligible variations with exposure rates or temperature increase compared to the reference coatings, with typical values of about 1.38 to 1.39. As for the thermo-optical coefficient (TOC), by deploying trimethoxy(vinyl)silane as a crosslinking agent, result in a relatively small degradation in property compared to its counterpart, from -2.57x10-4 K-1 to -2.55x10-4 K-1. In addition, optical loss values for combinations showed different behavior at various wavelengths selected for measurements in favor of dimethyl(divinyl)silane while used as a crosslinking agent.
Consequently, the transparent perfluoropolyether (PFPE)-urethane methacrylate coatings described herein exhibit viscoelastic and optical properties that make them suitable for demanding applications requiring a low refractive index cladding for optical fibers or polymeric waveguides with low optical losses. The experimentally measured optical properties obviously support the combination with dimethyl(divinyl)silane as a crosslinking agent, although the other proposal should not be disclosed.
Author Contributions
Conceptualization, C.D.; methodology, C.D, M.K.; software, M.K., D.L.M and L.G.; validation, C.D., M.K; formal analysis, C.D.; investigation, D.L.M., L.G.; resources, C.D.; data curation, D.L.M., M.K. and L.G.; writing—original draft preparation, D.L.M.; writing—review and editing, M.K., L.G. and D.L.M.; visualization, C.D.; supervision, C.D.; funding acquisition, C.D. All authors have read and agreed to the published version of the manuscript.