A scalable process for the generation of durable easy-to-clean coatings on foils

Materials

Chemicals for polymer synthesis CuBr (99.998%, Alfa Aesar), ethyl-2-bromoisobutyrate (EBiB, 98%, Sigma-Aldrich), acetonitrile (≥ 99.8%, Sigma-Aldrich), 2-propanol (99.5%, Acros Organics) 2-butanone (≥ 99.5%, Sigma-Aldrich), 2,2′-bipyridine (bipy, ≥ 99%, Sigma-Aldrich) and n-hexane (≥ 99%, Merck KGaA) were used without further purification. 2-Methacryloyloxyethyl phosphorylcholine (MPC, 97%, Sigma-Aldrich) was purified by recrystallization from acetonitrile. For all purposes, if it was necessary, Millipore^® water was used.

Foils As substrates PET foils (Mitsubishi Hostaphan RN 100) and PVC foils (typical overlay foil) are used. Both materials were applied with a thickness of 100 µm.

SiO₂substrates For the ellipsometric measurements, highly polished silicon wafers orientated in [100] direction and with ≈ 1.3 nm native SiO₂ were used, purchased from Si-Mat (Silicon Materials, Kaufering, Germany).

Synthesis of functional MPC polymer

The statistical copolymer consisting of 4-benzophenyl methacrylate (BPO) (for the covalent anchoring on the lacquer surface) and functional 2-methacryloyloxyethyl phosphorylcholine (MPC) units was synthesized by atom transfer radical polymerization using the methacrylates of these monomers. The 4-benzophenyl methacrylate is not commercially available and was synthesized by a Schotten-Baumann esterification15 of 4-hydroxybenzophenone and methacryloyl chloride with triethylamine according to the synthesis published previously.16,17 Since the synthesis of the MPC BPO copolymer was also demonstrated previously,10 the preparation will be only summarized briefly. All preparation steps are scalable and feasible with industrial synthesis techniques. 1.616 g (5.472 mmol) MPC, 8.7 mg (0.052 mmol) bipy and 4 mg (0.026 mmol) CuBr were placed under inert conditions in a Schlenk flask and dissolved in 22 mL isopropyl alcohol. Because of the insolubility of BPO in alcohols, 14.0 mg (0.0547 mmol) was dissolved in 0.3 mL 2-butanone and added to the MPC solution. Then the mixture was degassed three times by freeze–pump–thaw cycles to remove the oxygen. To start the polymerization, 4 µL (0.0273 mmol) of the initiator EBiB was added at room temperature. After 24 h, the reaction was stopped by air exposure. The removing of the catalyst was performed by passing through a column filled by neutral alumina. The polymer was precipitated in n-hexane and purified by twofold precipitation from the same solvent. After drying under vacuum for 24 h, the polymer (MPC-co-BPO) was obtained as a white solid powder (1.058 g, 64.9%). ¹H-NMR (500.13 MHz, methanol-d4): δ (ppm) = 0.97–1.97 (br, polymer backbone), 3.30 (br, –N⁺(CH₃)₃), 3.74, 4.08, 4.23, and 4.33 (br, –CH₂ of MPC side chain), 7.41, 7.47, 7.60, 7.70, 7.82, and 7.96 (br, aromatic); ¹³C-NMR (125.77 MHz, methanol-d4): δ (ppm) = 18.2, 19.9, 20.7 (br, CH₃-backbone), 46.1, 46.4 (C_quaternary-backbone), 54.8 (–N–CH₃), 55.6 (br, CH₂-backbone), 60.6 (–O–CH₂–CH₂–N–), 64.3 (–O–CH₂–CH₂–O–), 66.2 (–O–CH₂–CH₂–O–), 67.4 (br, –O–CH₂–CH₂–N–), 122.6, 129.8, 131.1, 132.9, 134.1, 136.6, 138.5, and 155.2 (aromatic carbons), 178.2, 179.1, and 179.3 (> C=O), 197.3 (> C=O ketone); ³¹P-NMR (202.46 MHz, methanol-d4): δ (ppm) = − 0.45 (br); GPC: M_n = 21 500 g·mol⁻¹, M_w = 38000 g·mol⁻¹, PDI 1.77; FTIR-ATR (bulk material): \(\bar{\varvec{\nu }}\) (cm⁻¹) = 3343, 3034, 2957, 1717, 1657, 1599, 1480, 1405, 1342, 1232, 1163, 1054, 954, 927, 875, 852, 828, 775, 746 (Scheme 1).

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Surface modification

Lacquer deposition and UV curing The lacquer application was performed via ZAA 2300 automatic film applicator made by Zehntner. The deposited films were cured in an irradiation chamber UVACUBE 2000 (Hönle UV Technology, Gräfelfing, Germany) equipped with a mercury lamp positioned in a distance of 20 cm to the samples for different times.

Polymer grafting A 0.5 wt% solution of MPC-co-BPO in ethanol was deposited on the plasma-functionalized as well as on non-functionalized foils (2 mL for an area of DIN A4, 210 × 297 mm) by a film squeegees process using a ZAA 2300 automatic film applicator (drawing speed 25 mm/s) equipped with a ZUA 2000 Universal Applicator (gap height 250 µm) made by Zehntner Testing Instruments (Germany). Afterward the films were dried and grafting was done by UV radiation using a UV lamp made by Vilber with a wavelength of 254 nm and a distance between lamp and foil of 1 cm for 15 s. The non-grafted polymer chains were removed by rinsing with water and drying in a stream of nitrogen. For the general investigation of the film properties, such as contact angle, stability, and easy-to-clean effect, smaller samples with a size of approximately 10 × 20 mm were used. The polymer solution was deposited by spin coating (v = 2500 r·min⁻¹, a = 1000 r·(min·s)⁻¹, V = 100 μL, tspin = 10 s) and grafted by the UV lamp of Vilber with the same conditions as in the case of the functionalization of DIN A4 foils. For the evaluation of the film thicknesses, the MPC polymers were also deposited on silicon wafers by spin coating using the same condition as above. Before polymer grafting, the substrates were purified with absolute ethanol in an ultrasonic bath three times for 5 min and dried under nitrogen flux. Afterward the samples were activated in an oxygen-plasma chamber for 1 min at 100 W.

Cleaning study

To investigate the oil repellence and easy-to-clean performance of the polymer-functionalized foils in comparison with uncoated foils without the MPC polymer, pure water (Millipore^® water) was hand-sprayed on the foils under an angle of approximately 20° by using a small glass hand sprayer (volume 10 mL). The angle was not controlled exactly during the investigations. A volume of about 0.5 mL was sprayed on all small foil substrates, demonstrated in Figs. 3 and 4. The oil from the larger A4 foil was removed by spraying 5 mL water (Fig. 5).

Characterization methods

NMR spectroscopy (NMR)¹H (500.13 MHz), ¹³C (125.77 MHz), and ³¹P (202.46 MHz) NMR spectra were recorded on an Avance III 500 spectrometer (Bruker, Germany) using methanol as solvent and internal reference for the MPC polymer (δ_methanol-d4 (¹H) = 3.31 ppm, δ_{methanol–d4} (¹³C) = 49.2 ppm).

Gel permeation chromatography (GPC) The determination of the molecular weight of the synthesized MPC polymer and its distribution was performed on an Agilent Technologies HP Agilent 110 HPLC system equipped with a refractive index detector K-2301 made by Knauer (Germany). The calibration of the measurement was realized with poly(ethylene glycol) standards. As eluent a mixture composed of 0.05 mol·L⁻¹ trizma^® buffer (pH = 8, Sigma-Aldrich) and 0.2 mol·L⁻¹ NaNO₃ solution with a flow of 1 mL·min⁻¹ was used in combination with a PolarGel-M column from Polymer Laboratories.

Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) The ATR spectrum of the bulk material was collected by a SPECAC spectrometer (single reflection Golden Gate Diamond ATR-unit; SPECAC, Orpington, United Kingdom). The spectral range was 4000–600 cm⁻¹ with 4 cm⁻¹ spectral resolution, and 300 scans were co-added to the ATR spectrum. The spectra were evaluated by the OPUS software (version 7.5).

Dynamic contact angle (CA) measurements The dynamic contact angle measurements were conducted with the optical contact angle instrument OCA40 fabricated by DataPhysics Instruments GmbH (Filderstadt, Germany). All contact angles were measured by sessile drop experiments as advancing (θ_adv) and receding contact angles (θ_rec). Millipore^® water (surface tension = 72.8 mN·m⁻¹) at 20°C was used as a test liquid.

Ellipsometry For the evaluation of the thicknesses of the dry MPC polymer films on SiO₂, a null-ellipsometer (SE-402, Sentech Instruments GmbH, Germany) was used. All measurements were performed at 632.8 nm at an angle of incidence (Φ₀) of 70°, which is close to the Brewster angle of silicon. For Si and SiO₂ indices of refractions of 3.858–0.018·k as well as 1.4598 was used. For the polymer a value of 1.4767 was assumed.17 The film thickness d was determined by an optical box model consisting of layers of silicon, SiO₂ (1.3 nm) and the MPC polymer. All data were acquired and evaluated using the SE400adv software (version 4.6.3.3).

X-ray photoelectron spectroscopy (XPS) XPS measurements were done using a Thermo K-Alpha K1102 system (Thermo Fisher Scientific, Waltham, USA) via monochromatic Al K_a radiation. The spectra were acquired at a detector angle of 0° (rectangular to the incident angle of the X-ray radiation) applying the constant analyzer energy (CAE) mode. Survey spectra were measured with a pass energy of 150 eV, while detailed spectra were acquired with a pass energy of 40 eV.

Scanning electron microscopy (SEM) SEM images were taken with a LEO 1530 system (Carl Zeiss Microscopy GmbH, Jena, Germany). Images were acquired with an acceleration voltage of 3 kV.

Process and easy-to-clean properties

To generate a flexible, transparent, and durable functional coating with easy-to-clean properties on foils, a three-step process was developed. The procedure includes a lacquer deposition with UV curing, a plasma surface modification, and a final grafting of the MPC-co-BPO polymer. Afterward the functionalized foils are available for the lamination of furniture surfaces. All steps as well as the used materials of clearcoat and thin functional polymer film are adjusted to each other. The whole process is illustrated in Scheme 2.

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Because of the usage of the foils as lamination material for different substrates, like furniture or automobiles, and the specific properties of foils, such as flexibility, the applied lacquer has to meet particular requirements. It must be transparent, flexible, durable, and commercially available. These properties are realized by using appropriate components summarized in Table 1. Essentially, the lacquer consists of different acrylate monomers and a colloidal silica sol. The listed formulation was cured by UV crosslinking. By this energetic input, a mixture of the photoinitiators Genocure MBF and Darocur 1173, both aromatic ketones, forms radicals and polymerizes the acrylates and incorporated SiO₂ particles resulting in a transparent homogenous lacquer film. Two representative SEM images are depicted in Figs. 1a and 1b and show resin-covered SiO₂ particles having a diameter between 25 and 50 nm. The flexibility of the coating can be adjusted by the degree of crosslinking, that means by the time of UV irradiation. It was determined that with the increase in time of UV curing the flexibility decreased and the film becomes brittle. However, a certain dosage of energy is necessary to form a stable and durable coating. Considering these requirements, 20 s as time of UV radiation at 2000 W were found to be the best parameters. By evaluating time-resolved infrared spectra of the lacquer, a yield of reacted double bonds of approximately 89% could be determined, resulting in a very flexible coated foil, which can be bent to 90°.

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After the successful lacquer curing, a plasma activation and etching of the surface was performed to partly uncover the silica particles from the lacquer resin and to increase the wettability of the surface via an implementation of oxygen-containing functional groups. This functionalization increases the adhesion and probably the mechanical stability of the subsequently applied functional polymer layer, as will be shown in the part regarding the investigations of the stability below.

Two representative SEM images of a lacquered and plasma-treated foil are depicted in Figs. 1c and 1d. In comparison with the SEM images acquired on the untreated reference surface (see Figs. 1a and 1b), the particles appear more distinct and show clearer defined borders after plasma treatment. This is a good indication that the covering resin layer was removed by the atmospheric plasma treatment with dry air. The effect of plasma treatment was further clarified by XPS measurements on untreated and plasma-treated lacquer surfaces (Table 3). The corresponding surface composition of the untreated reference surface shows mainly carbon and oxygen and only a minor amount of silicon, as the silica particles are still covered by the organic resin. An exemplary plasma treatment with parameter set 1PET and dry air as process gas lead to a strong decrease of carbon, while the oxygen and silicon portions were significantly increased. This result clearly shows that the covering resin layer is at least partly removed via the plasma process and silica particles are uncovered. Additionally, the plasma treatment results in a better surface wetting, as determined by contact angle measurements (see Fig. 2). For the given plasma parameters, the surface activation was still measurable three or more days after the surface treatment. Even though the water contact angle increased during that time, the samples were still usable for the subsequent application of the functional polymer layer.

Table 3

Surface composition, determined via XPS measurements, of untreated and plasma-treated lacquer surface

Surface state	Surface composition (at.%)
	C 1s	O 1s	Si 2p	N 1s
Reference (untreated lacquer)	72.1	27.8	< 0.1	–
Plasma treatment (1PET)	49.7	42.6	6.8	0.9

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Hence, the coating of the MPC-co-BPO polymer was investigated on non-activated lacquered PET and PVC foils, denoted as 0PET and 0PVC, and on coated foils which were plasma-treated with different parameters. The parameters are summarized in Table 2 and chosen in such a way that the treatment intensity decreases with the treatment number (e.g., from 1PET over 2PET to 3PET). The success of the polymer grafting was determined by the comparison of contact angles before (blue columns) and after plasma activation (red columns) which is demonstrated in Fig. 2a. The lacquer films before plasma treatment have a relative high contact angle of approximately 95° resulting in a low wettability regardless of the material of the foil. Films of MPC have a very low contact angle below 20°.17,18 Because of the zwitterionic structure of the synthesized MPC-co-BPO, the polymer is very hydrophilic and a complete wetting of the foil by the MPC ethanol solution is not possible. It seems that the generation of a homogeneous film of MPC, which is a necessary condition for oil repellent properties, on non-activated foils was not possible. Therefore, it is necessary to increase the wettability of the hydrophobic lacquered foil. The plasma treatment results in a decrease in the water contact angle and an increase in the wettability depending on the intensity of the applied plasma treatment. With the increase in intensity the contact angle decreases from approximately 95° to 55° in the case of PET and to 42° in the case of PVC. However, this difference has no significant influence on the deposited MPC. Regardless of the intensity, the contact angles of the polymer coated foils decrease to values fewer than 10°, indicating a successful functionalization.

The used polymer is made of a copolymer with the main component MPC and benzophenone with an amount of 1% as an anchor unit to graft the polymer covalently to the organic lacquer. This aromatic ketone can abstract hydrogens from a suitable hydrogen donor by forming a biradical triplet excited state of the benzophenone induced by UV irradiation.19,20 The process results in new C–C bonds.21 Furthermore, a crosslinking inside the polymer film can be observed which improves the stability of the coating. The benzophenone as a crosslinking agent as well as an anchor group is an attractive unit in functional polymers, because it can impede the oxidative damage of polymer and substrate.22 Furthermore, a very fast one-step generation of a thin film under mild conditions, like room temperature, is possible. Its kinetics were investigated previously.17,23 For a practicable implementation of the polymer grafting into industrial process, the grafting must be very fast in contrast to common laboratory procedures to generate functional polymer films, like polymer brushes, with grafting times of several hours.24^–26 To investigate the film thickness of the deposited film in relation to the time of UV radiation, thin films were prepared on silicon wafers as model substrates. Because of their reflective properties, in contrast to the used foils, it is possible to measure the film thickness with the sensitive optical method ellipsometry. It is assumed that the generated film on the silicon wafer is the same as on the lacquered foil. The results are illustrated in the grafting curve in Fig. 3a.

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The diagram demonstrates that the film thickness increases within the first 20 s because of a proceeding crosslinking reaction. Resulting from the increasing radiation dose, an irradiation (grafting time) of longer than 20 s causes a degradation of the polymer structure and a decrease in the film thickness. In summary, the establishment of a thin polymer film of MPC at room temperature in several seconds is possible on a plasma-activated UV-cured acrylate-based lacquer system on foils.

It is known from the literature that polymer films of MPC show very low contact angles with water as well as alkanes, such as hexadecane, and a high degree of swelling depending on the degree of crosslinking which results in oil repellent properties.10,11 The easy-to-clean effect was checked on functionalized PET and PVC foil, denoted with (“i”) in Fig. 3b, in comparison with the plasma-activated lacquer substrates (“ii”), and non-functionalized foils without any surface modification (“iii”) by spraying pure water on samples that were furnished with an oil droplet, inspired by the experiments of He et al.10 The pictures show, regardless of the foil material, the complete removal of the oil droplet from the MPC-functionalized samples in contrast to the uncoated foils and to the more hydrophilic plasma-activated lacquered substrates. That means that the polymer layer is essential for the easy-to-clean effect. Further images regarding the other two plasma treatments are demonstrated in the electronic supporting information (ESI).

The mechanism behind the oil repellent effect is not completely clarified yet. The hydrophilic and oleophilic properties of the zwitterionic polymers cannot be the only explanation for the effect, because other very hydrophilic surfaces do not show a behavior similar to the one demonstrated in this contribution. The other important property is probably a large degree of swelling depending on the degree of crosslinking,10,27,28 resulting in a substantial reduction of the force of interaction between the oil phase and the foil substrate. Because of the swelling behavior of MPC, the polymer takes up a high volume of water leading to a very high layer thickness of the swollen film in comparison with the dry film. In a first approximation, the oil droplets interact only with water molecules and not with the polymer film deposited on the lacquered foil after water spraying. Due to the lower density of oil in comparison with water, the oil swims on the water film and can be easily removed by gravitation for instance.

Investigation of the thermal, chemical, and mechanical stability

In the furniture industry, it is a common practice to investigate the thermal stability of foils after storage at − 20 and 90°C over a period of 2 weeks. These parameters are used in order to simulate long freight routes. Hence, the lacquered and polymer-functionalized foils were treated at − 20 and 90°C, respectively, to investigate the stability of the addressed easy-to-clean effect, illustrated by oil removing with pure water and demonstrated in the pictures of Fig. 4. The results show the successful purification without a loss of performance and therefore a high thermal stability in the practical range concerning real application.

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Furthermore, the mechanical stability is an important criterion for the application as furniture foil and was tested by a modified crock test using an abrasion tester of TQC. The test medium was an ISO crocking cloth concerning the standard ISO 105-F09 loaded by a 200 g weight. To evaluate the stability of the polymer coating and the easy-to-clean effect, the contact angles before testing and after 10 back and forth cycles were measured and compared with the pure UV-cured lacquer. For the contact angles of the pure lacquer, a value of 97° ± 1° before and a value of 96° ± 1° after the abrasion test were determined and it illustrated the high mechanical stability of the lacquer system. The foil functionalized by the MPC polymer had a contact angle of 20° ± 4°. After ten cycles of the modified crockmeter test, the contact angle increased to a value of 27° ± 9°. The growth of standard deviation is remarkable. Despite this increase, the easy-to-clean performance is not negatively influenced and the oil could be completely removed from the foil samples, as it was shown before. An influence of the different used plasma parameters, summarized in Table 2, on the adhesion of the polymer film and the stability of the easy-to-clean effect could not be observed in this study. The improved mechanical stability in comparison with the low stability of typically applied thin functional polymer films is probably a result of the embedded SiO₂ particles, which are uncovered after plasma treatment, i.e., they protrude a bit from the clearcoat matrix. This coating morphology, which should be preserved after the MPC film was applied, seems to be able to protect the polymer chains between the particles from abrasion. An additional hint for this assumption is that the degree of gloss did not show a significant change before and after the abrasion test. This test shows the feasibility of the developed functional coating system to combine the surface wetting properties of thin polymer coatings with a specific mechanical stability. Yet, it should be noted that for an assessment of the long term mechanical stability, more than 50 cycles or even better 100 cycles, or a different test procedure would be necessary.

In addition, the chemical stability was investigated according to the standard DIN 68861 part 1 and stress group 1B, which describes a common method to evaluate the stability of furniture coatings against different substances, like acetic acid, mustard, black tea, acetone, or ethanol. The testing criterions of this norm are summarized in table S1 of the ESI. After the test procedure, no changes in gloss, haptic, and surface appearance of the functionalized foils could be identified. The foils have shown an excellent resistance to the investigated test media.

Functionalization of larger foil substrates

In addition to the successful functionalization of small test samples, the verification of the easy-to-clean effect and its stability, the transfer of the coating system and the deposition procedure onto larger foil substrates are essential steps to demonstrate the transferability into a real application. Therefore, a foil with an area of 210 mm × 297 mm was functionalized with the clearcoat and the MPC polymer as shown above. The physical parameters of the lacquer curing, the plasma process, the material deposition, and the crosslinking of the MPC by UV irradiation were the same as in the case of the smaller samples (10 mm × 20 mm). Only the amounts of lacquer and polymer solution were adjusted to the larger size of the substrates. The performed scale-up corresponds to a factor of 310 in terms of the surface area. The verification of a successful functionalization was carried out by the proof of the oil repellency shown in Fig. 5b in comparison with a non-functionalized substrate in Fig. 5a. The pictures of Fig. 5b demonstrate the complete oil removing by pure water. Subsequently, the functionalized foil was laminated on a real furniture board. Also after this process, the easy-to-clean effect is still visible. The successful test of the oil repellency can be found in the ESI (Fig. S2). Furthermore, the contact angles of the foil before and after the lamination were measured. The results, also demonstrated in the ESI (Fig. S3), do not show changes, i.e., no influences of the lamination process. It should be noticed that the foil was pressed on the furniture plate with a roller during this process in order to ensure a complete adhesion. This generates a mechanical stress on the foil and its coating. Hence, the successful demonstration of the oil removal with pure water after the lamination is further evidence of the stability of the functionalization.

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