Thermal/Blue Light Induced Cross-Linking of Acrylic Coatings with Diazo Compounds

The thermal curing of industrial coatings (e.g., car painting, metal coil coatings) is accompanied by a substantial energy consumption due to the intrinsically high temperatures required during the curing process. Therefore, the development of new photochemical curing processes-preferably using visible light-is in high demand. In this work, we describe new diazo-based crosslinkers that can be used to photocure acrylic coatings using blue light. We demonstrate that the structure of the tethered diazo compounds influences the crosslinking efficiency, finding that side reactions are suppressed upon engineering greater molecular flexibility. Importantly, we show that these diazo compounds can be employed as either thermal or photochemical crosslinkers, exhibiting identical crosslinking performances. The performance of diazo-crosslinked coatings was evaluated to reveal excellent water resistance and demonstrably similar material properties to UV-cured acrylates. These studies pave the way for further usage of diazo-functionalized crosslinkers in the curing of paints and coatings. This article is protected by copyright. All rights reserved.

Materials.Diethyl 2,2'-(1,4)phenylenebis(2-diazoacetate) (1), diethyl 2,2'-(1,3)phenylenebisacetate (2), diheptyl 2,2'-(1,4)phenylenebis(2-diazoacetate) (3) were prepared according to a previously published procedure. [1]Ethane-1,2-diyl bis(2-diazo-2-phenylacetate) (4) was prepared according to a previously published procedure. [2]H (500, 400 MHz) and 13 C (125, 101 or 75 MHz) spectra were recorded on a Bruker DRX 500 MHz or a Bruker AVANCE 400 MHz spectrometer or on a Bruker DRX 300 MHz spectrometer. 1 H and 13 C spectra were referenced against residual solvent signal.FD-MS spectra were collected on an AccuTOF GC v 4g, JMS-T100GCV Mass spectrometer (JEOL, Japan) equipped with a Carbotec emitter or a LiFDi probe (FD) equipped with an FD Emitter, Linden CMS GmbH.A typical current rate of 51.2 mA min -1 over 1.2 min and a flashing current 40 mA on every spectrum of 30 ms was used. Sie exclusion chromatography (SEC) was used to determine the polymer number-average molecular weight ( ) and degree of dispersion ( ).SEC measurements were performed on a Shimadzu LC-20AD system with two PLgel 5 μm MIXED-C columns (Polymer Laboratories) in series and a Shimadzu RID-10A refractive index detector.DCM was used as mobile phase at a flow rate of 1 mL min -1 and T = 35 °C.Polystyrene standards in the range of 760 -1 880 000 g mol -1 (Aldrich) were used for calibration.The glass transition temperature (T g ) of the polymers was determined by differential scanning calorimetry (DSC).Measurements were performed on a Perkin Elmer Jade DSC.Samples were heated from 20°C to 180°C at a heating rate of 10°C min -1 followed by an isothermal step at 180°C for 5 min.A cooling cycle to 20°C at a rate of 10°C min -1 was performed prior to a second heating run to 180°C at the same heating rate.The was defined as the temperature of the midpoint of a heat capacity change on the second heating run.The Universal Analysis 2000 software was used for data acquisition.Hydroxyl values were determined according to ISO 4629-2.Solid contents were determined according to ISO 3251.UV/Vis spectra were recorded on a double beam Shimadzu UV-2600 spectrometer in a 0.2 cm quartz cuvette using ethyl acetate as a background at an approximate concentration of 10 mM.
Ethane-1,2-diyl bis(2-diazo-2-(4-methoxyphenyl)acetate) 5. Yield 10%. 1 H NMR (400 MHz, Chloroform-d) δ 7.37 (d, J = 8.9 Hz, 4H), 6.94 (d, J = 8.9 Hz, 4H), 4.52 (s, 4H), 3.80 (s, 6H). 13The polymerizations were carried out in an autoclave with heating and cooling capability, a stirrer, and a dosing pump.Approximately 50-70 wt% of the total amount of n-butyl acetate was placed in the autoclave.The solvent was heated to 160°C.and at this temperature a mixture of hydroxyethyl methacrylate (32.6% w/w), butyl methacrylate (23.1% w/w), butyl acrylate (35.2% w/w), methyl methacrylate (2.7% w/w), methacrylic acid (1.1% w/w) and styrene (5.0% w/w) and tertbutylperoxy-3,5,5-trimethylhexanoate (4.6 wt%, based on the total weight of the monomers), was added via the dosing pump over a period of 4 hours.The pump and tubing was rinsed with n-butyl acetate (about 1-5 wt-% of the total amount of solvent) and the temperature was lowered to 125 °C.A mixture of n-butyl acetate (approx.2-10 wt% of the total amount of solvent) and 0.3 wt%, based on the weight of the monomers, of di-tert-butyl peroxide were added and the reaction mixture was held at 125 °C.for one additional hour.The polymer was diluted with n-butyl acetate to a final solids content of 75% and cooled to room temperature.The hydroxyl value as such was determined to be 103 mg KOH g -1 .Solid contents were determined according to ISO 3251.The acrylic was characterized by 1 H and 13 C-NMR.

Thermochemical evaluation of diazo compounds.
DSC analysis was performed through a previously published procedure. [5]Thereafter, shock sensitivity was determined using a home-built drop-hammer setup using an adjustable weight on a rail that can be dropped from a height up to 1 meter onto a closed DSC pan containing ~10 mg of diazo compound (Figure S1).All diazo compounds were tested in triplicate with none demonstrating any signs of explosive decomposition.

Thermal Crosslinking
In a typical procedure, acrylic polyol (50 mg, 75 wt% solids in n-butyl acetate) was added to a preweighed 4 mL vial together with diazo cross-linker 1-5 (7.5 mg, 20 wt% doping based on solids acryl content).The n-butyl acetate was volatilized at 60 °C in a well-ventilated fumehood for two hours.Thereafter, the samples were heated to 120 °C for one hour to fully activate all diazo crosslinker.
After cooling down to room temperature, a sample (2-5 mg) was taken for DSC and IR measurements.Thereafter, the vial was weighed again before adding DCM (4 mL) to the vial.After vortexing, the vial was centrifuged to settle all solids and the supernatant was removed to analyze the soluble contents by GPC.The leftover solids were dried in vacuo and weighed to determine the gel content of the coating.

Photochemical Crosslinking
In a typical procedure, acrylic polyol (50 mg, 75 wt% solids in n-butyl acetate) was added to a preweighed 4 mL vial together with diazo cross-linker 1-5 (7.5 mg, 20 wt% doping based on solids acryl content).A few drops of ethyl acetate were added to solubilize the crosslinker if necessary.Thereafter, 90 µM thick films were cast onto glass plates using a drawbar.The plates were placed on a stirring plate heated at 40°C with a 50 W Kessil 456 nm PR160L lamp 10 cm above it.The films were cured at varying intensities of blue light and the curing was monitored by transmission IR.After curing, the films were removed from the substrate using a razor blade and analyzed by DSC and the gel contents were determined as described above.wt ratio (%)

Differential Scanning Calorimetry
The glass transition temperature ( ) of the polymers was determined by differential scanning calorimetry (DSC).Measurements were performed on a Perkin Elmer Jade DSC.Samples were heated from 20°C to 180°C at a heating rate of 10°C/min followed by an isothermal step for 5 min.A cooling cycle to 20°C at a rate of 10°C/min was performed prior to a second heating run to 180°C at the same heating rate.The was defined as the temperature of the midpoint of a heat capacity change on the second heating run.The Universal Analysis 2000 software was used for data acquisition.

Figure S14
. 1 H NMR of APO thermally reacted with 1:1 ethyl diazo(phenyl)acetate. Integrated signals can be assigned to the formation of ethyl phenylglyoxylate due to a side reaction with molecular oxygen. [7]1.UV-Vis spectra of crosslinkers

Kinetics of crosslinking by gas evolution measurements
Photochemical curing kinetics of 4 were studied using a homebuilt bubble counter of which the design [8] and data processing [9] is described in detail elsewhere.In a 10 mL vial a thin layer of film (100 mg) containing 30 wt% doping of 4 (22.5 mg) was cast and the solvent was volatilized in a wellventilated fumehood at 80 °C for one hour.Thereafter, the vial was closed off in a gas-tight setup and the gas evolution originating from dinitrogen release from the diazo monomer was detected by analyzing bubble formation from a hexadecane medium in the detection cell.Bubbles were detected with the aid of a laser and translated into an evolved volume of gas.The light intensity was tuned by using the intensity settings on the 456 nm PR160L Kessil LED lamp of which the 25%/50%/75% intensities correspond to 15/30/45 mW cm -2 output intensity respectively at the utilized distance of 10 cm.

Pendulum hardness testing
Pendulum hardness (Persoz hardness) of the cured coatings were determined in quadruplicate according to ISO 1522.The number of pendulum swings for the amplitude of the pendulum to decrease from 12° to 4° was determined and expressed in amount of swings.

Film water and MEK resistance testing
The chemical resistance to water and organic solvent (methyl ethyl ketone, MEK) was determined using the standard spot tests. [10]For the water resistance, a droplet of demineralized water was put on the cured coating and covered by a watch glass.After 60 minutes, the water droplet was wiped off and the effect on the coating was visually assessed using a scale of 0-5, where 5 means that the water droplet has no visible impact on the coating, and 0 means that the water droplet has a detrimental impact.For the solvent resistance a droplet of MEK was put on the cured coating.After 1 minute, the MEK droplet was wiped off and the effect on the coating was determined visually on a scale of 0 to 5, wherein 5 means that the solvent droplet has no visible impact on the coating, and 0 means that the solvent droplet has a detrimental impact.Table S3.Results of Persoz hardness and MEK resistance testing.

Single-crystal X-ray diffraction (SC-XRD) studies
The SC-XRD data of compounds 1-4 were measured on a Bruker D8 Quest Eco diffractometer using graphite-monochromated (Triumph) Mo K radiation ( = 0.71073 Å) and a CPAD Photon III C14 detector.The sample was cooled with N 2 to 100 K with a Cryostream 700 (Oxford Cryosystems).Intensity data were integrated using the SAINT software. [11]Absorption correction and scaling was executed with SADABS. [12]The structures were solved using intrinsic phasing with the program SHELXT 2018/2 [13] against F 2 of all reflections.Least-squares refinement was performed with SHELXL-2018/3. [14]All non-hydrogen atoms were refined with anisotropic displacement parameters.The hydrogen atoms were introduced at calculated positions with a riding model.The X-ray crystallographic data for 1-4 was deposited at the Cambridge Crystallographic Data Centre (CCDC), under the deposition numbers CCDC 2271590-2271593.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

Figure S2 .
Figure S2.Gel content of APO thermally crosslinked with increasing amounts of crosslinker as function of crosslinker doping by weight ratio to solid APO.

Figure S3 .
Figure S3.Glass transition temperature of APO thermally cross-linked with increasing amounts of crosslinker as function of crosslinker doping by weight ratio to solid APO.

Figure S4 .
Figure S4.Mass average molar mass of APO thermally crosslinked with increasing amounts of crosslinker as function of crosslinker doping by weight ratio to solid APO.

Figure S5 .
Figure S5.Number average molar mass of APO thermally crosslinked with increasing amounts of crosslinker as function of crosslinker doping by weight ratio to solid APO.

Figure S8 .
Figure S8.Zoom of ATR-IR spectra of APO thermally crosslinked with increasing amounts of crosslinker 2 (ratio N 2 /OH in legend).

Figure S9 .
Figure S9.Zoom of ATR-IR spectra of APO thermally crosslinked with increasing amounts of crosslinker 2 (ratio N 2 /OH in legend).

Figure S11 .
Figure S11.Zoom of ATR-IR spectra of APO thermally crosslinked with increasing amounts of crosslinker 4 (ratio N 2 /OH in legend).

Figure S12 .
Figure S12.Zoom of ATR-IR spectra of APO thermally crosslinked with increasing amounts of crosslinker 4 (ratio N 2 /OH in legend).

Figure S18 .
Figure S18.Schematic of the setup used for measuring the gas evolution from the films.

Figure S19 .
Figure S19.Diazo conversion as determined by gas evolution from the film, and processed data showing the relationship between lamp intensity and curing rate.

Figure S26 .
Figure S26.DSC thermal self-curing of APO by repeated isothermal steps at 120°C for 15 minutes, followed by a temperature scan (at 10 °C min -1 ) to and from −40 °C to determine the T g .Each trace corresponds to a temperature scan after 15 minutes at 120 °C (red trace t = 0 min, black trace t = 60 min).

Figure S27 .
Figure S27.Photographs of coatings using crosslinker 4 after water and MEK resistance tests.

Table S1 .
Thermochemical information derived from DSC data using a ramp of 5 °C min -1 .
*Inaccurate due to coincidence of melting endotherm with decomposition exotherm.T m,o onset of melting temperature.T d,o onset temperature of decomposition, T d,p peak temperature of decomposition, SS/EP shock sensitivity and explosive propagation as defined by Yoshida, ΔH D enthalpy of decomposition.