Preparation and Characterization of UV-Curable Cyclohexanone-Formaldehyde Resin and Its Cured Film Properties

UV-curable cyclohexanone-formaldehyde (UVCF) resin was prepared with cyclohexanone-formaldehyde (CF) resin, isophorone diisocyanate (IPDI), and pentaerythritol triacrylate (PETA) as base substance, bridging agent, and functional monomer, respectively. The structure of UVCF was characterized by Fourier transform infrared spectroscopy (FT-IR), H-nuclear magnetic resonance spectroscopy (H-NMR), and gel permeation chromatography (GPC). The viscosity and photopolymerization behavior of the UV-curable formulations were studied. The thermal stability and mechanical properties of the cured films were also investigated. The results showed that UVCF resin was successfully prepared, the number of average molecular weight was about 2010, and its molecular weight distribution index was 2.8. With the increase of UVCF resin content, the viscosity of the UVcurable formulations increased. After exposure to UV irradiation for 230 s, the photopolymerization conversion of the UV-curable formulations was above 80%. Moreover, when the UVCF content was 60%, the formulations had high photopolymerization rate, and the cured UVCF films showed good thermal stability and mechanical properties.


Introduction
Ketone-aldehyde (KA) resin, an important resins synthesized with ketone and aldehyde, is well known as multifunctional additives in coatings and inks [1,2].Owing to the existence of carbonyl and hydroxyl groups, in the molecular structure, KA resin has good compatibility with coating or ink resins, and can be used as dispersant for pigments in the coatings and inks [3][4][5].Meanwhile, KA resin can endow the products with high hardness and good adhesion to various substrates.In addition, the saturated chain in KA resin made contributions to the high gloss of the coating film [6,7].Therefore, the preparation of KA resin and its application in coatings and inks have been paid more and more attentions [8][9][10].Zhang et al. synthesized the urea-isobutyraldehyde-formaldehyde (UIF) resins by the condensation of urea, isobutyraldehyde, and formaldehyde.The results showed that the UIF resins had good yellowing resistance, UV resistance, and solubility in common organic solvents [11].A new melamine-formaldehyde-butanone (MFB) resin was prepared by Glowacz-Czerwonka and Kucharski [12], and they found that the coatings with MFB resin had transparent appearance, high hardness, and good resistance against boiling water.
Nowadays, due to its advantages of fast curing rate, energy saving, and environment protection, UV-curing has been considered to be one of the most promising technologies [13][14][15].For example, compared to traditional thermal-curable coatings, UV-curable coatings always show better comprehensive performance such as excellent film-forming property and high thermal stability.However, most of the KA resins have no C=C double bonds to react with UV-curable resin and it is easily to migrate from the UV-curable coatings, which will lead to the decrease of the mechanical properties and solvent-resistant properties [16].Therefore, it is necessary to introduce the curable groups into the structure of KA resin for UV-curing.Mishra et al. [17] prepared a radiationcurable resin through modifying the carbonyl-hydrogenated International Journal of Polymer Science KA resin with isophorone diisocyanate (IPDI) and hydroxy acrylate, and the UV-cured films showed good resistance to organic solvents.Thus, the carbonyl and hydroxyl reactive groups in KA resin provide potential modification with curable groups, which allows the KA resin to be cured under radiation condition.

Synthesis of UVCF Resin
. UVCF resin was synthesized by two steps as shown in Figure 1.Firstly, CF resin (50 g) and butyl acetate (143 g) were added into a four-neck glass reactor equipped with a reflux condenser, blender, and thermometer under dry nitrogen gas.When CF resin was dissolved, DBTDL (0.54 g) and IPDI (22.2 g, 0.1 mol) were added dropwise into the reactor and the temperature was heated to 50 ∘ C. The NCO value during the reaction was determined using the dibutylamine back-titration method every half an hour.When the NCO% decreased to about 50%, PETA (35.8 g, 0.12 mol) and hydroquinone (0.5 wt%) were dropwise into the reactor and the temperature raised to 60 ∘ C. The reaction continued until NCO% was below 0.5%.The reactant was washed with distilled water and then dissolved in acetone and purified via precipitation method by hexane.Finally, after drying, under a vacuum drying oven at 40 ∘ C, the light yellow powder product was obtained.The detailed formulations of UV-cured films were given in Table 1.

Size Exclusion Chromatography (SEC).
The molecular weight and molecular weight distribution of samples were determined with an equipment with a Waters 1515 pump and Waters 2414 differential refractive index detector (Waters, USA) using two columns HR4E and HR5 at 35 ∘ C, and tetrahydrofuran was used as the mobile phase at an elution rate of 1.0 mL/min.The calibration was made with low polydispersity polystyrene samples.

Fourier Transform Infrared (FT-IR) Spectroscopy
. FT-IR spectra were obtained from a Bruker Tensor 27 spectrometer (Bruker, Germany) in the range of 4000-400 cm −1 by KBr pellets method.

1 H-Nuclear Magnetic Resonance Spectroscopy ( 1 H-NMR)
. 1 H-NMR spectrum was recorded on a DRX400 NMR Spectrometer (Bruker, Germany) at 25 ∘ C using CDCl 3 as solvent and TMS as internal standard substance.

Viscosity. The viscosity was measured by an NDJ-7 model spinning viscometer (Shanghai Precision & Scientific
Instrument Company, China), with a speed of 750 rpm/min at different temperature.

Conversion of C=C Bonds.
The percent conversion of C=C bonds in the curing process was calculated by the area of the characteristic absorption peak of UV formulations via FT-IR according to the method in the literature, and the conversion () of C=C bonds was calculated according to the following equation [18]:  where  810 and  1724 were the calibration peak area at 810 and 1724 cm

Results and Discussion
3.1.FT-IR Analysis.FT-IR spectra of CF resin and UVCF resin are shown in Figure 2. In the spectrum of CF resin, the bands at 3526 cm −1 were the stretching vibration absorption peaks of -OH groups on CF resin.The peaks at 2934 cm −1 and 2860 cm −1 were attributed to the asymmetric stretching vibration and symmetrical stretching vibration of -CH 2 -, and the peak at 1448 cm −1 was attributed to the deformation vibration of -CH 2 -.The bands located at 1709 cm −1 and 1138 cm −1 were attributed to the stretching vibration of C=O and symmetrical stretching vibration of C-O, respectively.Compared with the FT-IR spectrum of CF resin, in Figure 2(b), new peaks of -NH group at 3377 cm −1 and 1531 cm −1 appeared, and the intensity of the peak attributed to -OH groups decreased obviously.Moreover, no absorption at 2270 cm −1 for -NCO group could be determined, indicating that IPDI had been reacted completely [19].In addition, the weak peaks at 3100-3000 cm −1 and strong peak at 810 cm −1 attributed to the stretching vibration and in-plane bending vibration of C-H in C=C double bonds were observed in the spectrum of UVCF, which conformed that PETA had been grafted onto CF resin.

3.2.
1 H-NMR Analysis.Figure 3 showed 1 H-NMR spectrum of UVCF resin.As shown in Figure 3, the peaks at 1.4-2.0ppm and 0.9-1.0ppm were attributed to the protons on methylene and methyl groups, respectively [20,21].The peaks located at 4.0-4.2ppm were attributed to H atoms of -O-CH 2 -groups.In addition, the peaks in the range of 5.8-6.5 ppm were attributed to the protons from C=C double bonds in PETA [22,23].Because the obtained UVCF had been purified, it can be further concluded that PETA had been grafted onto CF resin, and the results of 1 H-NMR were in accord with those of FT-IR.

SEC Analysis.
The molecular weight and molecular weight distributions of CF resin and UVCF resin were determined by SEC and the results are shown in Figure 4 and Table 2.It can be observed that the number average molecular weight (  ) of CF resin was 650, and its molecular weight distributions (PDI) were 1.2.After being modified with IPDI and PETA, the   of UVCF resin increased to 2010, which was close to the theory value.The PDI increased to 2.8 from 1.2, indicating that there was unreacted PETA in the system.

Viscosity.
The viscosity of the UVCF formulations had big effect on the liquid mobility, film-forming property, photopolymerization behavior, and the final mechanical properties of the cured film.Therefore, the viscosity should be adjusted to appropriate value before UV-curing.Figure 5 shows the viscosity of the formulations at different temperatures.In Figure 5, when the temperature was 30 ∘ C, the viscosity of UVCF formulation with 20% content of UVCF was about 1500 mPa⋅s.With the increase of UVCF content, the viscosity increased obviously.When the UVCF content reached 80%, the viscosity increased to 18000 mPa⋅s, which was mainly owing to the high inter-/intramolecular force of -NHCOO-groups and -COO-groups in UVCF.However, when the temperature increased from 30 ∘ C to 75 ∘ C, the viscosity of all formulations decreased to a low value of 200-300 mPa⋅s, which might be due to the decrease of inter-/ intramolecular force and the increase of molecular motion [24].

Photopolymerization Behavior.
Real time FT-IR was used to study the photopolymerization behavior of UVCF formulations under different UV irradiation time.As shown in Figure 6, before UV irradiation, the absorbance peak at about 810 cm −1 that attributed to C-H vibration from C=C double bonds was strong.With the increase of UV irradiation time, the C=C double bonds in the UVCF began to react and the absorbance peak at 810 cm −1 became weak.When the irradiation time was 230 s, the peaks at 810 cm −1 almost disappeared, indicating that the polymerization had been completely finished [18].Therefore, the appropriate irradiation time was about 230 s.

Effect of UVCF Content on Photopolymerization Conversion.
Figure 7 shows the effect of UV irradiation time on the conversion of UV-curable formulations.As shown in Figure 7, with the increase of irradiation time, the conversions increased rapidly, and when the irradiation time was about 230 s, except the UV-curable formulation with 95% content of UVCF, the conversions of the other formulations all exceed 80% and reached to the maximum value.However, with the further increase of irradiation time, the conversion increased a little.These results were consistent with the analysis of the real time FT-IR.In fact, in the later stage of the polymerization, some of the C=C double bonds might be trapped by the high cross-linking structures [25], and the active free radical from the UVCF was restricted to react completely, so the conversion under UV-curing condition was difficult to reach 100%.In addition, as shown in Figure 7, compared with other formulations, the UVCF formulation with 60% content of UVCF had higher photopolymerization rate and final conversion.The reasons could be explained as follows: when the UVCF content was low, the collision odds between reactive chains were little and it needed enough time to photopolymerized completely.When UVCF content was high, the viscosity of the formulations drastically increased, which led to International Journal of Polymer Science the limited mobility of reactive chains.Moreover, with high double bonds concentration, the photopolymerization easily occurred on the surface and formed the cross-linking structures, which might affect the mobility of reactive chain and the entrance of UV irradiation into the cured film [26].weight loss rate ( max ) of CF and UVCF were at 300-320 ∘ C. For cured UVCF formulation, with 60% content of UVCF,   and  max were at 250 ∘ C and 380 ∘ C, respectively.Compared with CF and UVCF, the obvious increase of  max of the cured UVCF formulation was mainly due to the formation of crosslinking structure.

Mechanical Properties.
The mechanical properties of cured UVCF films with different UVCF content are listed in Table 3.As shown in Table 3, all the cured films of UVCF formulations exhibited excellent adhesive force to the substrate.With the increase of UVCF content, the flexibility gradually decreased, especially for the cured film with 95% content of UVCF, and the flexibility attained 10 mm, owing to the excess cross-linking structures in the films.Furthermore, the cross-linking structures also restricted the mobility of the molecular chain, which led to the low impact strength at 31 kg⋅cm.As for the cured film with 60% content of UVCF, the pendulum hardness and impact strength were the highest.This can be explained that, in the cured film, C=C double bonds were well polymerized to form the appropriate cross-linking structures with good rubbery and rigid properties.

I r r a d i a t i oW a v e n u m b e r ( c m − 1 )AbsorbanceFigure 6 :Figure 7 :
Figure 6: 3D profile of UVCF formulations at bands of 810 cm −1 was recorded under different UV irradiation time by real time FT-IR.

Table 1 :
Formulations of UV-cured films.Preparation of UV-Cured Films.UVCF resin and Darocur 1173 were dissolved in TPGDA under magnetic stirring for about 15 min, and the UV-curable formulation was obtained.Subsequently, the formulation was coated on tinplate by an applicator with a 20 m gap and cured for 2 min under UV radiation (365 nm, 2 kW, 80 W/cm; Shenzhen Nengjia Automation Equipment Co., China).
−1attributed to in-plane bending vibration of C-H in C=C double bonds and stretching vibration of C=O in urethane unit, respectively.( 810 / 1724 ) 0 is the ratio of  810 to  1724 before UV radiation, and ( 810 / 1724 )  is the ratio of  810 to  1724 at different UV exposure time.

Table 2 :
Molecular weight and molecular weight distribution of CF resin and UVCF resin.

Table 3 :
Mechanical properties of cured UVCF films.