Effects of Adding Antioxidants on the Lightfastness Improvement of Refined Oriental Lacquer

polymers

Article Effects of Adding Antioxidants on the Lightfastness Improvement of Reﬁned Oriental Lacquer

Kun-Tsung Lu * and Jia-Jhen Lee

Department of Forestry, National Chung Hsing University, 250 Kuo-Kuang Rd., Taichung 402, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-4-2284-0345 (ext. 122); Fax: +886-4-2287-3628

Abstract: Refined oriental lacquer (ROL) is a natural polymeric material with a satiny texture, elegant beauty, and high durability for wood furniture and handicraft finishing. However, its poor lightfastness, which results from the photo-degradation or photo-oxidation of its main component, catechol derivatives, must be improved for its widespread utilization. In this study, two experiments were performed. First, five types of antioxidants, including three primary antioxidants, such as 2,20-methylenebis(6-nonyl-p-cresol) (coded as AO-1), 2,20-methylenebis(6-tert-butyl-4-methylphenol) (AO-2), and bis [4-(2-phenyl-2-propyl) phenyl] amine (AO-N), and two secondary antioxidants, such as tris (2,4-ditert-butylphenyl) phosphite (AO-P) and dilauryl thiodipropionate (AO-S), were investigated to determine which is the most effective for improving the lightfastness of ROL. Secondly, the appropriate quantity of the best antioxidant, including 0, 1, 2, 3, 5, and 10 phr, was also determined. The lightfastness parameters, such as brightness difference (∆L*), yellowness difference (∆YI), and color difference (∆E*), as well as other coating and film properties, were assessed. The results showed that the primary antioxidants had higher efficiency than secondary antioxidants for improving the lightfastness of ROL. Among the primary antioxidants, the 5 phr AO-N was the most effective at im- Citation: Lu, K.-T.; Lee, J.-J. Effects of proving the lightfastness of ROL; however, 1 phr addition had already shown significantly improved Adding Antioxidants on the efficiency. In addition, the drying time of ROL was extended and film properties decreased when Lightfastness Improvement of Refined increasing the content of AO-N, but the 1-phr-containing ROL displayed superior film properties, Oriental Lacquer. Polymers 2021, 13, especially adhesion and bending resistance, compared with the raw ROL film. 1110. https://doi.org/10.3390/ polym13071110 Keywords: refined oriental lacquer; antioxidants; lightfastness; coating and film properties

Academic Editors: Anna Szwajca and Radosław Pankiewicz

Received: 10 March 2021 1. Introduction Accepted: 28 March 2021 Oriental lacquer is a natural polymeric material with a water in oil (W/O) emulsion Published: 31 March 2021 sap, which is obtained by tapping Rhus trees, specifically Rhus succedanea, found in Taiwan and Vietnam [1,2]. The sap of raw oriental lacquer must be refined by stirring under a ◦ Publisher’s Note: MDPI stays neutral temperature below 45 C to reduce the water content to around 3% [3,4] or other refined with regard to jurisdictional claims in processes [5–8] for shortening the curing time and improving the gloss and other film published maps and institutional affil- properties of the raw oriental lacquer. It is defined as a refined oriental lacquer (ROL). The iations. ROL film possesses a wax-like gloss and has elegant beauty and high durability compared to synthetic coatings and is widely used for wood furniture and handicraft finishing in Taiwan. However, the poor lightfastness of ROL is a major limitation and it must be improved for the widespread utilization of ROL as a natural coating. Copyright: © 2021 by the authors. The poor lightfastness of ROL results from the main component, catechol derivatives, Licensee MDPI, Basel, Switzerland. such as urushiol, laccol, or thitsiol, of Rhus vernicifera (grown in China and Japan), R. This article is an open access article succedanea (in Taiwan and Vietnam), and Melanorrhoea usitata (in Thailand and Myanmar) distributed under the terms and saps, respectively [2,3]. The catechol derivatives have a 15 or 17 carbon side chains at conditions of the Creative Commons meta- or para-positions of the ortho-dioxy-benzol ring and with 0–3 unsaturated double Attribution (CC BY) license (https:// bonds—namely, 3-pentadecatrienylcatechols for urushiol, 3-heptadecatrienylcatechols for creativecommons.org/licenses/by/ laccol, and 4-heptadecatrienylcatechols for thitsiol. The catechol derivatives are dried first 4.0/).

Polymers 2021, 13, 1110. https://doi.org/10.3390/polym13071110 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1110 2 of 13

by the laccase-catalyzed dimerization of the ortho-dioxy-benzol and then aerobic auto- oxidative polymerization of the unsaturated side chains to form a network structure [1,3]. It is known that ultraviolet (UV) radiation provides strong energy to destroy the polymer materials by way of photo-degradation or photo-oxidation to produce excited state, free radical, peroxide, and singlet oxygen, etc. [9,10]. The catechol derivatives containing ROL coating networks were first photo-oxidated to form polymer oxy radicals (RO˙) and polymer peroxy radicals (ROO˙) under UV exposure and then the hydroxy (OH) and hydroperoxide (OOH) groups are formed by reacting these radicals with the same or neighboring polymer molecules (RH), respectively. Furthermore, the carbonyl groups, such as ketone groups, are formed by the internal rear- rangement of polymer alkoxy ally biradicals in the side chain of catechol derivatives [11]. In addition, cracking, chalking, gloss loss, and white spots are found on the film’s surface after UV exposure. These phenomena are due to the photo-degradation and cause a plant gum, which is another component of oriental lacquer resulting from the coating network structure [2,12,13]. Many photo-stabilizers, such as UV screeners, UV absorbers, hindered amine light stabilizers (HALS), antioxidants, singlet oxygen scavengers, and excited state quenchers, can be used for reducing the photo-degradation of polymer films [9,14–17]. Hong et al. [11] added HALS and a benzotriazole UV absorber to an oriental lacquer to enhance its photostabilization. In our previous reports [12,13,18], effects of a titanium dioxide (TiO2) UV screener and different types of HALS addition on the lightfastness improvement of a refined oriental lacquer were examined. In this study, the antioxidant additions were further investigated for improving the lightfastness of ROL. Five types of antioxidant were used, including primary antioxidants such as hindered phenol, whose function is to scavenge peroxy radical intermediates (ROO˙) in the photo-oxidation process [19]. Another is secondary antioxidants such as phosphite, which is frequently referred to as a hydroperoxide (OOH) decomposer and acts to convert hydroperoxides into nonradical, nonreactive, and thermally stable products [20].

2. Materials and Methods 2.1. Materials The oriental lacquer, harvested from the cultivar R. succedanea, was purchased from the Long-Nan Museum of Natural Lacquer Ware in Nantou, Taiwan. The composition of oriental lacquer was as follows: 54.1 ± 0.3% of laccol, 7.2 ± 0.4% of plant gum and laccase, 4.4 ± 0.7% of nitrogenous compounds, and 34.3 ± 0.1% of water. It was analyzed in our laboratory according to the CNS 2810 Standard [21]. Five types of antioxidant were used, including three primary antioxidants, such as 2,20-methylenebis(6-nonyl-p-cresol) (coded as AO-1), 2,20-methylenebis(6-tert-butyl-4- methylphenol) (coded as AO-2), and bis [4-(2-phenyl-2-propyl) phenyl] amine (coded as AO-N), and two secondary antioxidants, such as tris (2,4-ditert- butylphenyl) phosphite (coded as AO-P) and dilauryl thiodipropionate (coded as AO-S). All antioxidants were purchased from Easchem Co. Ltd., (Taipei, Taiwan), with the exception of the AO-N antioxidant, which was donated by Eutec Chemical Co. Ltd., Taiwan. The antioxidants were used as received without further purification. Figure1 shows the structures of all antioxidants. Cryptomeria japonica boards (with a moisture content of 11.0%, radial section), glass sheets, S-16 wear-resistant steel plates, tin-coated iron sheets, and Teflon sheets were used as substrates for specified experiments. All of the substrates were pretreated according to the CNS 9007 Standard [22]. Polymers 2021, 13, 1110 3 of 13 Polymers 2021, 13, x FOR PEER REVIEW 3 of 14

FigureFigure 1. 1. TheThe structures structures of of the the five ﬁve types of antioxidants. 2.2. Preparation of ROL 2.2. Preparation of ROL First, 400 g of oriental lacquer was placed in a glass container and stirred at 60 rpm, at 40First,◦C, with 400 g the of mixingoriental blades lacquer 5 mmwas fromplaced the in bottoma glass container of the container, and stirred until at the 60 water rpm, atcontent 40 °C, was with reduced the mixing to 3.5% blades and 5 the mm ROL from was the obtained. bottom of the container, until the water content was reduced to 3.5% and the ROL was obtained. 2.3. Preparation of Antioxidant-Containing ROL 2.3. PreparationFor determining of Antioxidant the best-Containing antioxidant ROL regarding the lightfastness of ROL, the different antioxidants,For determining AO-1, AO-2, the best AO-N, antioxidant AO-P, and regarding AO-S, with the 2 lightfastness phr (by the solid of ROL, content the of differ- ROL) entwere antioxidants, dissolved in AO 10- mL1, AO xylene-2, AO and-N, then AO- addedP, and AO into-S the, with ROL, 2 phr respectively. (by the solid The content mixtures of ROL)were stirredwere dissolved evenly at 120in 1 rpm0 mL for xylene 10 min. and The then lightfastness added into of the the ROL, antioxidant-containing respectively. The mixturesROL was were evaluated stirred and evenly the best at 120 antioxidant rpm for selected.10 min. Finally,The lightfastness for determining of the theantioxi- most dantappropriate-containing quantity ROL ofwas antioxidant evaluated to and add, the amounts best antioxidant of 0, 1, 2, 3, 5,selected. and 10 phrFinally, were for added de- termininginto the ROL, the respectively.most appropriate The preparation quantity of process antioxidant was the to sameadd, asamounts that mentioned of 0, 1, 2, above 3, 5, andand 10 the phr coating were andadded ﬁlm into properties the ROL, were respectively. investigated. The preparation process was the same as that mentioned above and the coating and film properties were investigated. 2.4. Measurement of Coating Properties

The pH value was determined at 25 ◦C using a Suntex sp-701 probe (Suntex In- struments, Taipei, Taiwan). The viscosity was measured at 25 ◦C by a Brookﬁeld DV-E Viscometer (Brookﬁeld Engineering Laboratories, Middleboro, MA, USA). The drying

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time was measured at 25 ◦C, 80% RH, and with a wet ﬁlm thickness of 76 µm using a three-speed BK Drying Time Recorder (BYK Additives & Instruments, Wesel, Germany). The drying states of touch-free dry (TF) and hardened dry (HD) were recorded as described by Lu et al. [23] and Chang et al. [4].

2.5. Preparation and Determination of Film Properties The selected substrates were finished with different ROLs using a universal applicator (B-3530, Elcometer, Manchester, UK) with a wet film thickness of 100 µm. The specimens were first placed in a chamber with a constant temperature of 25 ◦C and humidity of 80% RH for one day, followed by placing them in an air-conditioned environment of 26 ◦C and 65% RH for another week. The film properties were evaluated after this drying process. The lightfastness of the film was evaluated using a Paint Coating Fade Meter (Suga Test Instruments Co., Tokyo, Japan) at a chamber temperature of 32 ± 4 ◦C and using a H400-F mercury lamp (SUGA Test Instruments Co., Ltd., Tokyo, Japan) as a light source. According to CIE L*, a*, b* color system, after 0, 12, 24, 48, 96, 144, and 192 h exposure, the color changes, including brightness difference (∆L*), yellowness difference (∆YI), and color difference (∆E*), were obtained using a spectrophotometer (CM-3600d, Minolta. ◦ Osaka, Japan) measurement with a D65 light source, test angle of 10 , and test window diameter of 8 mm and computed from the Minolta MCS software system. Nine points were measured and averaged for each specimen. The scanning electron microscope (SEM) inspection was performed using an electron microscope (SM-200, Topcon, Tokyo, Japan) with a magnification of 1350×. The Fourier transform infrared spectroscopy (FTIR) analysis was conducted using a Perkin-Elmer Spectrum 100 instrument (Perkin-Elmer, Shelton, CT, USA) with single-spot attenuated total reflection (ATR) at a range of 4000 to 650 cm−1 at a resolution of 4 cm−1 and averaged over 16 scans for each specimen. The hardness of the film on glass sheets was evaluated according to the DIN 53157 using a König/Persoz Pendulum Hardness Tester (Braive Instruments, Liège, Belgium). Seven points were measured and averaged for each specimen. The mass retention was determined by placing approximately 0.3 g free film, which was separated from the Teflon sheet into a Soxhelt extractor (Dogger Co., New Taipei City, Taiwan) containing 250 mL acetone. The solution was siphoned four times per hour (total 6 h), and the soaked film was dried in an oven at 50 ◦C for 6 h and the weight retention was calculated. Five repetitions were performed and averaged. For determining the glass transition temperature (Tg) of film, dynamic mechanical analysis was carried out with a Perkin-Elmer DMA 8000 (Perkin- Elmer, Waltham, MA, USA) using tension mode at a frequency of 1 Hz, temperature of 0–180 ◦C, and a heating rate of 5 ◦C min−1 in a nitrogen atmosphere. The impact resistance of the film on wood was investigated based on the falling weight of 300 g and an impact needle diameter of 1/2 inch by using the DuPont Impact Tester IM-601 (DuPont Co., Wilmington, DE, USA) and the height of the intact film was recorded. The adhesion of film on wood was evaluated by using the crosscut method according to CNS 10756 K 6800 [24]. The best adhesion is Grade 10, followed by Grades 8, 6, 4, 2, and 0. The bending resistance of film on tin-coated iron sheet was carried out according to JIS-K-5400 [25] by using a bending tester (Ueshima Seisakusho Co., Ltd., Tokyo, Japan) with steel bars with diameters of 2, 3, 4, 6, 8, and 10 mm. The tensile strength and elongation at break were evaluated according to ASTM D-638 Standard [26] by using a Shimadzu EZ Test Series Tensile Tester (Shimadzu, Kyoto, Japan) at a speed of 5 mm min−1 and with a fixture distance of 40 mm. Seven samples were tested and averaged for each specimen. Thermogravimetric analysis (TGA) of film was conducted by using a Perkin-Elmer STA 6000 (Perkin-Elmer, Waltham, MA, USA) in a nitrogen atmosphere with the temperature increasing from 50 to 700 ◦C at a heating rate of 10 ◦C min−1. Polymers 2021, 13, 1110 5 of 13

3. Results and Discussion 3.1. Lightfastness of ROL Films with Different Antioxidants The time-dependent brightness difference (∆L*), yellowness difference (∆YI), and color difference (∆E*) of ROL films with different antioxidants after UV exposure test are listed in Tables1–3, respectively. All of the ∆L*, ∆YI, and ∆E* values increased with increasing UV exposure time. However, the films containing primary antioxidants AO-2 and AO-N had ∆L* values of 2.7 and 0.5, ∆YI values of 45.7 and 22.3, and ∆E* values of 11.1 and 5.3 after a UV exposure time of 192 h, which are significantly lower than those of blank (∆L*, ∆YI, and ∆E* of 12.3, 99.9, and 32.4) and other antioxidants. The results also showed that the primary antioxidants (AO-1, AO-2, and AO-N) had higher efficiency than secondary antioxidants (AO-P and AO-S) for improving the lightfastness of ROL. In principle, the photo-oxidation of catechol derivatives containing ROL could be stopped to form polymer oxy radicals (RO˙) and polymer peroxy radicals (ROO˙) under UV exposure if the primary antioxidants acted as efficient radical scavengers. Furthermore, the primary antioxidants also interrupted peroxy radicals (ROO˙) to abstract a proton from the same or neighboring polymer (RH) to form the hydroxy (OH) and hydroperoxide (OOH) groups [19,27].

Table 1. Time-dependent brightness difference (∆L*) of reﬁned oriental lacquer (ROL) ﬁlms with different antioxidants after UV exposure.

Antioxidants ∆L* after UV Exposure (h) (2 phr) 12 24 48 96 144 192 Blank a 1.5 2.7 5.2 9.5 11.1 12.3 AO-1 b 1.6 2.6 5.1 9.1 11.6 11.9 AO-2 c 0.3 0.1 0.9 1.8 2.0 2.7 AO-N d −0.2 −0.4 −0.2 0.0 0.1 0.5 AO-P e 2.2 4.4 7.7 12.7 15.1 16.6 AO-S f 0.9 1.9 4.6 8.7 9.4 12.5 a ROL without antioxidant; b 2,20-methylenebis (6-nonyl-p-cresol); c 2,20-methylenebis (6-tert-butyl-4-methylphenol); d Bis [4-(2-phenyl-2-propyl)phenyl] amine; e Tris(2,4-ditert-butylphenyl) phosphite; f Dilauryl thiodipropionate.

Table 2. Time-dependent yellowness difference (∆YI) of ROL ﬁlms with different antioxidants after UV exposure.

Antioxidants ∆YI after UV Exposure (h) (2 phr) 12 24 48 96 144 192 Blank a 32.4 49.2 72.4 91.3 98.2 99.9 AO-1 b 34.3 49.8 69.7 92.3 98.3 101.4 AO-2 c 8.3 11.5 19.8 31.9 38.7 45.7 AO-N d 1.5 2.9 5.9 11.7 16.5 22.3 AO-P e 39.6 63.2 82.4 97.9 102.0 102.8 AO-S f 20.4 39.7 65.8 91.4 94.8 102.1 a, b, c, d, e, f the same as Table1.

Table 3. Time-dependent color difference (∆E*) of ROL films with different antioxidants after UV exposure.

Antioxidants ∆E* after UV Exposure (h) (2 phr) 12 24 48 96 144 192 Blanka 8.3 12.5 19.4 27.4 30.8 32.4 AO-1 b 8.7 12.5 18.4 26.7 30.7 31.8 AO-2 c 2.3 2.9 5.0 7.9 9.4 11.1 AO-N d 0.6 0.9 1.5 2.8 3.9 5.3 AO-P e 10.2 16.7 23.7 32.6 36.8 39.0 AO-S f 5.3 10.1 17.4 26.6 28.0 33.0 a, b, c, d, e, f the same as Table1. Polymers 2021, 13, 1110 6 of 13

In this study, bis [4-(2-phenyl-2-propyl) phenyl] amine (AO-N) was the most efficient for improving the lightfastness of ROL, followed by 2,20-methylenebis (6- tert-butyl-4- methylphenol) (AO-2) and 2,20-methylenebis(6-nonyl-p-cresol) (AO-1). This result is due to the -NH group of the amine antioxidant (AO-N) reacting more easily with peroxy radicals (ROO˙) than the -OH group of phenolic antioxidants (AO-1 and AO-2) and providing a more easily abstracted proton than those of the polymer (RH) to form a stable, sterically hindered structure, thus interrupting the further photo-oxidation of the ROL [19,27]. In addition, as the AO-1 possesses long alkyl chains, which reduce the reaction of -OH groups with peroxy radicals (ROO˙), it was inferior to AO-2 in improving the lightfastness of ROL. Among the secondary antioxidants, dilauryl thiodipropionate (AO-S) had also an ability to improve the lightfastness of ROL. However, after long-term UV exposure, it lost the improving ability; for example, after more than 144 h UV exposure, the ∆L*, ∆YI, and ∆E* values of ROL were higher than those of the blank group. It could be elucidated that the secondary antioxidants acted as hydroperoxide (OOH) decomposers. The hydroperoxide was produced from the initial stage products, peroxy radicals (ROO˙), during photo-degradation; therefore, the improvement in the efficiency of the secondary antioxidant is inferior to that of the primary antioxidant. In addition, among the five types of antioxidants used in this study, the secondary antioxidant AO-P, which contained the phosphite group, was the least efficient for the lightfastness improvement of ROL. Thsi may be because the AO-P possesses three units of benzene structure, and the steric hindrance decreases the reactivity with ROOH, and a quinone structure is formed after UV irradiation. The result shows that the ROL with AO-P had the highest ∆L*, ∆YI, and ∆E* values.

3.2. Coating and Film Properties of ROL with Different Contents of Antioxidant In Section 3.1, it is reported that the primary antioxidant of the bis [4-(2-phenyl-2- propyl) phenyl] amine (AO-N)-containing ROL ﬁlm had the best lightfastness. Therefore, for determining the best content of AO-N, the coating and ﬁlm properties of ROL were further investigated.

3.2.1. Coating Properties of ROL The coating properties of ROL with different contents of AO-N are listed in Table4. The ROL with and without AO-N had the same pH values of 3.4–3.5, meaning that the activity of laccase was be affected by the addition of the antioxidant, and the optimum pH value of ROL with the best activity of laccase is 3.0~5.0 [11,12]. The viscosity of ROL increased with increasing amount of AO-N added. This result may be due to the –N-H group in AO-N, and the greater hydrogen bonding increases the viscosity of ROL. However, the viscosity of ROL from 0 phr of 113 cps increased to 10 phr of 148 cps. The slight increase in viscosity did not affect the ﬁnishing operation of ROL.

Table 4. Coating properties of ROL with different contents of AO-N.

◦ Contents of Viscosity Drying Time (h (25 C, 80% RH) pH ◦ AO-N (phr) (cps, 25 C) TF a HD b 0 3.5 113 5.0 11.0 1 3.5 116 5.0 12.5 2 3.5 129 5.5 13.5 3 3.4 131 5.5 13.5 5 3.4 136 6.0 15.0 10 3.5 148 6.0 20.0 a TF: Touch-free dry; b HD: Hardened dry.

The drying time of ROL without AO-N was the touch-free dry (TF) of 5.0 h and hardened dry (HD) of 11.0 h whereas it extended with increasing content of AO-N. The ROL with 10 phr AO-N had the longest drying time of 6.0 and 20 h for TF and HD, respectively. It is known that the catechol derivatives of ROL are dried ﬁrst by the laccase-catalyzed Polymers 2021, 13, 1110 7 of 13

dimerization of the ortho-dioxy-benzol and then aerobic auto-oxidative polymerization of the unsaturated side chains to form a network structure. In the auto-oxidative polymerization stage, the formed hydro-peroxide radicals (ROO˙), which polymerize to form a network structure, are captured by the Ar-NH groups of AO-N to form a hydro-peroxide (ROOH) and undergo retarded auto-oxidative polymerization of ROL, exhibiting a longer drying time after adding AO-N.

3.2.2. Lightfastness of ROL Films The time-dependent brightness difference (∆L*), yellowness difference (∆YI), and color difference (∆E*) of ROL films with different contents of AO-N after UV irradiation are shown in Figures2–4, respectively. The ∆L* of ROL films increased with prolonged exposure time under UV irradiation. This phenomenon results from the photo-degradation and photo-oxidation to destroy the network structure of ROL and white particle plant gum migrated to the surface [28]. The ROL film without AO-N (0 phr) had the largest change in the brightness of the film. The result showed that with only 1 phr addition, the ∆L* value could be lowered significantly; however, 5 phr content yielded the best improvement in the brightness of the ROL film. The yellowness difference (∆YI) of ROL films also increased with increasing UV exposure time; this may be due to the chormophoric group of quinone structures formed from the benzene rings in the ROL film after photo-oxidation. The ROL film without AO-N yellowed seriously after only UV irradiation of 12 h, and a twofold increase in the ∆YI value after UV exposure of 48 h was found. The color difference (∆E*) is a synergistic effect of ∆L* and ∆YI, and the trend was the same as the ∆L* and ∆YI. The color change parameters of ∆L*, ∆YI, and ∆E* values after the lightfastness test of 192 h showed that the values of the ROL film without AO-N were 17.1, 99.0, and 39.2, respectively, and all of the values collected with different contents of AO-N were lower than those without AO-N. However, the decrease in the color change parameters was not proportionate to the content of AO-N, i.e., the ROL film with 5 phr AO-N had the lowest ∆L*, ∆YI, and ∆E* values of 3.2, 65.4, and 14.4, and with 10 phr, AO-N increased to 6.4, 81.9, and 22.1, respectively. In practice, the ∆L*, ∆YI, and ∆E* values of the ROL film with Polymers 2021, 13, x FOR PEER REVIEW 8 of 14 1 phr AO-N were 5.8, 84.4, and 22.9, respectively, and it already demonstrated significant efficiency in improving the lightfastness.

FigureFigure 2. 2.TimeTime-dependent-dependent brightness brightness difference difference (ΔL*) (∆L*) of of ROL ROL films ﬁlms with with different different contents contents of of AO-N AOafter-N after UV exposure UV exposure test. test.

Figure 3. Time-dependent yellowness difference (ΔYI) of ROL films with different contents of AO-N after UV exposure test.

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Polymers 2021, 13, 1110 8 of 13 FigureFigure 2. 2. Time Time-dependent-dependent brightness brightness difference difference (ΔL*) (ΔL*) of of ROL ROL films films with with different different contents contents of of AOAO-N-N after after UV UV exposure exposure test. test.

FigureFigureFigure 3. 3. 3.Time TimeTime-dependent-dependent-dependent yellowness yellowness yellowness difference difference difference (ΔYI) (ΔYI) (∆YI) of of ROL ROL films ﬁlmsfilms with with different differentdifferent contents contentscontents of of AO-N AOAOafter-N-N after UVafter exposure UV UV exposure exposure test. test. test.

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Figure 4. Time-dependent color difference (ΔE*) of ROL films with different contents of AO-N after UVFigure exposure 4. Time-dependent test. color difference (∆E*) of ROL ﬁlms with different contents of AO-N after UV exposure test.

3.2.3. SEM Insp Inspectionection of ROL Films The SEM images of ROL filmsfilms with different contents of AO-NAO-N before and after UV exposure of 192 h are shown in FigureFigure5 5.. TheThe ROLROL filmfilm withoutwithout AO-NAO-N hadhad moremore whitewhite particles of plant gum (as Figure5 5b)b),,which which waswas duedue toto severesevere photo-degradation photo-degradation underunder UV exposure [2,12,13,28 [2,12,13,28].]. However, the white spots were less prevalent in the ROL filmsfilms with AO-NAO-N afterafter UVUV irradiation. irradiation. These These results results were were also also confirmed confirmed by theby decreasingthe decreasing∆L* valuesΔL* values of the ofROL the ROL films films after after adding adding the AO-N the AO antioxidant,-N antioxidant as shown, as shown in Figure in Figure2. 2.

（a）AO-N-0 phr-0 h （b） AO-N-0 phr-192 h

Figure 5. Cont （c） AO-.N-1 phr-0 h （d） AO-N-1 phr-192 h

（e） AO-N-2 phr-0 h （f） AO-N-2 phr-192 h

（g） AO-N-3 phr-0 h （h） AO-N-3 phr-192 h

（i） AO-N-5 phr-0 h （j） AO-N-5 phr-192 h

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Figure 4. Time-dependent color difference (ΔE*) of ROL films with different contents of AO-N after UV exposure test.

3.2.3. SEM Inspection of ROL Films The SEM images of ROL films with different contents of AO-N before and after UV exposure of 192 h are shown in Figure 5. The ROL film without AO-N had more white particles of plant gum (as Figure 5b), which was due to severe photo-degradation under UV exposure [2,12,13,28]. However, the white spots were less prevalent in the ROL films with AO-N after UV irradiation. These results were also confirmed by the decreasing ΔL* values of the ROL films after adding the AO-N antioxidant, as shown in Figure 2.

（a）AO-N-0 phr-0 h （b） AO-N-0 phr-192 h

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（c） AO-N-1 phr-0 h （d） AO-N-1 phr-192 h

（e） AO-N-2 phr-0 h （f） AO-N-2 phr-192 h

（g） AO-N-3 phr-0 h （h） AO-N-3 phr-192 h

（i） AO-N-5 phr-0 h （j） AO-N-5 phr-192 h

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（k） AO-N-10 phr-0 h （l） AO-N-10 phr-192 h

Figure 5. Scanning electronelectron microscopic microscopic images images (1350 (1350x)×) of of ROL ROL films films with with different different contents contents of AO-N.of 3.2.4.AO-N. FTIR Analysis of ROL Films 3.2.4.The FTIR FTIR Analysis spectra of ofROL ROL Films films with different contents of AO-N before and after UV exposure of 192 h are shown in Figure6. The functional groups of the ROL film without The FTIR spectra of ROL films with different contents of AO-N before and after UV AO-N (0 phr) showed the -OH at 3200–3400 cm−1; -C-H stretching vibration of -C=C-H at exposure of 192 h are shown in Figure 6. The functional groups of the ROL film without 3010 cm−1; -CH asymmetric and symmetric stretching vibration of the urushiol side chain 2 -1 atAO 2924-N ( cm0 phr)−1 and showed 2854 cmthe− -1OH, respectively; at 3200–3400 -C-O-C- cm ; -C stretching-H stretching vibration vibration at 1715 of - cmC=C−1-Hand at -1 -C=O3010 cm stretching; -CH2 asymmetric vibration at and 1700 symmetric cm−1; -C=C- stretching stretching vibration vibration of the and urushiol -C-H out side of planechain -1 -1 -1 bendingat 2924 cm vibration and 2854 of the cm urushiol, respectively; benzene - ringC-O- atC-1615 stretching cm−1 andvibration 730 cm at− 11715, respectively; cm and −1 the-C=O conjugated stretching triene vibration of the at urushiol 1700 cm side; -C=C chain- stretching also can be vibration seen at 990 and cm -C−-1H[ 5out,12 ,29of ].plane The −1 −1 FTIRbendi spectrang vibration of the of ROL the urushiol films with benzene different ring contents at 1615 ofcm AO-N and were730 cm similar, respectively; to that of −1 the ROLconjugated film without triene of AO-N the urushiol before UV side exposure. chain also However, can be seen after at the990 192-h cm [ lightfastness5,12,29]. The experiment,FTIR spectra theof the peak ROL at 3010films cm with−1 disappeareddifferent contents and those of AO at-N 2924 were cm similar−1, 2854 to cmthat−1 of, and the 990ROL cm film−1 decreased.without AO The-N characteristicbefore UV exposure. peaks at 1715However, cm−1 (-C-O-C-)after the 192 and-h 1700 lightfastness cm−1 (-C=O) ex- increasedperiment, andthe consolidatedpeak at 3010 incm a− new1 disappeared broad peak and at 1707those cm at− 29241. The cm results−1, 2854 showed cm−1, and that 990 the cm−1 decreased. The characteristic peaks at 1715 cm−1 (-C-O-C-) and 1700 cm−1 (-C=O) increased and consolidated in a new broad peak at 1707 cm−1. The results showed that the -CH bonding in the side chain fractured by photo-degradation and the peroxides formed. Furthermore, the degradation caused damage to the films and the plant gum migrated to the surface of the film [19,30]. In addition, the peak at 730 cm−1 also decreased due to the benzene ring degradation, and quinone compounds formed. From the FTIR analysis, it could be concluded that the photo-degradation of the ROL film was a result of the side chain and catechol structure of the network film. This result is also confirmed by the previous study by Hong et al. [11]. Figure 6 also shows that the ROL films with different contents of AO-N had a similar FTIR spectrum after UV exposure of 192 h, meaning that the peak at 3010 cm−1 disappeared, those at 2924 cm−1, 2854 cm−1, and 990 cm−1 decreased, and that at 1707 cm−1 increased. However, among the spectra, the 5-phr-AO-N-containing film showed the lowest increase in the peak at 1707 cm−1, demonstrating that it possessed the best lightfastness, and this result is also confirmed in Section 3.2.2.

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-CH bonding in the side chain fractured by photo-degradation and the peroxides formed. Furthermore, the degradation caused damage to the films and the plant gum migrated to the surface of the film [19,30]. In addition, the peak at 730 cm−1 also decreased due to the benzene ring degradation, and quinone compounds formed. From the FTIR analysis, it could be concluded that the photo-degradation of the ROL film was a result of the side chain and catechol structure of the network film. This result is also confirmed by the previous study by Hong et al. [11]. Figure6 also shows that the ROL films with different contents of AO-N had a similar FTIR spectrum after UV exposure of 192 h, meaning that the peak at 3010 cm−1 disappeared, those at 2924 cm−1, 2854 cm−1, and 990 cm−1 decreased, and that at 1707 cm−1 increased. However, among the spectra, the 5-phr-AO-N-containing −1 Polymers 2021, 13, filmx FOR showedPEER REVIEW the lowest increase in the peak at 1707 cm , demonstrating that it possessed11 of 14

the best lightfastness, and this result is also conﬁrmed in Section 3.2.2.

1707 ↓

2924 730 2854 990 AO-N-10 phr-192 h ↓ ↓ 1615 ↓ 3010 ↓ AO-N-10 phr-0 h ↓

AO-N-5 phr-192 h

AO-N-5 phr-0 h

AO-N-3 phr-192 h

AO-N-3 phr-0 h

AO-N-2 phr-192 h Absorbance

AO-N-2 phr-0 h

AO-N-1 phr-192 h

AO-N-1 phr-0 h

AO-N-0 phr-192 h

AO-N-0 phr-0 h

3750 3500 3250 3000 2750 2500 2250 2000 1750 1500 1250 1000 750 500 Wavenumvers （cm -1）

Figure 6. FTIRFigure spectra 6. FTIR of spectra ROL ﬁlms of ROL with films different with different contents contents of AO-N.of AO-N.

3.2.5. Film Properties3.2.5. Film Properties of ROL of ROL The fundamentalThe fundamental film properties film properties of ROL of areROL listed are listed in Tablein Table5. 5. The The ROL ROL filmfilm without without AO-N had theAO highest-N had the hardness highest hardness of 78 sec of 78 and sec the and one the one containing containing 10 10 phr phr AO-NAO-N show showeded the lowest hardness of 67 s. The other films with AO-N content had a hardness of 72~75 the lowest hardnesssec, which of was 67 s.slightly The otherlower than films that with of the AO-N raw ROL content film. had The amass hardness retention of and 72~75 Tg s, which was slightlyhad the lowersimilar than trends that regarding of the rawthe hardness. ROL film. The The rawmass ROL film retention had the and highest Tg hadmass the similar trendsretention regarding and Tg the of hardness.93.7% and 103 The °C, raw which ROL decreased film had with theincreasing highest content mass of retention AO-N. and Tg of 93.7%In addition, and 103 the◦C, raw which ROL film decreased also had with the best increasing impact resistance content of of 10 AO-N. cm, compared In addition, to the raw ROLonly film also5 cm had for the bestthe impactother AO resistance-N-containing of 10 cm,films. compared The raw to onlyROL 5 cmand for 1-phr-AO-N-containing films showed the best adhesion of Grade 10; however, there the other AO-N-containing films. The raw ROL and 1-phr-AO-N-containing films showed were no differences in bending resistance for all samples except the the best adhesion10-phr of-AO Grade-N-containing 10; however, film, which there yielded were noa rather differences poor value in bendingof 10 mm. resistance The results for all samples exceptmentioned the above 10-phr-AO-N-containing show that the auto-oxidative film, polymerization which yielded of the a rather unsaturated poor side value chains of ROL was inhibited by adding the AO-N antioxidant, which decreased the cross-linking density of the network structure. The same trends were also found in tensile strength. The raw ROL film had a tensile strength and elongation at break of 16.5 Mpa and 10.8%, while the tensile strength decreased and the elongation at break increased

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of 10 mm. The results mentioned above show that the auto-oxidative polymerization of the unsaturated side chains of ROL was inhibited by adding the AO-N antioxidant,

Polymers 2021, 13which, x FOR PEER decreased REVIEW the cross-linking density of the network structure. The same trends were 12 of 14 also found in tensile strength. The raw ROL ﬁlm had a tensile strength and elongation at break of 16.5 Mpa and 10.8%, while the tensile strength decreased and the elongation at break increased with the addition of AO-N. The 10-phr-AO-N-containing ﬁlm had with the addition of AO-N. The 10-phr-AO-N-containing film had the lowest tensile the lowest tensile strength and elongation at break of 4.9 Mpa and 3.3% due to its low strength and elongation at break of 4.9 Mpa and 3.3% due to its low cross-linking density. cross-linking density. Table 5. Film properties of ROL with different contents of AO-N. Table 5. Film properties of ROL with different contents of AO-N. Contents of Mass Impact Bending Tensile Contents MassHardness Impact Tg BendingAdhesion Tensile Elongation at HardnessAO-N Tg Retention AdhesionResistance Resistance StrengthElongation at of AO-N Retention(König, s) Resistance(°C) Resistance(Grade) Strength Break (%) (König, s)(phr) (◦C) (wt. %) (Grade)(300 g, cm) (mm) (MPaBreak) (%) (phr) (wt. %) (300 g, cm) (mm) (MPa) 0 78 ± 2 93.7 ± 0.8 103 10 10 6 16.5 ± 0.9 10.8 ± 1.0 ± ± ± ± 0 78 21 93.7 0.872 ± 4 10392.7 ± 1.4 1098 105 610 16.56 0.915.4 10.8 ± 0.21.0 11.6 ± 1.3 1 72 ± 4 92.7 ± 1.4 98 5 10 6 15.4 ± 0.2 11.6 ± 1.3 2 74 ± 12 93.0 ± 1.474 ± 1 9793.0 ± 1.4 597 85 68 15.36± 0.115.3 11.8 ± 0.1± 0.9 11.8 ± 0.9 3 74 ± 13 91.8 ± 0.374 ± 1 9591.8 ± 0.3 595 65 66 13.96± 0.913.9 13.2 ± 0.9± 0.0 13.2 ± 0.0 5 75 ± 15 90.3 ± 0.675 ± 1 8890.3 ± 0.6 588 65 66 12.36± 0.512.3 10.9 ± 0.5± 0.7 10.9 ± 0.7 10 67 ± 310 86.7 ± 0.867 ± 3 8686.7 ± 0.8 586 65 86 4.98± 0.24.9 3.3 ± 0.2± 0.3 3.3 ± 0.3

The TGA curvesThe of TGA ROL curves films of with ROL different films with contents different of AO-Ncontents are of displayedAO-N are indisplayed in Figures7 and8 Figures, respectively. 7 and 8, The respectively. ROL films The with ROL or without films with AO-N or without showed AO similar-N showed behav- similar be- ior in terms ofhavior thermal in degradation.terms of thermal They degradation. consisted They of three consisted dominant of three steps dominant at around steps at around 100–200 ◦C, 200–350100–200◦C, °C and, 200 350–550–350 °C, ◦andC,corresponding 350–550 °C, corresponding to the water to vaporization the water vaporization and and decompositiondecomposition of the low-molecular-weight of the low-molecular compounds,-weight the compounds, degradation the of thedegradation side chain of the side of urushiol, andchain the decompositionof urushiol, and of the the decomposition network structure of the of network the ROL structure film, respectively. of the ROL film, re- This result is alsospectively. confirmed This byresult the is previous also confirmed study by the Niimura previous et al.study [1, 6by]. Niimura The results et al. [1,6]. The results showed that in the second step, the raw ROL film had a temperature of maximum showed that in the second step, the raw ROL film had a temperature of maximum decom- decomposition rate (Tmax) and derivative weight at Tmax around 313 ℃ and -1.9 %.min−1. position rate (T ) and derivative weight at T around 313 ◦C and −1.9 %.min−1. The Themax other films with AOmax-N showed similar results, except for the other films with AO-N showed similar results, except for the 10-phr-AO-N-containing one, 10-phr-AO-N-containing one, which had the− 1largest derivative weight at Tmax of which had the largest derivative weight at Tmax of −2.9%.min . In the third step, the Tmax −2.9%.min−1. In the third step, the Tmax of raw ROL and 1-phr-AO-N-containing films was of raw ROL and 1-phr-AO-N-containing films was 446 ◦C, while those of the other films 446 ℃, while those of the other films were slightly lower than the temperature; for ex- were slightly lower than the temperature; for example, that of the 10-phr-AO-N-containing ◦ ample, that of the 10-phr-AO-N-containing film was 440 ℃. The results showed that the film was 440 C.thermal The results stability showed of the ROL that film the thermalbecame inferior stability after of theadding ROL AO film-N and became decreased with inferior after addingincreasing AO-N content and decreasedof AO-N due with to increasingthe decrease content in the ofcross AO-N-linking due density to the of the net- decrease in thework cross-linking structure. density of the network structure.

100 90 0 phr 80 1 phr ） 70

% 2 phr 60 （ 3 phr 50 5 phr 40 10 phr Weight Weight 30 20 10 0 50 150 250 350 450 550 650 750 Temperature （℃）

Figure 7.FigureThermogravimetric 7. Thermogravimetric diagrams diagrams of ROL of ﬁlmsROL films with with different different contents contents of AO-N. of AO-N.

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） -1 -2 0 phr 1 phr -3 2 phr （ %/min -4 3 phr 5 phr -5 10 phr -6 -7 -8 Derivative Weight Weight Derivative -9 50 150 250 350 450 550 650 750 Temperature （℃）

FigureFigure 8. 8.DerivativeDerivative thermogravimetric thermogravimetric (DTG) (DTG) diagrams diagrams of ofROL ROL films films with with different different contents contents of ofAO AO-N.-N.

4. Conclusions4. Conclusions For improvingFor the improving lightfastness the lightfastness of ROL, five of types ROL, of five antioxidants, types of antioxidants including, threeincluding three primary antioxidantsprimary (AO-1,antioxidants AO-2, (AO and-1, AO-N) AO-2, andand twoAO-N) secondary and two antioxidantssecondary antioxidants (AO-P (AO-P and AO-S), wereand used.AO-S), The were best used. antioxidant The best andantioxidant most suitable and most amount suitable of amount antioxidant of antioxidant were investigated.were Theinvestigated. results showed The results that showed the efficiency that the of efficiency lightfastness of lightfastness improvement improvement of the primaryof antioxidants the primary wasantioxidants higher than was thathigher of thethan secondary that of the antioxidants. secondary antioxidants. Among Among the primary antioxidants,the primary bisantioxidants, [4-(2-phenyl-2-propyl) bis [4-(2-phenyl phenyl]-2-propyl) amine phenyl] (AO-N) amine was (AO the-N) best was the best for improvingfor the improving lightfastness the oflightfastness ROL. Among of ROL. the differentAmong the amounts different of amounts AO-N, the of AO 5 phr-N, the 5 phr addition was theaddition best choices; was the however, best choices 1 phr; however AO-N addition, 1 phr AO already-N addition showed already a significant showed a signifi- improvementcant efficiency. improv Inement addition, efficiency. the drying In addition, time ofthe ROL drying was time extended of ROL and was the extended film and the properties decreasedfilm properties with increasing decreased amountwith increasing of AO-N amount added. of AO However,-N added. among Howeve ther, among the different AO-N-containingdifferent AO- ROLN-containing films, the ROL one films with, 1the phr one showed with 1 superior phr showed film superior properties, film proper- especially regardingties, especial the adhesionly regarding and bending the adhesion resistance, and comparedbending resistance with the, rawcompar ROLed film. with the raw ROL film. Author Contributions: Conceptualization, K.-T.L.; Investigation, K.-T.L. and J.-J.L.; Methodology, K.-T.L. and J.-J.L.;Author Project Contributions: administration, Conceptualization, K.-T.L.; Resources, K.-T.L.; K.-T.L.; Investigat Software,ion, K. J.-J.L.;-T.L. and Supervision, J.-J.L.; Methodology, K.-T.L. and J.-J.L.; Project administration, K.-T.L.; Resources, K.-T.L.; Software, J.-J.L.; Supervision, K.-T.L.; Writing—original draft, J.-J.L.; Writing—review and editing, K.-T.L. Both authors have read K.-T.L.; Writing—original draft, J.-J.L.; Writing—review and editing, K.-T.L. All authors have read and agreed to the published version of the manuscript. and agreed to the published version of the manuscript. Funding: This investigationFunding: This was investigation financially was supported financially by supported the Ministry by the of ScienceMinistry and of Science Technology and Technology (MOST), Taiwan.(MOST), Taiwan. Institutional ReviewInstitutional Board Statement:Review BoardNot Statement: applicable. Not applicable. Informed ConsentInformed Statement: ConsentNot Statement: applicable. Not applicable. Conflicts of Interest:ConflictsThe of authors Interest: declare The authors no conflict declare of interest.no conflict of interest.

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