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Synthesis and Developing Properties of Functional Phenolic Polymers for Eco-Friendly Thermal Papers Kang-Hoon Choi, Hyun-Jin Kwon, and Byeong-Kwan An Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03129 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Industrial & Engineering Chemistry Research

Synthesis and Developing Properties of Functional Phenolic Polymers for Eco-Friendly Thermal Papers Kang-Hoon Choi, Hyun-Jin Kwon, and Byeong-Kwan An* Department of Chemistry, The Catholic University of Korea, 43 Jibong-ro, Wonmi-gu, Bucheon, Gyeonggi-do, 14662, Korea.

*

Corresponding author. E-mail: [email protected]

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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Owing to multiple potential health risks caused by exposure to BPA and the strengthened regulation of BPA-containing thermal papers, much concern has been raised recently regarding a BPA-type color developer in thermal papers. However, few efforts have so far been focused on the development of safer alternative materials to BPA. In this paper, we examined the use of polymeric materials as a safer replacement for BPA that is used in thermal papers. For this objective, a series of phenolic polymers, phenol-formaldehyde novolac and resole (PF-N and PFR), and BPA-formaldehyde novolac and resole (BPAF-N and BPAF-R), were prepared. Among these polymers, a BPAF-N-type polymer was determined as the most appropriate candidate for a developer material in thermal papers. Through printing tests of the prepared thermal papers, it was found that BPAF-N_8 polymer has a similarly high static sensitivity and dark contrast of thermal papers as BPA, whereas the BPAF-N_16 and BPAF-N_30 polymer have a lower static sensitivity than BPFA-N_8 and BPA.

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Introduction Commercial printing applications, such as cash register receipts, tickets, diagnostic sheets, and labels, widely use thermal papers because of their simple and rapid usability, inexpensiveness, and high reliability.1–4 In general, thermal papers comprise a base paper with a thermosensitive layer, which is impregnated with two key molecules, a fluoran-type dye (leuco dye) acting as a thermal dye, and a color developer acting as a proton donor; owing to these chemicals, the color of thermal paper changes upon heating. During the color developing process, the ring of the lactone group of fluoran dyes opens via the protonation from the weak acidic developer, and then the colorless neutral fluoran structure is transformed into the colored cationic fluoran structure which has an extended conjugated double bond system with a charge-transfer character.4–8 Owing to its affordability, efficacy, and low cost, bisphenol A (4,4'-(propane-2,2diyl)diphenol (BPA)) has been intensively used as a developer material.1–4 However, much concern has been recently raised about BPA and its structural analogs as developer materials of thermal papers9–14 because exposure to BPA can potentially cause various detrimental health effects.15–26 Accordingly, in 2016, the European Commission (EU) published a regulation (EU) to restrict BPA in thermal papers to no more than 200 mg/kg after 2020.27 In 2012, the US Environmental Protection Agency (EPA) evaluated 19 chemical alternatives to BPA in thermal papers including bisphenol S (4,4'-sulfonyldiphenol (BPS)) through its Design for the Environment (DfE) program, and the final report in 2014 found that none of these molecules clearly address the health and environmental hazards of BPA.28,

29

The main failure can be

attributed to their low molecular weights as well as the structural similarity to BPA. In particular, 3



these molecules can readily transport through biological membranes via intercellular or transcellular routes owing to their small size. In this respect, polymeric materials can be considered as a suitable alternative because not much molecules with high molecular weights transport through biological membranes.30, 31 However, in fact, the development of polymeric materials to be used for a developer in commercial thermal papers is challenging owing to other crucial requirements such as colorless solid morphology, inexpensive synthesis and good miscibility with thermal dyes. Therefore, not all polymers containing proton donor groups are appropriate candidates for developer materials in thermal papers. In previous work, we prepared hyperbranched polyester copolymers for use as a developer material in thermal papers and investigated their fundamental developing properties in thermal papers.32, 33 Particularly, the effect of the flexibility of the hyperbranched polymer backbone on the thermal sensitivity of thermal papers was examined by varying the proportion of flexible alkyl chains in the hyperbranced polymer backbone.33 However, it was found that these hyperbranched copolymers have limitations with respect to directly replacing a conventional BPA developer owing to their inferior developing properties compared to BPA. In searching for more suitable polymeric developer materials for practical application in commercial thermal papers, we focused on phenolic resin derivatives because of their intriguing advantages such as easy synthesis, low cost, and especially repeating linked phenol groups that are essential for color-forming reactions with leuco dyes. Phenolic resins are broadly divided into two classes, i.e., novolacs and resoles, which are prepared by the reaction of phenol with formaldehyde under acidic and alkaline conditions, respectively.34, 35 In both resins, the phenol units are mainly linked by methylene and/or dimethylene ether groups. Unfortunately, however, 4


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common commercial phenolic resins may not be used directly as developer materials because they are often viscous liquids that appear yellow to dark brown, and this coloration can become intense during storage and processing.35 In this study, for the first time, we investigated the feasibility of using these phenolic resintype materials as developers for thermal papers. To do this, a series of novolac- and resole-based phenolic polymers, phenol-formaldehyde novolac and resole (PF-N and PF-R), and BPAformaldehyde novolac and resole (BPAF-N and BPAF-R) (Scheme 1), were prepared by a simple one-pot polymerization using phenol and BPA with formaldehyde under acid and/or base catalysis. The structural features, thermal and optical properties, and color-generating reactions of the obtained phenolic polymers were carefully examined to determine the most suitable polymers for use as a developer material for thermal papers. The preparation of thermal papers with the selected phenolic polymers in the thermally active layer was tested, and the developing capability of the polymers was also compared with that of BPA. OH

OH

OH

OH

OH

CH 2

CH 2

CH2

OH CH2

n

OH

CH 3

OH

OH

OH

OH

CH 3

H 3C

CH 3

H 3C

CH3

OH

OH CH 2OCH 2

H3 C

CH 3

CH2 OH

m

CH2 H 3C

CH 2OH

Phenol-Formaldehyde Resole (PF-R)

CH2 H 3C

CH 2OCH 2 n

Phenol-Formaldehyde Novolac (PF-N)

OH

OH

H 3C

CH3

H 3C

CH3

CH 2 n

OH

OH

CH 2OH

OH

OH n

BPA-Formaldehyde Resole (BPAF-R)

BPA-Formaldehyde Novolac (BPAF-N)

5


m

OH


Scheme 1. Chemical structures of the novolac- and resole-type phenolic polymers.

Experimental Materials. Phenol, BPA, and formaldehyde were purchased from Aldrich and used as received. 2-anilino-6-dibutylamino-3-methylfluoran (ODB-2) was purchased from ChemPacific and used as received. Polyvinyl alcohol (PVA, Mw = 89,000~98,000, 99+% hydrolyzed) for the binder polymer of the thermal ink solution was purchased from Aldrich and used as received. Methods. 1H-NMR spectra were recorded on an Avance 500 spectrometer (Bruker BioSciences Korea Co. Ltd.). Fourier transform-infrared (FT-IR) spectra were obtained with TENSOR27 (Bruker). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were performed at a heating rate of 10°C/min in nitrogen using a DSC 4000 instrument (Perkin Elmer) and a TGA 4000 analyzer (Perkin Elmer), respectively. UV/Vis absorption spectra were recorded on a Lambda 1050 spectrometer (Perkin Elmer) by using a diffuse reﬂection mode with an integrating sphere. The optical density (OD) of developed images on thermal papers was measured on a X-Rite 504 spectrodensitometer. ODB-2 dyes, BPA and polymers were ground with a mill (Planetary Micro Mill Pulverisette 7, Fritsch). The particle size measurements were carried out with a Zetasizer Nano ZS instrument (Malvern Instruments Ltd.).

Polymerization. Phenol-formaldehyde novolac (PF-N). To a 500 mL two-necked round flask, phenol (10 g, 106.3 mmol), formaldehyde solution (35%, 9.5 mL, 90.4 mmol), and oxalic acid dihydrate (0.05 g, 0.4 mmol) were added and stirred at 90°C for 1 h. The reaction mixture was cooled down to 6


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room temperature and the product was washed with water several times to remove impurities and unreacted phenol and formaldehyde. Finally, water was removed under vacuum, and a light orange solid product was obtained (yield: 14.7 g). 1H-NMR (500 MHz, DMSO-d) δ (ppm) 9.25–9.03 (br, phenolic protons), 7.04–6.47 (br, aromatic protons), 3.83–3.51 (br, methylene bridge protons). FT-IR (ATR, cm-1): 3570–3014 (phenolic–OH, strong, br). BPA-formaldehyde novolac (BPAF-N). A series of BPAF-N polymers were prepared by the following procedure. To a 100 mL one-necked round flask, BPA (5 g, 106.3 mmol), formaldehyde solution (35%, 6.3 mL, 76.7 mmol), and oxalic acid dehydrate (0.05 g, 0.4 mmol) were added and stirred at 100°C for 8 h. The reaction mixture was cooled down to room temperature and the product was washed with water several times to remove impurities and unreacted formaldehyde. Finally, water was removed under vacuum, and a white solid product was obtained. BPAF-N_8: yield: 4.0 g (as a white solid). 1H-NMR (300 MHz, DMSO-d) δ (ppm) 9.30–9.05 (br, phenolic protons), 7.50–6.30 (br, aromatic protons), 3.90–3.66 (br, methylene bridge protons), 1.89–0.63 (br, isopropyl protons). FT-IR (ATR, cm-1): 3672–3050 (phenolic–OH, strong, br). BPAF-N_16: yield: 5.8 g (as a hard gel). FT-IR (ATR, cm-1): 3679–3014 (phenolic–OH, strong, br). BPAF-N_30: yield: 6.4 g (as a hard gel). FT-IR (ATR, cm-1): 3652–3014 (phenolic–OH, strong, br). 7



The numbers in the names of BPAF-N polymers imply the polymerization time. Phenol-formaldehyde resole (PF-R). To a 250 mL one-necked round flask, phenol (5 g, 53.2 mmol), formaldehyde solution (35%, 11.0 mL, 104.5 mmol), and NaOH (0.04 g, 0.9 mmol) were added and stirred at 90°C for 23 h. Thereafter, three drops of phosphoric acid (42.5%) were added, and the resulting mixture was cooled down to room temperature. A solid mass of product was obtained in the flask. This was ground to fine powder and washed with water several times. To a 250 mL one-necked round flask, the powdered product was added and stirred with 150 mL of water at 50°C for 12 h. After cooling, the powdered product was filtered and washed with water. Finally, water was removed under vacuum, and a yellow solid powder was obtained (yield: 2.4 g). 1H-NMR (500 MHz, DMSO-d) δ (ppm) 9.28–9.08 (br, phenolic protons), 8.57–8.25 (br, phenolic protons), 7.44–6.61 (br, aromatic protons), 4.84–4.26 (br, dimethylene ether protons), 4.06–3.71 (br, methylene bridge protons). FT-IR (ATR, cm-1): 3698–3031 (phenolic–OH, strong, br). BPA-formaldehyde resole (BPAF-R). To a 100 mL one-necked round flask, phenol (5 g, 21.9 mmol), formaldehyde solution (35%, 4.5 mL, 54.8 mmol), and NaOH (0.02 g, 0.4 mmol) were added and stirred at 90°C for 12 h. Thereafter, three drops of phosphoric acid (42.5%) were added and the resulting mixture was cooled down to room temperature. A solid mass of product was obtained in the flask. This was ground to fine powder and washed several times with water. To a 250 mL one-necked round flask, the powdered product was added and stirred with 150 mL of water at 50°C for 12 h. After cooling, the powdered product was filtered and washed with water. Finally, water was removed under vacuum, and a white solid product was obtained (yield: 8


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3.0 g). 1H-NMR (500 MHz, DMSO-d) δ (ppm) 9.30–8.98 (br, phenolic protons), 8.55–8.20 (br, phenolic protons), 7.40–6.45 (br, aromatic protons), 4.77–4.26 (br, dimethylene ether protons), 3.98–3.71 (br, methylene bridge protons), 1.86–1.14 (br, isopropyl protons). FT-IR (ATR, cm-1): 3676–3064 (phenolic–OH, strong, br).

Results and discussion Polymerization and polymer structure. The target novolac-type phenolic polymers, PF-N and BPAF-N (Scheme 1), were prepared via the reaction of phenol or BPA with formaldehyde in the presence of oxalic acid as a catalyst. On the other hand, sodium hydroxide was used as a catalyst for preparing the resole-type phenolic polymers, PF-R and BPAF-R (Scheme 1), with the same monomer mixture as that used for novolac-type phenolic polymers. The polymerization was carried out until complete transformation of the solid had occurred. PF-N and PF-R were orange and yellow, respectively, whereas BPAF-N and BPAF-R were obtained as white solids. All these polymers were insoluble in common organic solvents, such as chloroform, tetrahydrofuran, ethanol, and ethyl acetate, but partially soluble in DMSO. However, from the viewpoint of a health hazard, the poor solubility of the polymers is beneficial because such insolubility decreases significantly the bioaccumulation of the polymers through dermal absoprtion.30,31 The structures of the resulting polymers were confirmed by

1

H-NMR and FT-IR

measurements. Figure 1(a) shows the assignment of the 1H-NMR resonances in the obtained phenolic polymers and BPA. Compared with the 1H-NMR spectrum of BPA, the aromatic proton 9



peaks in the polymers appeared as broad multiplets in the region 7.70–6.30 ppm because of the polymerization reactions. Multiplets at around 3.70 ppm were assigned to methylene bridge protons, which are characteristic of all phenolic polymers. For the resole-type polymers, additional new multiplets corresponding to methylol or dibenzyl ether protons appeared at around 4.50 ppm. In all the polymers, most importantly, the peaks of the phenolic protons, which are key functional groups required for the reactions with leuco dyes, were clearly identified between 8.00 and 9.00 ppm. The phenolic groups of the obtained polymers were also confirmed by FT-IR spectroscopy (Figure 1(b)). The peak at around 3700–3100 cm-1 corresponds to phenolic–OH stretching vibration of the polymers.

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(a)

-CH2OH -CH2OCH2- -CH2-

Aromatic -H

Phenolic -H

BPA

PF-N

PF-R

BPAF-N

BPAF-R

11

10

9

8

7

6

5

4

3

ppm

(b) Phenolic -OH BPA

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PF-N

PF-R

BPAF-N

BPAF-R

4000

3000

2000

1000 -1

Wavenumbers (cm )

Figure 1. (a) 1H-NMR (in DMSO-d) and (b) FT-IR spectra of BPA and phenolic polymers (PF-N, PF-R, BPAF-N, and BPAF-R).

In order to obtain quantitative information on the phenolic groups of each polymer, in addition, the hydroxyl values of the obtained polymers were determined by acetylation method using a pyridine/acetic anhydride mixture solution,33 and the results are summarized in Table 1. According to the hydroxyl titration, it was found that the hydroxyl value of PF-R (688.6 mg KOH/g) is higher than that of BPA (660.6 mg KOH/g), but the other polymers have slightly 11



lower hydroxyl values (502.2~551.7 mg KOH/g) than BPA. Thermal properties. The thermal transitions, including glass transition temperatures (Tg) and melting temperatures (Tm), of the polymers were carefully determined by DSC because the thermal morphology of polymers strongly affects the processability for thermal ink solutions and the developing properties of thermal papers.33 In the DSC as shown in Figure 2(a), none of the polymers have a melting point between room temperature and 250°C. This implies that all the polymers can have a solid morphology at the processing temperature during the preparation of thermal papers. Materials that are fluid or viscous at room temperature are generally unsuitable as a developer material for thermal papers owing to viscous-flow problems. Unlike the resoletype phenolic polymers, PF-R and BPAF-R, interestingly, the novolac-type phenolic polymers, PF-N and BPAF-N, had Tgs at 135°C and 124°C, respectively. Additionally, the thermal stabilities of these polymers were examined by the TGA method (in N2 atmosphere). The TGA traces in Figure 2(b) show that the initial temperature for 5% weight loss of all the phenolic polymers is higher than 200°C. Therefore, it is evident that all the polymers are thermally stable at end-use temperatures and printing temperatures ranging from room temperature to 200°C.

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(a)

m.p.

Endo (up)

BPA

PF-N

Tg PF-R

BPAF-N

Tg

BPAF-R

50

100

150

200

250

(b) 100

Weight loss (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

BPA PF-N PF-R BPAF-N BPAF-R

40

20

0 100

200

300

400

500

600

o

Temperature ( C)

Figure 2. (a) DSC and (b) TGA curves of BPA and phenolic polymers. Table 1 Properties of BPA and phenolic polymers. BPA PF-N a 158/ND ND/135 Tm/Tg (°C) 252 363 Td (°C)b Hydroxyl valuec 660.6 502.2

PF-R ND/ND 243 688.6

a)

Not detectable up to 250°C. The degradation temperature for 5% weight loss. c) mg KOH/g (sample).

b)

13


BPAF-N ND/124 269 541.3

BPAF-R ND/ND 318 551.7


Color-forming reaction with a thermal dye. In order to investigate the color-forming ability of the phenolic polymers with a leuco dye, we examined the color changes in mixtures of the polymers and leuco dyes upon heating. All the polymers and BPA were ground to fine particles with a ball mixer because smaller particles are expected to increase the interaction between the developer molecules and the leuco dye, and thus enhance the proton transfer process from the developer to the leuco dye in the eutectic mixture system.4 However, with PF-R, very coarse and large particles were obtained because it is too hard and tough to be ground to a fine powder (see the microscope image of the ground PF-R polymer powder in Figure S1 in the Supporting Information). Samples containing 20 mg of BPA (sample 1), the polymers (samples of PF-N, PFR, BPAF-N, and BPAF-R are numbered 2–5, respectively) as developers, and 10 mg of ODB-2 as the black thermal dye were mixed thoroughly with 100 mg of 1-octadecanol (mp = 56°C~59°C) binder. Additionally, sample 6 (developer-free mixture), containing only ODB-2 in 1-octadecanol, was prepared for comparison. After heating at 160°C for 30 s, the white 1octadecanol solid in the prepared samples completely melted into a transparent liquid. The samples were returned to their solid form when cooling down to room temperature. This cooling process was accompanied by drastic color change in the samples containing developer materials. The photographs in Figure 3 show the color changes in samples 1–6 upon heating. Before thermal treatment, all the samples (1–6) are white (upper photos) but they turned black after heating, except for samples 3 and 6 (bottom photos). The color changes indicate that the ODB-2 molecule undergoes the process of the structural change from the closed-lactone form to the open-lactone form (i.e., the carboxylic acid form) as a consequence of proton release from the developer molecules upon heating, which cause extension of the conjugated double bond and 14


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charge-transfer between the electron donor (–NH–ph) and electron acceptor (=N+) units of the ODB-2 dye molecule (see the structural change of ODB-2 dyes upon protonation in Figure S2 in the Supporting Information), and thus turns black.4, 6–8 However, the ODB-2 dye in sample 6 had no chemical structural change owing to the absence of a developer material. In the case of sample 3, in which a very weak color change was observed, PF-R is unable to transfer protons efficiently from the polymer to the ODB-2 dyes due to the effect of the large particles, as mentioned above.

Figure 3. (Upper photographs (the bottom side of the vial)) Mixtures of octadecanol (binder, 100 mg), leuco dye (ODB-2, 10 mg), and the developer compounds (BPA or polymers, 20 mg) in vials. Samples 1– 5 contain BPA, PF-N, PF-R, BPAF-N, and BPAF-R as developer materials, respectively. In sample 6, there was no developer material. (Bottom photographs) Molten mixtures of the upper samples after heat treatment at 160°C for 30 s.

The open-lactone form in the ODB-2 dye in the molten samples was also confirmed by monitoring the lactone stretching band of ODB-2 by FT-IR spectroscopy (Figure 4(a)). The intense C=O stretching band of lactone group at 1750 cm-1 disappears or decreases in samples 1, 2, 4, and 5 after heating due to ring opening reaction of ODB-2 dyes by protonation. However, the C=O stretching band of lactone ring is still present in samples 3 and 6 even after heating. The color-forming reaction capability of the polymers with ODB-2 dyes upon heating was further investigated quantitatively by measuring OD values of the molten mixed samples (Figure 4(b)). It was found that the ODs of samples 4 and 5 are higher than those of samples 2 and 3, indicating that the BPA-based phenolic polymers, BPAF-N and BPAF-R, induce a better color-forming 15



reaction with an ODB-2 dye than the phenol-based phenolic polymers, PF-N and PF-R.

(a) Transmittance (a.u.)

BPA (1) PF-N (2) PF-R (3) BPAF-N (4) BPAF-R (5) No developer (6)

C=O stretching of lactone ring (ODB-2)

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) 3.0

(b)

2.5

0.0

NO Developer (6)

BPAF-R (5)

0.5

BPAF-N (4)

1.0

PF-R (3)

1.5

PF-N (2)

2.0

BPA (1)

Optical Density (O.D.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 4. (a) FT-IR spectra and (b) ODs of samples 1–6 in Figure 3 after heating.

Polymer selection for printing tests of thermal papers. Based on the characterization of the obtained polymers, including powder colors, thermal properties, and color-forming reaction capabilities, we chose BPAF-N as the best candidate polymer to be applied as a developer material for thermal papers. Moreover, to investigate the effect of the degree of polymerization of BPAF-N on the developing capability of thermal papers, two additional BPAF-N polymers, 16


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i.e., BPAF-N_16 and BPAF-N_30, were prepared by increasing the polymerization time. The numbers in the names of the BPAF-N polymers imply the polymerization time. Unlike the initial BPAF-N polymer (BPAF-N_8), BPAF-N_16 and BPAF-N_30 were obtained as hard gels. This is probably due to inevitable crosslinking that occurs through the addition-condensation of two phenolic units of BPA with formaldehyde.34, 35 As shown in the DSC measurement of BPAF-N_8 (Figure 2(a)), the newly obtained BPAF-N_16 and BPAF-N_30 also had no melting points around 158°C (Figure S3 in the Supporting Information), indicating that all the BPA monomer was consumed in the polymerization. Notably, however, the Tg of 124°C for the BPAF-N_8 polymer (Figure 2(a)) was not exhibited by the BPAF-N_16 and BPAF-N_30 polymers as a result of the crosslinking reaction. In the color-forming reaction tests of the BPAF-N series polymers in the eutectic mixture system, it was revealed that all the BPAF-N polymer samples (2–4) have similar ODs (2.07~2.30) to the BPA sample (1) (OD = 2.41) upon heating (Figure 5(a) and inset). This result confirms that all the prepared BPAF-N polymers have color-forming reaction capabilities comparable to BPA. Furthermore, according to the dependence of the OD of the molten mixture on the ratio of BPAFN polymers to ODB-2 dye (w/w) (Figure 5(b)), the 1:1 ratio of BPAF-N polymers to ODB-2 dyes (w/w) was found to be suitable to achieve a full color-forming reaction with ODB-2 dyes.

17



(a)

0 3.0

NO Developer (5)

BPAF-N_30 (4)

BPAF-N_8 (2)

1

BPAF-N_16 (3)

2

BPA (1)


3

(b)

2.5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0

1.5

1.0

BPA BPAF-N_8 BPAF-N_16 BPAF-N_30

0.5

0.0 0.0

0.5

1.0

1.5

2.0

The ratio of developer to ODB-2 (w/w)

Figure 5. (a) ODs of molten mixtures of 1-octadecanol (binder, 100 mg), leuco dye (ODB-2, 10 mg), and the developer compounds (20 mg) in vials. Samples 1–4 contain BPA, BPAF-N_8, BPAF-N_16, and BPAF-N_30 as developer materials, respectively. Sample 5 contains no developer material. (Inset photographs) Mixed samples 1–5 before (upper) and after (bottom) heat treatment at 160°C for 30 s. (b) OD changes of molten mixed samples 1–4 depending on the ratio of BPAF-N polymers to ODB-2 dye (w/w).

Developing capability and static sensitivity of the thermal papers. To examine their use as a developer material for thermal papers, the BPAF-N series polymers were ground by ball milling for 3 hours. All the polymers had a uniform particle size of ca. 1.2~1.4 µm (see Table S1 in the 18


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Supporting Information). BPAF-N series polymer-based thermal papers were successfully prepared by bar coating method using thermal ink solutions containing ODB-2 and the polymers (1:2 (w/w)); the preparation of the thermal inks and thermal papers are described in detail in the Supporting Information. In the thermosensitive layers of the resulting thermal papers, ground particles of ODB-2 dye and BPAF-N polymers were coated with PVA matrix. BPA and developer-free thermal papers were also prepared for comparison. In order to evaluate the performance of the prepared thermal papers, firstly we checked whether a black image developed on them upon heating. As anticipated, the BPAF-N thermal papers produced a black image upon heating (180°C) by using a printer with a rectangular thermal head (Figure 6(a) inset). The developed black image on the BPAF-N thermal papers was further characterized by UV/vis diffuse reflectance spectroscopy (Figure 6(a)). It was shown that the BPAF-N thermal papers exhibit strong absorption bands at around 450 and 590 nm, which are very weak before thermal printing. This drastic change is mainly attributed to the structural transformation of the ODB-2 dye molecule from the closed-lactone form to the open-lactone form via proton transfer from a phenolic unit in the polymer. It was also found that the image contrast of the thermal papers depends on the developer material. The contrast values of the developed images were quantitatively measured by two different methods: (i) integrating the absorption spectrum in the region 400–800 nm (σa400–800) and (ii) measuring the OD with a spectrodensitometer (Figure 6(b) and Table S2 in the Supporting Information). The BPA thermal paper exhibited the highest σa400–800 and OD of 376.6 and 1.37, respectively, whereas the σa400–800 and OD for the developer-free thermal paper were only 18.4 and 0.08, respectively. Among the thermal papers using BPAF-N polymers, the BPAF19



N_8 thermal paper had the highest σa400–800 and OD of 346.4 and 1.35, respectively. The higher contrast of the BPAF-N_8 thermal paper can be attributed to the softening of the BPAF-N_8 developer in response to the applied thermal energy. It is generally believed that the melting point of the developer material profoundly affects the thermal sensitivity of a thermal paper since the homogeneous blending of the leuco dye and the developer molecules upon melting can provide optimal conditions for the proton transfer from developer molecules to leuco dye molecules to trigger the color-forming reaction.4 In this sense, BPAF-N_8 is potentially a better and more effective developer material than BPAF-N_16 and BPAF-N_30 because BPAF-N_8 has a Tg of around 124°C and thus can be softened at printing temperatures, but BPAF-N_16 and BPAF-N_30 are infusible at printing temperatures owing to their cross-linked structures. It is also noteworthy that the OD (1.35) of BPAF-N_8 thermal paper was higher than not only that of our previous hyperbranched polyester-based thermal paper (1.00)33 but also the value required for commercialization (1.10).36 In addition, it was also found that the initial ODs of printed images of all the thermal papers are little changed, although the storage temperature is increased to 100°C (Figure S4 in the Supporting Information).

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1.8

Absorbance (Reflectance)

1.6

(a) 1

2

3

4

BPA (1) BPAF-N_8 (2) BPAF-N_16 (3) BPAF-N_30 (4) No developer (5)

5

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm) 400

(b)

σa400-800

1.8

OD 1.5

300

0.9

200

OD

1.2

400-800

σa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6 100 0.3

0

0.0 BPA

BPAF-N_8

BPAF-N_16 BPAF-N_30 No developer

Figure 6. (a) UV/Vis absorption spectra and (b) σa400–800 and ODs of the developed black images of the thermal papers coated with ODB-2 dyes and developers (BPA (1), BPAF-N_8 (2), BPAF-N_16 (3), BPAF-N_30 (4), and no developer (5)). Inset photos in (a) show the developed images of thermal papers obtained by using a thermal printer with a rectangular thermal head upon heating (180°C).

In order to practically apply BPAF-N polymers as a developer in thermal papers, the static sensitivity of the BPAF-N thermal papers must be investigated because the static sensitivity can provide not only the temperature at which thermal paper begins imaging, but also the maximum resistance temperature of the thermal papers before they blacken, which is quite important when 21



selecting thermal papers for a specific application. The static sensitivity of the BPAF-N thermal papers was measured by determining the ODs of the generated image by varying the temperature of (rectangle-shaped) thermal printing head. Figure 7 shows the static sensitivity for BPA, BPAFN_8, BPAF-N_16, and BPAF-N_30 thermal papers. It was found that a black image begins to develop at around 100°C in BPA and BPAF-N_8 thermal papers, whereas BPAF-N_16 and BPAF-N_30 thermal papers begin to blacken at much higher temperatures (above 160°C), indicating that BPA and BPAF-N_8 thermal papers have a higher static sensitivity than BPAFN_16 and BPAF-N_30 thermal papers. Considering that BPAF-N_8 thermal paper has a similarly high static sensitivity and developing OD to BPA thermal paper, it appears that BPAFN_8 polymer is potentially a suitable replacement for BPA developer materials that can be used for general-purpose thermal papers. In contrast, BPAF-N_16 and BPAF-N_30 thermal papers have a lower static sensitivity, i.e., these polymers are more resistant to degradation at high temperatures than BPA and BPAF-N_8 thermal papers. Therefore, BPAF-N_16 and BPAF-N_30 polymers are more appropriate as developer materials for use in thermal papers for applications that require recorded images to be thermally stable.

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(a) No heat

100oC

120oC 140oC 160oC 180oC

BPA BPAF-N_8 BPAF-N_16 BPAF-N_30 1.6

(b)

1.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BPA BPAF-N_8 BPAF-N_16 BPAF-N_30

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

20

40

60

80

100

120

140

160

180

200

o

Temperature ( C)

Figure 7. (a) Photographs of developed images of thermal papers coated with BPA, BPAF-N_8, BPAFN_16, and BPAF-N_30 for various printing temperatures. (b) Static sensitivity curves of the thermal papers.

Conclusions A series of phenolic polymers, phenol-formaldehyde novolac and resole (PF-N and PF-R), and BPA-formaldehyde novolac and resole (BPAF-N and BPAF-R) were prepared to investigate their feasibility as developer materials for thermal papers. Among these polymers, a BPAF-N-type 23



polymer was determined as the most appropriate candidate for a developer material in thermal papers by examining its solid-state color properties, thermal properties, and color-forming reaction capabilities. Through printing tests of the prepared thermal papers, it was found that BPAF-N_8 polymer, with a Tg of around 124°C, has a similarly high static sensitivity and dark contrast of thermal papers a BPA. On the other hand, the cross-linked BPAF-N polymers (BPAFN_16 and BPAF-N_30) were found to have a lower static sensitivity than BPFA-N_8 and BPA and were thus suitable for use as a developer material in thermal papers that needed to be used or stored in hot environments. This successful demonstration of BPAF-N polymers as a developer material for thermal papers opens a new pathway for developing polymer-type developer materials, not only to replace a problematic BPA developer material but also to improve the heat resistance of thermal papers.

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Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A1A2059180). This work was also supported by the Catholic University of Korea, Research Fund, 2016 (M2016-B0002-00113).

Supporting Information The image of the ground PF-R polymer powder, structural changes of ODB-2 dyes upon protonation, DSC curves and ground particle sizes of BPAF-N series polymers, and quantitative information and stability of the developed images of BPAF-N thermal papers have been provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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