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Preparation and characterization of ambient-temperature self-crosslinkable water-soluble acrylic resin for PE film ink.

Abstract An ambient-temperature self-crosslinkable acrylic resin was synthesized by solution polymerization, with ethanol as solvent; butyl acrylate (BA), methyl methacrylate (MMA), acrylic acid (AA), hydroxypropyl acrylate (HPA), and diacetone acrylamide (DAAM) as monomers; and adipic acid dihydrazide (ADH) as crosslinker. The resin could be diluted by water. The resin was used for PE film ink, and its curing behavior and film properties were studied. FTIR and DSC results indicated that the reaction between ketone carbonyl and hydrazine groups could occur during film curing at ambient temperature and improved glass transition temperature (7g) of the film by 12.67[degrees]C. Crosslinking density of film increased with DAAM content and m(ADH)/m(DAAM) ratio. Adhesion on PE film increased with DAAM content, while it first increased then decreased with nz(ADH)/m(DAAM) ratio. Water absorption of film decreased with DAAM content, while it first decreased then increased with m(ADH)/m(DAAM) ratio. The optimal m(ADH)/m(DAAM) ratio and DAAM content in this experiment are 0.8:1 and 2%, respectively. Compared with ambient-temperature self-crosslinkable emulsion, this water-soluble resin shows film not only with the same adhesion, but also without any shrinkage void, indicating better film-forming ability on PE film. The ambienttemperature self-crosslinkable water-soluble acrylic resin shows excellent potential application in water-based ink for PE film because of good film-forming ability and adhesion and low water absorption.

Keywords Water-soluble acrylic resin, Ambienttemperature self-crosslinkable, Film-forming ability, Adhesion, PE film

Introduction

With environmental regulations prohibiting volatile organic compounds (VOC), waterborne coatings and ink, including waterborne polyurethane, waterborne polyacrylate, and waterborne epoxy, have received increasing attention and developed rapidly recently. (1-6) Acrylic resin is particularly known for its excellent adhesion, color retention, and water and solvent resistance. However, it is difficult to apply waterborne acrylic resin to PE film ink because of the great difference of polarity and surface tension between resin and PE film, which leads to bad film performances, e.g., weak adhesion and easy shrinkage void.

The problem of weak adhesion could be solved by introducing crosslinkable groups into the resin polymer. Ketone-hydrazide crosslinking reaction which can occur at ambient temperature was often introduced to prepare a low-temperature self-crosslinkable acrylic polymer. Emulsion polymerization is a general approach to introduce ketone-hydrazide crosslinking system into acrylic polymer using diacetone acrylamide (DAAM) as a crosslinkable monomer. (7-11)

In our previous work, (12) a low-temperature selfcrosslinkable acrylic emulsion was synthesized by introducing ketone-hydrazide crosslinking reaction. The emulsion was applied for PE film ink and showed films with low water absorption and good adhesion. Unfortunately, film-forming ability of the emulsion on PE film was obviously affected by the surface tension of PE film. When the PE film was not corona-treated or its surface tension was less than 40 mN/m after corona treatment, there were shrinkage voids on the coating films.

The problem of shrinkage voids is attributed to the dispersion style of resin. In emulsion, emulsion particles dispersed in water, during film formation process, water evaporated first and then followed deformation and syncretization of emulsion particles, which easily leads to shrinkage void. Water-soluble acrylic resin can disperse in water with the help of hydrophilic organic solvents due to its molecules with hydrophilic groups such as carboxyl and hydroxyl. It can be prepared using solution polymerization. (13) Because its molecules stretch in water and solvent, we think that watersoluble acrylic resin could be used for PE film ink with good film-forming ability and none of shrinkage void.

In this paper, a kind of ambient-temperature self-crosslinkable water-soluble acrylic resin was synthesized via solution polymerization, with ethanol as solvent, butyl acrylate (BA), methyl methacrylate (MMA), acrylic acid (AA), hydroxypropyl acrylate (HPA), and diacetone acrylamide (DAAM) as monomers, and adipic acid dihydrazide (ADH) as crosslinker. The procedure for preparation of acrylic resin and film-forming process is shown in Scheme 1. The acrylic resin was characterized by gel permeation chromatography (GPC), Fourier transform infrared (FTIR), and differential scanning calorimeter (DSC) analyses. The acrylic resin was used for main resin of PE film ink, and its film performances were investigated and compared with those of emulsion.

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Experimental

Materials

MMA, BA, and AA were supplied by Guangdong Chengfang Chemical Co. Ltd. (China). HPA, DAAM, and ADH were purchased from Shanghai Aladdin Reagent Co. Ltd. Ammonium water (25%) was purchased from Guangzhou Chemical Co. Ltd. (China). Azobisisobutyronitrile (AIBN) was purchased from Shandong Xiya Chemical Co. Ltd. (China). De-ionized water was self-prepared.

Synthesis of ambient-temperature self-crosslinkable acrylic resin

Ethanol (60 g) was added into a 250-mL four-necked flask equipped with a mechanical stirrer, a thermometer, and a reflux condensing tube and heated to 78[degrees]C. Then a mixture of 20 g MMA, 12 g BA, 2 g HPA, 6 g AA, 0.4 g-1.6 DAAM, and 0.4 g AIBN was added into the flask dropwise for 3 h at 78[degrees]C and kept for another 4 h at 78[degrees]C. After that, the reaction mixture was cooled to about 40[degrees]C, the pH value was adjusted in the range of 7-8 by ammonia, and then 60 g de-ionized water and ADH were added with m(ADH)/m(DAAM) ratio from 0:1 to 1:1. The resulting products were a series of water-soluble acrylic resins with a solid content of about 25% and different ADH/DAAM ratios.

Preparation of ink film

The above-mentioned resin was mixed with aqueous color paste (provided by Foshan Mangrove Waterbased Coating Material Co. Ltd. China) at a mass ratio of 7:3 under a stirring rate of 300 rpm for 15 min, then coated on the surface of PE film (provided by Foshan Mangrove Water-based Coating Material Co. Ltd. China) by an ink proofer (JO, Shanghai Yinze Equipment Co. Ltd. China), and placed at ambient temperature for 1 h or in an oven at 50[degrees]C for 20 min.

Characterization

Fourier transformation infrared (FTIR) spectroscopy (Nicolet 380, US) was performed in the 40(1-4000 cnT1 region. Differential scanning calorimetry (DSC, Netzsch 204F1, Germany) measurements were carried out in the temperature range from -50[degrees]C to 150[degrees]C at [N.sub.2] atmosphere with a heating rate of 10[degrees]C/min. Molecular weight was measured by a gel permeation chromatography (GPC, Agilent 1100 GPC) equipped with a refractive index detector (DAWN HELEOS- II from Wyatt Co. Ltd.) using THF as the mobile phase at a flow rate of 1 mL/min at 30[degrees]C and polystyrene as a relative marker.

The water absorption was performed by immersing a dried resin film (approximately lcm x 1 cm, W0 of weight) into water for 24 h. Then the film was taken out of the water and the water on the surface was removed with filter papers, and then the film was weighed immediately ([W.sub.1]). The water absorption of the film was calculated as follows:

Water absorption (%) = [W.sub.1] - [W.sub.0]/[W.sub.0] x 100.

Three runs were made for each sample and the average value was taken.

The crosslinking density was measured through a typical process. A dried resin film ([W.sub.0] of weight) was placed in a Soxhlet extractor and extracted with methylbenzene under reflux for 7 days. After extraction, linear polymer was dissolved away by methylbenzene, while crosslinking polymer remained and was weighed ([W.sub.1]) after dried for 72 h at 30[degrees]C. The crosslinking density of the film was calculated as follows:

Crosslinking density (%) = [W.sub.1]/[W.sub.0] x 100.

A dried ink film on the PE thin film (approximately 20 cm x 5 cm) was fixed on millimeter grid paper (grid: 1 mm x 1 mm), and a scotch tape (3 M, width of 1.5 cm) was pasted on the ink film tightly. The number of grids covered by tape was recorded as Aq. Then the tape was pulled off quickly with the angle of 180[degrees], and the number of grids covered by the rest of ink film was recorded as A. The adhesion of ink film was calculated as follows:

Adhesion (%) = A/[A.sub.0] x 100.

Three runs were made for each specimen and average was taken. To show the adhesion clearly, we define the adhesion range from grade 0 to grade 5 based on the value of adhesion shown in Table 1 and Fig. 1.

Results and discussion

FTIR spectra analysis

ADH exists stably in the prepared self-crosslinkable water-soluble acrylic resins because of the inhibition of ammonia. With the volatilization of water, ethanol, and ammonia during film formation, the water-soluble polyacrylic resin becomes weakly acidic, and therefore the active carbonyl group can condense with hydrazine.

Figure 2 displays FTIR spectra of films with (a) and without (b) crosslinking reaction in which ADH was not added. Peaks at 3266 [cm.sup.-]1 are hydroxyl group stretching vibration; the peaks at about 2957,1450, and 1389 [cm.sup.-1] are attributed to the stretching vibration of -C[H.sup.3] and -C[H.sup.2]; the peak at 1735 [cm.sup.-1] is stretching vibration of C=0. Compared with curve a, the absorption peak of N-H bending vibration of amide at 1543 [cm.sup.-1] disappeared in curve b, and characteristic absorption peak of C=N stretching vibration at 1655 [cm.sup.-1] appeared in curve a indicates that ketonehydrazide crosslinking reaction has occurred during film formation.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

DSC analysis

Figure 3 shows the DSC curves of films with (a, b) and without (c) crosslinking reaction. Curve a is the first scanning and curve b is the second scanning for crosslinked film. From Fig. 3, we can find that the glass transition temperature ([T.sub.g]) of film with crosslinking reaction (45.42[degrees]C of curve a and 46.93[degrees]C of curve b) is much higher than that of film without crosslinking reaction (32.75[degrees]C of curve c). The increase of [T.sub.g] indicates that ketone-hydrazide crosslinking reaction has occurred during film formation and a crosslinked network structure was obtained. Moreover, the Tg of film with crosslinking reaction of first scanning (45.42[degrees]C of curve a) is very near to that of the second scanning (46.93[degrees]C of curve b), illustrating that ketonehydrazide crosslinking reaction occurred at ambient temperature during film formation.

GPC analysis

Figure 4 shows molecular weight and distribution of acrylic resin. Number average molecular weight ([M.sub.n]) of resin is 37,425 and molecular weight distribution (PD) is 1.4563. The appropriate molecular weight gives acrylic resin good water solubility and film-forming ability.

Influence of DAAM content on the properties of films

The DAAM content in acrylic resin could influence crosslinking density of the film and therefore the properties of the film. Figure 5 shows the crosslinking density and water absorption of the film with different DAAM content at a constant m(ADH)/w(DAAM) ratio of 0.8:1. The crosslinking density of film increased with DAAM content. When DAAM content increased from 0% to 2%, the crosslinking density of film increased rapidly from 0% to 86.7%. When DAAM content further increased from 2% to 4%, the crosslinking density increased slightly, just from 86.7% to 88.6%. The water absorption of film decreased with DAAM content. When DAAM content increased from 0% to 2%, the water absorption decreased rapidly from 37.14% to 10.3%. And when DAAM content further increased from 2% to 4%, it decreased slightly from 10.3% to 9.2%.

[FIGURE 5 OMITTED]

The adhesion of ink film on PE film (38 mN/m) with different DAAM content is shown in Table 2. It can be seen from Table 2 that all the ink films with crosslinking reaction had better adhesion than those without crosslinking reaction, and the adhesion of ink film increased with DAAM content, indicating that ketonehydrazide crosslinking reaction can obviously improve adhesion. When DAAM content increased from 0% to 2%, the adhesion increased rapidly from grade 5 to grade 0. When DAAM content is increased from 2% to 4%, the adhesion remained constant at grade 0. It could be explained that the increase of DAAM content improved the degree of crosslinking reaction and therefore the adhesion. However, the excessive DAAM content could not improve crosslinking density greatly again and therefore the adhesion of ink film on PE film. The optimal DAAM content is 2% in this experiment.

Influence of ADH/DAAM ratio on the properties of films

The ratio of ADH to DAAM can affect the crosslinking reaction between ketone carbonyl and hydrazine groups and therefore the crosslinking density and properties of films. When the DAAM content was set at 2%, a series of films with different m(ADH)/m(DAAM) ratios ranging from 0:1 to 1:1 were prepared. Their water absorption and crosslinking density are shown in Fig. 6, and adhesion on PE film is shown in Table 3.

It can be seen from Fig. 6 that as m(ADH)/m(DAAM) ratio increased from 0 to 0.8:1, the crosslinking density increased from 0% to 86.7% and water absorption decreased from 37.1% to 10.3%, indicating that ketone-hydrazide crosslinking reaction improved the crosslinking density of film and therefore the water resistance. When m(ADH)/m(DAAM) ratio increased from 0.8:1 to 1:1, crosslinking density did not change but water absorption increased. It can be explained that when m(ADH)/m(DAAM) ratio is 0.5:1, ADH could react completely with the quantitative DAAM in theory. However, the -NHN[H.sub.2] groups of dissociative ADH probably could not access -CO-groups of polymer molecules completely during film formation process because of steric hindrance and dispersion, resulting in incomplete crosslinking curing reaction of film. Therefore, a little more ADH makes -NHN[H.sup.2] groups and -CO- groups react entirely. When m(ADH)/m(DAAM) ratio was 0.8:1 in this experiment, the crosslinking density reached maximum and water absorption was minimum. When m(ADH)/m(DAAM) ratio was 1:1, the excessive ADH could not react and exist as free molecules, which caused the increase of water absorption because of its hydrophilicity.

[FIGURE 6 OMITTED]

From Table 3, it can be found that the adhesion of ink film on PE film (38 mN/m) increased with m(ADH)/m(DAAM) ratio when it is increased from 0 to 0.8:1. When m(ADH)/m(DAAM) ratio was 0, there was no ketone-hydrazide crosslinking reaction during film formation process, and adhesion of ink film was the worst at grade 5. When m(ADH)/m(DAAM) ratio was 0.8:1, adhesion of ink film was the best at grade 0, indicating that ketone-hydrazide crosslinking reaction during film formation can obviously improve the adhesion of ink film on the PE film. But as m(ADH)/m(DAAM) ratio increased from 0.8:1 to 1:1, the adhesion decreased from grade 0 to grade 1, which indicated that excessive ADH reduced the adhesion. The optimal m(ADH)/m(DAAM) ratio is 0.8:1 in this experiment.

Influence of surface tension of PE film

The surface tension of PE film also affects film performances. Higher surface tension can result in better film performance. To obtain higher surface tension, PE films are usually corona-treated in advance. Generally, PE films are corona-treated with a surface tension of 38 mN/m in practical application. The adhesion on PE film with different surface tensions was tested, and the result is listed in Table 4.

It can be seen from Table 4 that the adhesion of ink film increased with the surface tension of PE film. When the surface tension of PE film was 31 mN/m (without corona treatment), the adhesion was grade 5. When the surface tension was 38 mN/m, the adhesion was grade 0. Corona treatment can produce some polar groups on the surface of PE film, and these polar groups can improve the surface tension of PE film and enhance the interaction force between PE film and ink film and therefore the adhesion.

Another influence of surface tension of PE film is film-forming ability of resin on PE film. In our previous work, we found that when the surface tension of PE film was less than 40 mN/m after corona treatment, there were shrinkage voids on the coating films of acrylic emulsion. The film-forming ability of watersoluble resin on PE film with different surface tensions is shown in Fig. 7. For comparison, the film-forming ability of acrylic emulsion is also shown in Fig. 7.

From Fig. 7, it can be found that when the surface tension of PE film is 38 and 36 mN/m, there are lots of shrinkage voids on the film of emulsion, while there is no shrinkage void on the film of water-soluble resin on all PE films. The appearance of shrinkage voids indicates weak film-forming ability of acrylic emulsion. That is to say, the film-forming ability of acrylic resin is better than that of acrylic emulsion on PE film. The great difference between acrylic emulsion and watersoluble resin is the dispersion style of resin molecules, which leads to different film-forming processes when drying. In emulsion, emulsion particles dispersed in water, during film formation process, water evaporated first and then followed deformation and syncretization of emulsion particles, which easily leads to shrinkage voids. Water-soluble acrylic resin molecules can disperse and stretch in water with the help of hydrophilic organic solvents due to its molecules with hydrophilic groups such as carboxyl and hydroxyl; during film formation process, the resin molecules can spread out on the PE film as water evaporates, which avoids shrinkage voids.

Curing temperature and time

Ketone-hydrazide crosslinking reaction can occur at low temperature even at ambient temperature. The curing temperature can influence the speed of ketonehydrazide crosslinking reaction. Complete curing and drying at different temperature needs different time durations. The curing time needed at different temperatures in this experiment is shown in Table 5. It can be seen from Table 5 that when curing temperature increased from 30[degrees]C to 60[degrees]C, curing time decreased from 30 min to 5 min. Furthermore, after drying for 30 min at 30[degrees]C, the excellent adhesion performance of ink films on PE film can be obtained, indicating that ketone-hydrazide crosslinking reaction can occur at ambient temperature.

[FIGURE 7 OMITTED]

Conclusions

A kind of ambient-temperature self-crosslinkable acrylic resin was synthesized by solution polymerization, with ethanol as solvent, BA, MMA, AA, HPA, and DAAM as monomers, and ADH as crosslinker. The reaction between ketone carbonyl and hydrazine occurred during film curing process at ambient temperature. Crosslinking density and adhesion of film increased, while water absorption decreased with DAAM content. Crosslinking density increased and water absorption and adhesion of film had an optimum value with m(ADH)/m(DAAM) ratio. The optimal m(ADH)/m(DAAM) ratio and DAAM content in this experiment are 0.8:1 and 2%, respectively. Due to low water absorption and good film-forming ability and adhesion on PE film, the ambient-temperature selfcross-linking water-soluble acrylic resin shows excellent potential application in water-based ink for PE film.

DOI: 10.1007/s11998-015-9742-8

P. Pi ([mail]), X. Chen, X. Wen, S. Xu, J. Cheng

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong. China

e-mail: phpi@scut.edu.cn

Acknowledgments The work has been financially supported by the project of the Natural Science Foundation of China (No. 21376093) and the project of science and technology of Guangdong Province (No. 2012B061700062).

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Table 1: The relationship between adhesion
grade and value of adhesion

Adhesion grade    Value of adhesion (%)

0                        100
1                         95-100
2                         85-95
3                         65-85
4                         35-65
5                          0-35

Table 2: Adhesion of films at different DAAM
content (m(ADH)/m(DAAM) ratio is 0.8:1)

DAAM content/%   Adhesion on PE/grade

0                         5
1                         2
2                         0
3                         0
4                         0

Table 3: Adhesion of films at different
m(ADH)/m(DAAM) ratios (DAAM content is 2%)

m(ADH)/m(DAAM) ratio    Adhesion on PE/grade

0                                5
0.4:1                            2
0.6:1                            1
0.8:1                            0
1:1                              1

Table 4: Adhesion on PE films with different
surface tensions

Surface tension/mN/m       Adhesion/grade

31                               5
34                               3
36                               1
38                               0
40                               0

Table 5: Curing time and adhesion of ink
for PE film at different curing temperatures

Curing                   Curing time/min   Adhesion/grade
temperature/[degrees]C

30                             30                0
40                             21                1
50                             14                0
60                              5                0
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Article Details
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Title Annotation:polyethylene
Author:Pi, Pihui; Chen, Xi; Wen, Xiufang; Xu, Shouping; Cheng, Jiang
Publication:Journal of Coatings Technology and Research
Geographic Code:9CHIN
Date:Jan 1, 2016
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