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Synthesis and characterization of multiple-crosslinkable polyacrylate emulsion for PVC film ink
Progress in Organic Coatings 137 (xxxx) xxxx
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Synthesis and characterization of multiple-crosslinkable polyacrylate emulsion for PVC film ink Gan Gaoa, Guanzhou Luoa, Mengyi Xub,*, Shouping Xua, Pi Pihuia,*, Xiufang Wena,* a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China School of Chemical Engineering and Technology, Guangdong Industry Polytechnic, Guangzhou, 510300, China
ARTICLE INFO
ABSTRACT
Keywords: Multiple-crosslinkable polyacrylate emulsion Hard core Soft shell Ink film PVC film
Multiple-crosslinkable polyacrylate emulsion (MCPE) with high content of crosslinkable monomers was successfully synthesized via core-shell emulsion polymerization. Specially, cross-linkable monomers MAA and NMA were designed as the core layer monomers, vinyl triethoxysilane (A151) and epoxy resin (BC2060) were designed as the shell layer monomers, and aziridine (XR-100) was added as crosslinker. The hard core can be benefit for rapid drying of the film at room temperature, the soft shell can exhibit good film-forming ability. Epoxy resin was grafted on the chain of polyacrylate, which can endow excellent adhesion performance. The stability of MCPE was excellent and the core-shell structures of MCPE particles were maintained after stored for 15days at 50 °C, because those different crosslinkable monomers were introduced into different layers of the emulsion particles. The crosslinking density, tensile strength and glass transition temperature (Tg) of MCPE films were obviously higher than those of pure polyacrylate emulsion films due to the modification of several crosslinkable monomers. The MCPE ink film on PVC film has rapid surface drying performance, excellent adhesion and alcohol scrubbing resistance after multiple cross-linking at room temperature. All these advantages of this emulsion may contribute to its wide applications in the field of PVC film packaging and decoration.
1. Introduction PVC film is a very promising plastic material for packaging and decoration due to relatively low cost, high mechanical strength, high-low temperature resistance, aging resistance and corrosion resistance. However, when PVC film is printed with environment friendly waterbased ink, bad ink film performances, such as weak adhesion, easy shrinkage void and wetting problems [1], often occur due to surface migration of low molecular weight plasticizers with low surface energy [2,3]. The lower polarity segments are often introduced into polyacrylate molecular chains to improve the properties of emulsion film on low surface energy materials. Li et al. [4] reported poly (acrylates-co-urethane) with lower polarity soft segment of urethane oligomer, which showed excellent adhesion to low surface energy materials. Schellekens et al. [5] used a selected controlled radical polymerization technique to obtain water-dispersible iBOA-based block copolymers for adhesion promotion of waterborne acrylics on untreated PP film. Hu et al. [6] prepared chlorinated polypropylene waterborne emulsion grafted by polyacrylate. The film had excellent adhesion to BOPP film and printing quality due to the different movement trend of the polar and un-polar chains during the baking step.
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Introducing crosslinkable functional groups into acrylate polymer is another solution to improve the performances of water-based ink on plastic film. Some crosslinkable monomers, such as methacrylate glycidyl methacrylate (GMA) [7], hydroxyethyl methacrylate (HEMA) [8], N-methylol acrylamide (NMA) [9], epoxy-amine [10] and hydroxylacid [11], are often used to copolymerize with acrylate monomers to synthesize self-crosslinkable acrylate polymer. Pi et al. [12] prepared a low-temperature self-crosslinkable polyacrylic emulsion by introducing ketone-hydrazide crosslinking groups into polyacrylic polymer. The ink film of the emulsion had low water absorption and good adhesion on PE thin film. However, these self-crosslinkable acrylic emulsions are easy to disintegrate in the alcohol condition and their films have low selfcrosslinked density, resulting in poor resistance to alcohol scrubbing. Slow surface drying rate at room temperature is also a serious drawback to limit their use in fast printing technology. To our best knowledge, there are less reports about synthesis of low-temperature self-crosslinkable emulsion with rapid surface drying performance, excellent adhesion and alcohol scrubbing resistance for PVC film ink. Epoxy (EP) resin with excellent adhesive property, water resistance, impermeability and chemical resistance [13,14], and polysiloxanes with low-surface energy, high flexibility and low-glass transition
Corresponding authors. E-mail addresses: [email protected] (M. Xu), [email protected] (P. Pi), [email protected] (X. Wen).
https://doi.org/10.1016/j.porgcoat.2019.06.036 Received 5 April 2019; Received in revised form 13 June 2019; Accepted 21 June 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Gan Gao, et al., Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.06.036
Progress in Organic Coatings 137 (xxxx) xxxx
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Scheme 1. Synthesis and curing process of MCPE.
temperature [15–17] were often introduced in polyacrylate molecular chains to increase the crosslinking density and improve performance of polymer film. In this study, a multiple-crosslinkable polyacrylate emulsion (MCPE) for PVC film ink was prepared by core-shell emulsion polymerization and followed by adding crosslinking agent XR-100 (Scheme 1). Specially, cross-linkable monomers MAA and NMA were used as the core layer monomers, vinyl triethoxysilane (A151) and epoxy emulsion (BC2060) were used as the shell layer monomers. The emulsion was designed with a hard core/soft shell structure, which can benefit for rapid drying of the film at room temperature and good filmforming ability. Epoxy resin was grafted on the chain of polyacrylate to endow excellent adhesion performance. Different crosslinkable monomers were introduced into different layers of emulsion particles, which not only increased the crosslinking density of the ink film, but also improved the stability of emulsion. The ink film has excellent alcohol scrubbing resistance after multiple cross-linking at room temperature. This novel MCPE exhibits significant potential application in waterbased ink for PVC film.
equipped with a mechanical stirrer (fitted with a crescent Teflon blade), a temperature control system, an inlet for feeding pre-emulsion, another inlet for feeding initiator solution and a reflux condenser. A mixture of 0.75 g SDS, 0.75 g OP-10, 18 g distilled water, 4 g MAA, 4 g NMA and 30 g MMA were placed in a graduated beaker and stirred with a magnetic stirrer to get a core pre-emulsion. A shell pre-emulsion containing 1.0 g SDS, 1.0 g OP-10, 27 g distilled water, 4 g A151, 5 g BC2060, 30 g MMA and 30 g BA was prepared by the same method. The flask was charged with 1.0 g SDS, 1.0 g OP-10, 0.1 g NaHCO3 and 60 g distilled water and was heated to 80 °C, then the core pre-emulsion and a initiator solution (0.2 g of APS dissolved in 10 ml water) were dropwise added in 30 min. After the addition was completed, the shell preemulsion and another initiator solution (0.4 g of APS dissolved in 20 ml water) were dropwise added in 150 min. The reaction was maintained at 85 °C for another150 min after the addition finished. The mixture in the flask was cooled to around 40 °C, and ammonia was added to adjust its pH value to the range of 7–8. Finally, 10 g XR-100 was added to the flask and stirred for 15 min to obtain MCPE (P5 emulsion). For comparison, P0 emulsion (pure polyacrylate emulsion), P1 emulsion (without adding NMA, A151, BC2060 and XR-100), P2 emulsion (without adding A151, BC2060 and XR-100), P3 emulsion (without adding BC2060 and XR-100), P4 emulsion (without adding XR-100) and P-core emulsion (the core emulsion of P5) were prepared respectively using the same approach. The feeding composition of monomers in various emulsions is shown in Table 1. These emulsions have excellent stability in the synthesis process and no gel is produced.
2. Experimental 2.1. Materials Methyl methacrylate (MMA), butyl acrylate (BA), methyl acrylic acid (MAA), N-methylol acrylamide (NMA), nonyl phenyl, vinyl triethoxysilane (A151), polyoxyethylene ether-10 (OP-10), sodium lauryl sulfate (SDS) and sodium bicarbonate (NaHCO3) were obtained from Tianjin Damao Chemical Reagent Co., Ltd. (Tianjin, China). Bisphenol A Epoxy (EP) resin BC2060 (EP value of 0.48–0.54) and crosslinker (aziridine XR100) were purchased from Guangzhou Zono Trading Co.Ltd. (Guangzhou, China). Ammonium persulfate (APS) and ammonia water were the products of Shenzhen Jitian Chemical Reagent Co., Ltd. (Shenzhen, China). Aqueous color paste was obtained from Guangzhou Keytec Colours Co., Ltd (Guangzhou, China). All the materials were used without further purification and deionized water was prepared in our lab.
2.3. Preparation of ink films The preparation process of ink films is shown in Fig. 1. MCPE was mixed with aqueous color paste at a mass ratio of 4:1 under stirring with a rate of 200 rpm for 30 min, then coated on the surface of PVC film by a winding bar and placed at room temperature. The alcohol scrubbing resistance of ink films was detected after 7days. The curing process of MCPE film is shown in Scheme 1, the multiple cross-linked networks can be formed by condensation reaction between carboxyl groups of MAA with amino of NMA, epoxy groups of BC2060 or aziridine ring of XR-100, and hydrolysis-condensation reaction between A151 and hydroxyl of NMA.
2.2. Synthesis of MCPE The preparation procedure of MCPE is schematically illustrated in Scheme 1. Polymerization was performed in a 250 mL, 4-neck flask 2
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The adhesion ratio was measured as following process. A dried ink film on PVC film (approximately 10 cm × 5 cm) was fixed on millimeter grid paper (grid: 1 mm × 1 mm), and a scotch tape (3 M, width of 1.5 cm) was pasted onto the ink film tightly. The number of grids covered by tape was recorded as A0. Then the tape was pulled off transiently and quickly with the angle of 180°, and the number of grids covered by deciduous ink film was recorded as A. The adhesion ratio of ink films was calculated as follows:
Table 1 The feeding composition of monomers in various emulsions. Components
P0
P1
P2
P3
P4
P5
P-core
MMA/g BA/g MAA/g NMA/g A151/g BC2060/g XR-100/g
60 30 – – – – –
60 30 4 – – – –
60 30 4 4 – – –
60 30 4 4 4 – –
60 30 4 4 4 5 –
60 30 4 4 4 5 10
60 – 4 4 – – –
Adhesion ratio (%) = (A0 – A)/A0 × 100 The alcohol scrubbing resistance was measured as following process. A 1000 g weight was covered with absorbent cotton and soaked by 70% ethanol aqueous solution. The soaked cotton was squeezed until there was no droplet, and applied and dragged back and forth onto an ink film (120 mm × 25 mm) with horizontal force. Each forward and backward scrub was a reciprocating scrub, and one reciprocating scrub was 1 s. According to this method, the ink film was reciprocally wiped until the PVC film was exposed, and the number of alcohol scrubbing was recorded. Surface drying test of ink film on PVC film was carried out followed the Chinese national standard of GB/T9278-2008 protocol. The ink film was dried at room temperature, and the surface was touched with finger at appropriate time intervals. The surface of the ink film was considered dry when no ink was sticked to the finger. Recording the time for the ink film reached the surface drying. For all performance test, three runs were made for each sample and average values were taken.
2.4. Characterization Fourier transform infrared (FT-IR) spectra of samples were performed on a NEXUS-380 spectrometer (Nicolet, USA) in the range from 400 cm−1 to 4000 cm−1 with a resolution of 6 cm−1 and 32scans. Emulsion polymer samples were obtained by emulsion demulsification, freeze and drying firstly, then it was frozen in liquid nitrogen and grinded into powder. Emulsion film on flat glasses was obtained by drying for 7days at room temperature. Dynamic light scattering (DSL) (ZS Nano S dynamic light scattering particle size analyzer, Malvern, UK) was used to measure particle size and size distribution. JEM-2100 F TEM (JEOL, Japan) transmission electron microscopy (TEM) was operated at 200 kV to characterize the dimensions and morphology of the sample. A drop of diluted dispersion was dropped on the hydrophilic carbon-coated copper grid. The samples were stained with 1.5% phosphotungstic acid solution. After 3 min, excessive phosphotungstic acid solution on grids was cleaned with distilled water [18–20]. Glass transition temperature (Tg) of the cured films were measured by differential scanning calorimeter (DSC) (Q20, TA instruments, USA) at a heating rate of 10 °C/min from −20 °C to 150 °C. The tensile experiments were carried out on a tensile tester (WDW3100, Changchun Kexin Co., China) at a cross-head speed of 10 mm/min at room temperature according to ASTM-D638. The dimensions of the tensile specimens were 10 mm × 4 mm in the working section. At least three specimens of each sample were tested. After the tensile tests, the broken samples were collected, and the morphologies and elemental analyses of the fracture surfaces were characterized by a field emission scanning electron microscope (SEM, SU-70, Hitachi, Japan) at an acceleration voltage of 30 kV. The crosslinking density was measured through a typical process. A dried emulsion film was placed in a Soxhlet extractor and extracted with methylbenzene under reflux for 7days. After extraction, linear polymer was dissolved away by methylbenzene, while crosslinked polymer was remained and weighed after dried (the weight of samples was constant). The crosslinking density of the films was calculated as follows:
3. Results and discussion 3.1. FTIR analysis FTIR spectra of the P0 emulsion after demulsification (curve a), P4 emulsion after demulsification (curve b) and P5 film (curve c) are shown in Fig. 2. No peak is observed at 1650 cm−1 in curve a, b and c, which indicates the C]C groups of monomer polymerize completely. In curve a, the peaks at 2957cm−1 and 2874 cm−1 are the stretching vibration of eCH2e and eCH3 groups, respectively, and the peak at 1453 cm−1 is the bending vibration peak of CeH. The peak at 1733 cm−1 is ascribed to the stretching vibration of C]O groups. The peaks at 1242cm−1 and 1126 cm−1 are attributed to the stretching vibration of CeOeC groups. So it can be deduced that the emulsion polymer is the random copolymer of acrylate monomers [12,21]. Comparing curve b with curve a, the peak at 3442 cm−1 is contributed to the NeH groups of NMA [21]. The peak at 1070 cm−1 is ascribed to the stretching vibration of Si–O–Si [19]. The important characteristic peak of Si-C at 1260 cm−1 is not observed because the peak of CeOeC groups at 1242 cm−1 is so wide and covers the characteristic peak of SiC at 1260 cm−1 of A151. The peaks at 1596 cm−1 and 1510 cm−1 are ascribed to the stretching vibration of para-substituted benzene rings of BC2060, the peaks at 3543 cm−1 and 943 cm−1 are ascribed to the
Crosslinking density (%) = W1/ W0×100% W0 was the weight of the dried emulsion film before extraction; W1 was the weight of the emulsion film after extraction.
Fig. 1. Preparation process of ink film. 3
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Table 2 Particle size and size distribution of various emulsions. Subject
P0
P1
P2
P3
P4
P5
P-core
Particle size/nm PDI
113.5 0.037
125.3 0.053
134.9 0.134
168.3 0.042
186.0 0.042
184.7 0.166
126.1 0.017
have a clear core-shell structure (Fig. 4b), which illustrates that the storage stability of MCPE is excellent. These phenomena can be explained that the crosslinkable monomers are evenly distributed in the core and shell layer respectively, which reduces the probability of crosslink reaction in storage process and improves the stability of MCPE. Fig. 5 shows the crosslinking density, tensile strength and elongation at break of different emulsion films. The tensile strength of the films of P0, P1, P2, P3, P4 and P5 are 7.99 MPa, 13.14 MPa, 15.00 MPa, 17.03 MPa, 19.27 MPa and 23.40 MPa, and their elongations at break are 269%, 186%, 163%, 112%, 69% and 30%, respectively. The crosslinking density of the films increases from 0% (P0) to 93% (P5). The increased reaction active sites make the multiple network structure easier to form and the network density increases when crosslinkable monomers MAA, NMA, A151 and BC2060 are gradually introduced into emulsion polymers, resulting in an improvement in tensile strengths and a reduction in the elongation at break. The mechanism of the increased tensile strength is studied from the morphology of fracture surfaces [25], and the SEM images of the fractures surfaces of samples are shown in Fig. 6. The cured P0 emulsion film is very smooth and river-like (Fig. 6a), indicating that pure emulsion film is very brittle, as typical thermoplastic polymers [25]. For samples P1, P2, P3, P4 and P5, the fracture surfaces become rougher and rougher with the increase of crosslinking density. It suggests that multiple cross-linked network structure and high crosslinking density is formed in MPCE film. Fig. 7 shows the DSC analysis of sample P0 (a), P1 (b), P2 (c), P3 (d), P4 (e), P5 (f) and P-Core (g) emulsion films. Tg of the P0, P1, P2, P3, P4, P5 and P-Core emulsion films are 12.5 °C, 17.2 °C, 19.9 °C, 21.2 °C, 25.7 °C, 38.6 °C and 77.5 °C, respectively. The crosslinked network in the polymer backbone can restrict the movement of polymer chains, the higher cross-linking density of films gets, the higher Tg is [12,26]. Tg of P-Core emulsion film is the highest in our research because it is composed only by hard monomers of NMA, MAA and MMA. TEM analysis and Tg of P-Core emulsion film indicate that the MCPE particles have the hard core-soft shell structures.
Fig. 2. FTIR spectra of the P0 emulsion (a), P4 emulsion (b) and MCPE film (c).
stretching vibration of −OH and terminal EP groups of BC2060 [22,23]. These results imply that NMA, A151 and BC2060 are polymerized in the P4 emulsion. Compared with curve b, the characteristic peak for EP at 943 cm−1 disappeared in curve c because of crosslinking between EP groups and aziridine [24] ring in XR-100 during film formation. The FTIR results suggest that P5 is MCPE composed by MAA, NMA, A-151, BC2060, and XR-100. 3.2. Emulsion performance DLS parameters of various emulsions are illustrated in Fig. 3. Particle size and size distribution of various emulsions are shown in Table 2. It can be seen from Fig. 3 and Table 2 that the particle size of Pcore emulsion and P1 is 126.1 nm and 125.3, respectively. The particle size of the emulsions increases with the introduction of various crosslinked monomers, for the P5 emulsion, it is up to 184.7 nm due to the coalescence of emulsion particles. The TEM micrographs of MCPE particles before (a) and after (b) 15days of storage at 50 °C are illustrated in Fig. 4. The appearance of MCPE is no visible change, no precipitation and flocculation are found after 15days of storage at 50 °C (Fig. 4d), and its emulsion particles still
3.3. Performance of emulsion ink films on PVC film Adhesion ratio and alcohol scrub numbers of different emulsion ink films on PVC film are shown in Fig. 8. The adhesion ratio of P0, P1, P2, P3, P4 and P5 emulsion ink films on PVC film are 0%, 30%, 60%, 85%, 100% and 100%, respectively. It increases as crosslinkable monomers gradually introduces, which is because the carboxyl groups, hydroxymethyl and epoxy groups in MAA, NMA and BC2060 can interact with the functional groups on the surface of substrate on one hand, and one the other hand, the surface tension of the ink film decreases with A151 introduced. They are all benefit to improve the adhesion of ink film on PVC film, and the higher content of crosslinkable monomers is, the higher adhension ratio of ink film is. It also can be found from Fig. 8 that the number of alcohol scrubbing of P0, P1, P2, P3, P4 and P5 ink films on PVC film are 0, 0, 12, 20, 35 and 50, respectively. The pictures of emulsion film before alcohol scrubbing of P0, P1, P2, P3, P4 and P5, and after alcohol scrubbing of P0 (1 scrub), P1 (1 scrub), P2 (13 scrubs), P3 (21 scrubs), P4 (36 scrubs) and P5 (51 scrubs) are shown in Fig. 9. It can be seen from Fig. 9 that the performance of alcohol resistance from P0 to P5 are gradually better. It indicates that the increase of cross-linking density of the film
Fig. 3. DLS measurement on the size distribution of various emulsions. 4
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Fig. 4. The TEM micrographs of the MCPE particles before (a) and after (b) 15days of storage at 50 °C.
Fig. 5. Crosslinking density, tensile strength and elongation at break of P0, P1, P2, P3, P4 and P5 emulsion films.
Fig. 6. SEM morphologies of the fracture surface of the cured films for P0 (a), P1 (b), P2 (c), P3 (d), P4 (e) and P5 (f). 5
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Fig. 7. DSC curves of the films for P0 (a), P1 (b), P2 (c), P3 (d), P4 (e), P5 (f) and P-Core (g) emulsion.
Fig. 10. Effect of BA distribution ratio within core and shell on the surface drying time of the ink films.
the effect of BA distribution ratio within core and shell on the surface drying time of the ink films. It can be seen from Fig. 10 that when all of BA monomers are distributed within the shell of MCPE particles, the surface drying time of the ink film is the shortest. Conversely, the ink film with all of BA monomers within the core takes the longest time to reach the surface drying. This can be explained that when BA monomers are completely polymerized in the shell part, the MCPE particles form hard core-soft shell structures. It is benefit to decrease the MFT of the emulsion [27] and shorten the surface drying time of the ink film. When BA monomers are completely polymerized within the core part, the MCPE particles possess soft core-hard shell structures, which raise the MFT of the MCPE and increase the surface drying time of the ink film. 4. Conclusion MCPE with high content of crosslinkable monomers was successfully synthesized via core-shell emulsion polymerization. Specially, cross-linkable monomers MAA and NMA were designed as the core layer monomers, vinyl triethoxysilane (A151) and epoxy emulsion (BC2060) were designed as the shell layer monomers, and aziridine (XR-100) was added as crosslinker. The stability of MCPE was excellent, and the core-shell structures of MCPE particles were kept after stored for 15 days at 50 °C because several crosslinkable monomers were
Fig. 8. Adhesion ratio and number of alcohol scrubbing of different emulsion ink films on PVC film.
is propitious to improve their alcohol resistance. In general, in order to build film with core-shell emulsion at room temperature, it requires not only the low Tg of the shell polymer, but also enough shell polymer to form a continuous phase. Fig. 10 shows
Fig. 9. The pictures of emulsion film before alcohol scrubbing of P0, P1, P2, P3, P4 and P5, and after alcohol scrubbing of P0 (1 scrub), P1 (1 scrub), P2 (13 scrubs), P3 (21 scrubs), P4 (36 scrubs) and P5 (51 scrubs). 6
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introduced into different layers of emulsion particles. The MCPE film displayed excellent tensile properties that the tensile strength is 23.40 MPa and the elongation at break is 30%. The crosslinking density of MCPE film reached up to 93%. Tg of the MCPE film was about 38.6 °C, which was obviously higher than that of the pure polyacrylate emulsion film (12.5 °C). When the MCPE ink film was applied to PVC film, the surface drying time of the film was shortened to 6 s dramatically and the adhesion ratio of the ink film could reach 100%. After drying at room temperature for 7days, the film could resist 50 times of alcohol scrubbing. All the above facts prove that this emulsion has wide applications in the field of PVC film packaging and decoration.
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Acknowledgements This work was financially supported by National Natural Science Foundation of China (Grant No. 21576097, No. 21776100 and No. 21878110). References [1] M. Rentzhog, A. Fogden, Print quality and resistance for water-based flexography on polymer-coated boards: dependence on ink formulation and substrate pretreatment, Prog. Org. Coat. 57 (2006) 183–194, https://doi.org/10.1016/j.porgcoat. 2006.08.003. [2] M.R. Li, Z. Zheng, S.J. Liu, W. Wei, X.L. Wang, Effect of prepolyurethane on the performances of poly(acrylates-co-urethane) copolymer, J Appl. Polym. Sci. 118 (2010) 3203–3210, https://doi.org/10.1002/app.32761. [3] M.F. Sonnenschein, S.P. Webb, R.C. Cieslinski, B.L. Wendt, Poly(acrylate/epoxy) hybrid adhesives for low-surface-energy plastic adhesion, J Polym. Sci. Pol. Chem. 45 (2007) 989–998, https://doi.org/10.1002/pola.21843. [4] M.R. Li, Z. Zheng, S.J. Liu, Y.Z. Su, W. Wei, X.L. Wang, Synthesis and properties of poly(acrylates-co-urethane) adhesives for low surface energy materials, Int. J. Adhes. Adhes. 31 (2011) 565–570, https://doi.org/10.1016/j.ijadhadh.2011.05. 006. [5] M. Schellekens, D. Twene, A. van der Waals, Block copolymers for waterborne coatings-A novel eco-friendly approach for improved coating adhesion to untreated polypropylene based plastics, Prog. Org. Coat. 72 (2011) 138–143, https://doi.org/ 10.1016/j.porgcoat.2011.03.007. [6] W.B. Hu, Y.P. Bai, C. Zhang, N. Li, B.Q. Cheng, Coating based on the modified chlorinated polypropylene emulsion for promoting printability of biaxially oriented polypropylene film, J. Adhes. Sci. Technol. 32 (2018) 50–67, https://doi.org/10. 1080/01694243.2017.1341191. [7] O. Yilmaz, M. Karesoja, A.C. Adiguzel, G. Zengin, H. Tenhu, Nanocomposites based on crosslinked polyacrylic Latex/Silver nanoparticles for waterborne High-performance antibacterial coatings, J. Polym. Sci. Pol. Chem. 52 (2014) 1435–1447, https://doi.org/10.1002/pola.27130. [8] C.Y. Zhang, Z.W. Zhu, S.L. Gong, Synthesis of stable high hydroxyl content selfemulsifying waterborne polyacrylate emulsion, J. Appl. Polym. Sci. 134 (2017), https://doi.org/10.1002/app.44844. [9] P. Volfova, V. Chrastova, L. Cernakova, J. Mrenica, J. Kozankova, Properties of polystyrene/poly(butyl acrylate) core/shell polymers modified with N-methylol acrylamide, Macromolecular Symposia. 170 (2001) 283–290, https://doi.org/10. 1002/1521-3900(200106)170:1<283::AID-MASY283>3.0.CO;2-N. [10] M.D. Shalati, J.H. McBee, W.J. DeGooyer, F. Thys, L. van der Ven, R.T.M. Leijzer, High performance accelerated all acrylic coatings Kineticskinetics and mechanistic aspects of non-isocyanate coatings, Prog. Org. Coat. 48 (2003) 236–250, https://
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Synthesis and characterization of multiple-crosslinkable polyacrylate emulsion for PVC film ink Gan Gaoa, Guanzhou Luoa, Mengyi Xub,*, Shouping Xua, Pi Pihuia,*, Xiufang Wena,* a b
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China School of Chemical Engineering and Technology, Guangdong Industry Polytechnic, Guangzhou, 510300, China
ARTICLE INFO
ABSTRACT
Keywords: Multiple-crosslinkable polyacrylate emulsion Hard core Soft shell Ink film PVC film
Multiple-crosslinkable polyacrylate emulsion (MCPE) with high content of crosslinkable monomers was successfully synthesized via core-shell emulsion polymerization. Specially, cross-linkable monomers MAA and NMA were designed as the core layer monomers, vinyl triethoxysilane (A151) and epoxy resin (BC2060) were designed as the shell layer monomers, and aziridine (XR-100) was added as crosslinker. The hard core can be benefit for rapid drying of the film at room temperature, the soft shell can exhibit good film-forming ability. Epoxy resin was grafted on the chain of polyacrylate, which can endow excellent adhesion performance. The stability of MCPE was excellent and the core-shell structures of MCPE particles were maintained after stored for 15days at 50 °C, because those different crosslinkable monomers were introduced into different layers of the emulsion particles. The crosslinking density, tensile strength and glass transition temperature (Tg) of MCPE films were obviously higher than those of pure polyacrylate emulsion films due to the modification of several crosslinkable monomers. The MCPE ink film on PVC film has rapid surface drying performance, excellent adhesion and alcohol scrubbing resistance after multiple cross-linking at room temperature. All these advantages of this emulsion may contribute to its wide applications in the field of PVC film packaging and decoration.
1. Introduction PVC film is a very promising plastic material for packaging and decoration due to relatively low cost, high mechanical strength, high-low temperature resistance, aging resistance and corrosion resistance. However, when PVC film is printed with environment friendly waterbased ink, bad ink film performances, such as weak adhesion, easy shrinkage void and wetting problems [1], often occur due to surface migration of low molecular weight plasticizers with low surface energy [2,3]. The lower polarity segments are often introduced into polyacrylate molecular chains to improve the properties of emulsion film on low surface energy materials. Li et al. [4] reported poly (acrylates-co-urethane) with lower polarity soft segment of urethane oligomer, which showed excellent adhesion to low surface energy materials. Schellekens et al. [5] used a selected controlled radical polymerization technique to obtain water-dispersible iBOA-based block copolymers for adhesion promotion of waterborne acrylics on untreated PP film. Hu et al. [6] prepared chlorinated polypropylene waterborne emulsion grafted by polyacrylate. The film had excellent adhesion to BOPP film and printing quality due to the different movement trend of the polar and un-polar chains during the baking step.
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Introducing crosslinkable functional groups into acrylate polymer is another solution to improve the performances of water-based ink on plastic film. Some crosslinkable monomers, such as methacrylate glycidyl methacrylate (GMA) [7], hydroxyethyl methacrylate (HEMA) [8], N-methylol acrylamide (NMA) [9], epoxy-amine [10] and hydroxylacid [11], are often used to copolymerize with acrylate monomers to synthesize self-crosslinkable acrylate polymer. Pi et al. [12] prepared a low-temperature self-crosslinkable polyacrylic emulsion by introducing ketone-hydrazide crosslinking groups into polyacrylic polymer. The ink film of the emulsion had low water absorption and good adhesion on PE thin film. However, these self-crosslinkable acrylic emulsions are easy to disintegrate in the alcohol condition and their films have low selfcrosslinked density, resulting in poor resistance to alcohol scrubbing. Slow surface drying rate at room temperature is also a serious drawback to limit their use in fast printing technology. To our best knowledge, there are less reports about synthesis of low-temperature self-crosslinkable emulsion with rapid surface drying performance, excellent adhesion and alcohol scrubbing resistance for PVC film ink. Epoxy (EP) resin with excellent adhesive property, water resistance, impermeability and chemical resistance [13,14], and polysiloxanes with low-surface energy, high flexibility and low-glass transition
Corresponding authors. E-mail addresses: [email protected] (M. Xu), [email protected] (P. Pi), [email protected] (X. Wen).
https://doi.org/10.1016/j.porgcoat.2019.06.036 Received 5 April 2019; Received in revised form 13 June 2019; Accepted 21 June 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Gan Gao, et al., Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.06.036
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Scheme 1. Synthesis and curing process of MCPE.
temperature [15–17] were often introduced in polyacrylate molecular chains to increase the crosslinking density and improve performance of polymer film. In this study, a multiple-crosslinkable polyacrylate emulsion (MCPE) for PVC film ink was prepared by core-shell emulsion polymerization and followed by adding crosslinking agent XR-100 (Scheme 1). Specially, cross-linkable monomers MAA and NMA were used as the core layer monomers, vinyl triethoxysilane (A151) and epoxy emulsion (BC2060) were used as the shell layer monomers. The emulsion was designed with a hard core/soft shell structure, which can benefit for rapid drying of the film at room temperature and good filmforming ability. Epoxy resin was grafted on the chain of polyacrylate to endow excellent adhesion performance. Different crosslinkable monomers were introduced into different layers of emulsion particles, which not only increased the crosslinking density of the ink film, but also improved the stability of emulsion. The ink film has excellent alcohol scrubbing resistance after multiple cross-linking at room temperature. This novel MCPE exhibits significant potential application in waterbased ink for PVC film.
equipped with a mechanical stirrer (fitted with a crescent Teflon blade), a temperature control system, an inlet for feeding pre-emulsion, another inlet for feeding initiator solution and a reflux condenser. A mixture of 0.75 g SDS, 0.75 g OP-10, 18 g distilled water, 4 g MAA, 4 g NMA and 30 g MMA were placed in a graduated beaker and stirred with a magnetic stirrer to get a core pre-emulsion. A shell pre-emulsion containing 1.0 g SDS, 1.0 g OP-10, 27 g distilled water, 4 g A151, 5 g BC2060, 30 g MMA and 30 g BA was prepared by the same method. The flask was charged with 1.0 g SDS, 1.0 g OP-10, 0.1 g NaHCO3 and 60 g distilled water and was heated to 80 °C, then the core pre-emulsion and a initiator solution (0.2 g of APS dissolved in 10 ml water) were dropwise added in 30 min. After the addition was completed, the shell preemulsion and another initiator solution (0.4 g of APS dissolved in 20 ml water) were dropwise added in 150 min. The reaction was maintained at 85 °C for another150 min after the addition finished. The mixture in the flask was cooled to around 40 °C, and ammonia was added to adjust its pH value to the range of 7–8. Finally, 10 g XR-100 was added to the flask and stirred for 15 min to obtain MCPE (P5 emulsion). For comparison, P0 emulsion (pure polyacrylate emulsion), P1 emulsion (without adding NMA, A151, BC2060 and XR-100), P2 emulsion (without adding A151, BC2060 and XR-100), P3 emulsion (without adding BC2060 and XR-100), P4 emulsion (without adding XR-100) and P-core emulsion (the core emulsion of P5) were prepared respectively using the same approach. The feeding composition of monomers in various emulsions is shown in Table 1. These emulsions have excellent stability in the synthesis process and no gel is produced.
2. Experimental 2.1. Materials Methyl methacrylate (MMA), butyl acrylate (BA), methyl acrylic acid (MAA), N-methylol acrylamide (NMA), nonyl phenyl, vinyl triethoxysilane (A151), polyoxyethylene ether-10 (OP-10), sodium lauryl sulfate (SDS) and sodium bicarbonate (NaHCO3) were obtained from Tianjin Damao Chemical Reagent Co., Ltd. (Tianjin, China). Bisphenol A Epoxy (EP) resin BC2060 (EP value of 0.48–0.54) and crosslinker (aziridine XR100) were purchased from Guangzhou Zono Trading Co.Ltd. (Guangzhou, China). Ammonium persulfate (APS) and ammonia water were the products of Shenzhen Jitian Chemical Reagent Co., Ltd. (Shenzhen, China). Aqueous color paste was obtained from Guangzhou Keytec Colours Co., Ltd (Guangzhou, China). All the materials were used without further purification and deionized water was prepared in our lab.
2.3. Preparation of ink films The preparation process of ink films is shown in Fig. 1. MCPE was mixed with aqueous color paste at a mass ratio of 4:1 under stirring with a rate of 200 rpm for 30 min, then coated on the surface of PVC film by a winding bar and placed at room temperature. The alcohol scrubbing resistance of ink films was detected after 7days. The curing process of MCPE film is shown in Scheme 1, the multiple cross-linked networks can be formed by condensation reaction between carboxyl groups of MAA with amino of NMA, epoxy groups of BC2060 or aziridine ring of XR-100, and hydrolysis-condensation reaction between A151 and hydroxyl of NMA.
2.2. Synthesis of MCPE The preparation procedure of MCPE is schematically illustrated in Scheme 1. Polymerization was performed in a 250 mL, 4-neck flask 2
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The adhesion ratio was measured as following process. A dried ink film on PVC film (approximately 10 cm × 5 cm) was fixed on millimeter grid paper (grid: 1 mm × 1 mm), and a scotch tape (3 M, width of 1.5 cm) was pasted onto the ink film tightly. The number of grids covered by tape was recorded as A0. Then the tape was pulled off transiently and quickly with the angle of 180°, and the number of grids covered by deciduous ink film was recorded as A. The adhesion ratio of ink films was calculated as follows:
Table 1 The feeding composition of monomers in various emulsions. Components
P0
P1
P2
P3
P4
P5
P-core
MMA/g BA/g MAA/g NMA/g A151/g BC2060/g XR-100/g
60 30 – – – – –
60 30 4 – – – –
60 30 4 4 – – –
60 30 4 4 4 – –
60 30 4 4 4 5 –
60 30 4 4 4 5 10
60 – 4 4 – – –
Adhesion ratio (%) = (A0 – A)/A0 × 100 The alcohol scrubbing resistance was measured as following process. A 1000 g weight was covered with absorbent cotton and soaked by 70% ethanol aqueous solution. The soaked cotton was squeezed until there was no droplet, and applied and dragged back and forth onto an ink film (120 mm × 25 mm) with horizontal force. Each forward and backward scrub was a reciprocating scrub, and one reciprocating scrub was 1 s. According to this method, the ink film was reciprocally wiped until the PVC film was exposed, and the number of alcohol scrubbing was recorded. Surface drying test of ink film on PVC film was carried out followed the Chinese national standard of GB/T9278-2008 protocol. The ink film was dried at room temperature, and the surface was touched with finger at appropriate time intervals. The surface of the ink film was considered dry when no ink was sticked to the finger. Recording the time for the ink film reached the surface drying. For all performance test, three runs were made for each sample and average values were taken.
2.4. Characterization Fourier transform infrared (FT-IR) spectra of samples were performed on a NEXUS-380 spectrometer (Nicolet, USA) in the range from 400 cm−1 to 4000 cm−1 with a resolution of 6 cm−1 and 32scans. Emulsion polymer samples were obtained by emulsion demulsification, freeze and drying firstly, then it was frozen in liquid nitrogen and grinded into powder. Emulsion film on flat glasses was obtained by drying for 7days at room temperature. Dynamic light scattering (DSL) (ZS Nano S dynamic light scattering particle size analyzer, Malvern, UK) was used to measure particle size and size distribution. JEM-2100 F TEM (JEOL, Japan) transmission electron microscopy (TEM) was operated at 200 kV to characterize the dimensions and morphology of the sample. A drop of diluted dispersion was dropped on the hydrophilic carbon-coated copper grid. The samples were stained with 1.5% phosphotungstic acid solution. After 3 min, excessive phosphotungstic acid solution on grids was cleaned with distilled water [18–20]. Glass transition temperature (Tg) of the cured films were measured by differential scanning calorimeter (DSC) (Q20, TA instruments, USA) at a heating rate of 10 °C/min from −20 °C to 150 °C. The tensile experiments were carried out on a tensile tester (WDW3100, Changchun Kexin Co., China) at a cross-head speed of 10 mm/min at room temperature according to ASTM-D638. The dimensions of the tensile specimens were 10 mm × 4 mm in the working section. At least three specimens of each sample were tested. After the tensile tests, the broken samples were collected, and the morphologies and elemental analyses of the fracture surfaces were characterized by a field emission scanning electron microscope (SEM, SU-70, Hitachi, Japan) at an acceleration voltage of 30 kV. The crosslinking density was measured through a typical process. A dried emulsion film was placed in a Soxhlet extractor and extracted with methylbenzene under reflux for 7days. After extraction, linear polymer was dissolved away by methylbenzene, while crosslinked polymer was remained and weighed after dried (the weight of samples was constant). The crosslinking density of the films was calculated as follows:
3. Results and discussion 3.1. FTIR analysis FTIR spectra of the P0 emulsion after demulsification (curve a), P4 emulsion after demulsification (curve b) and P5 film (curve c) are shown in Fig. 2. No peak is observed at 1650 cm−1 in curve a, b and c, which indicates the C]C groups of monomer polymerize completely. In curve a, the peaks at 2957cm−1 and 2874 cm−1 are the stretching vibration of eCH2e and eCH3 groups, respectively, and the peak at 1453 cm−1 is the bending vibration peak of CeH. The peak at 1733 cm−1 is ascribed to the stretching vibration of C]O groups. The peaks at 1242cm−1 and 1126 cm−1 are attributed to the stretching vibration of CeOeC groups. So it can be deduced that the emulsion polymer is the random copolymer of acrylate monomers [12,21]. Comparing curve b with curve a, the peak at 3442 cm−1 is contributed to the NeH groups of NMA [21]. The peak at 1070 cm−1 is ascribed to the stretching vibration of Si–O–Si [19]. The important characteristic peak of Si-C at 1260 cm−1 is not observed because the peak of CeOeC groups at 1242 cm−1 is so wide and covers the characteristic peak of SiC at 1260 cm−1 of A151. The peaks at 1596 cm−1 and 1510 cm−1 are ascribed to the stretching vibration of para-substituted benzene rings of BC2060, the peaks at 3543 cm−1 and 943 cm−1 are ascribed to the
Crosslinking density (%) = W1/ W0×100% W0 was the weight of the dried emulsion film before extraction; W1 was the weight of the emulsion film after extraction.
Fig. 1. Preparation process of ink film. 3
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Table 2 Particle size and size distribution of various emulsions. Subject
P0
P1
P2
P3
P4
P5
P-core
Particle size/nm PDI
113.5 0.037
125.3 0.053
134.9 0.134
168.3 0.042
186.0 0.042
184.7 0.166
126.1 0.017
have a clear core-shell structure (Fig. 4b), which illustrates that the storage stability of MCPE is excellent. These phenomena can be explained that the crosslinkable monomers are evenly distributed in the core and shell layer respectively, which reduces the probability of crosslink reaction in storage process and improves the stability of MCPE. Fig. 5 shows the crosslinking density, tensile strength and elongation at break of different emulsion films. The tensile strength of the films of P0, P1, P2, P3, P4 and P5 are 7.99 MPa, 13.14 MPa, 15.00 MPa, 17.03 MPa, 19.27 MPa and 23.40 MPa, and their elongations at break are 269%, 186%, 163%, 112%, 69% and 30%, respectively. The crosslinking density of the films increases from 0% (P0) to 93% (P5). The increased reaction active sites make the multiple network structure easier to form and the network density increases when crosslinkable monomers MAA, NMA, A151 and BC2060 are gradually introduced into emulsion polymers, resulting in an improvement in tensile strengths and a reduction in the elongation at break. The mechanism of the increased tensile strength is studied from the morphology of fracture surfaces [25], and the SEM images of the fractures surfaces of samples are shown in Fig. 6. The cured P0 emulsion film is very smooth and river-like (Fig. 6a), indicating that pure emulsion film is very brittle, as typical thermoplastic polymers [25]. For samples P1, P2, P3, P4 and P5, the fracture surfaces become rougher and rougher with the increase of crosslinking density. It suggests that multiple cross-linked network structure and high crosslinking density is formed in MPCE film. Fig. 7 shows the DSC analysis of sample P0 (a), P1 (b), P2 (c), P3 (d), P4 (e), P5 (f) and P-Core (g) emulsion films. Tg of the P0, P1, P2, P3, P4, P5 and P-Core emulsion films are 12.5 °C, 17.2 °C, 19.9 °C, 21.2 °C, 25.7 °C, 38.6 °C and 77.5 °C, respectively. The crosslinked network in the polymer backbone can restrict the movement of polymer chains, the higher cross-linking density of films gets, the higher Tg is [12,26]. Tg of P-Core emulsion film is the highest in our research because it is composed only by hard monomers of NMA, MAA and MMA. TEM analysis and Tg of P-Core emulsion film indicate that the MCPE particles have the hard core-soft shell structures.
Fig. 2. FTIR spectra of the P0 emulsion (a), P4 emulsion (b) and MCPE film (c).
stretching vibration of −OH and terminal EP groups of BC2060 [22,23]. These results imply that NMA, A151 and BC2060 are polymerized in the P4 emulsion. Compared with curve b, the characteristic peak for EP at 943 cm−1 disappeared in curve c because of crosslinking between EP groups and aziridine [24] ring in XR-100 during film formation. The FTIR results suggest that P5 is MCPE composed by MAA, NMA, A-151, BC2060, and XR-100. 3.2. Emulsion performance DLS parameters of various emulsions are illustrated in Fig. 3. Particle size and size distribution of various emulsions are shown in Table 2. It can be seen from Fig. 3 and Table 2 that the particle size of Pcore emulsion and P1 is 126.1 nm and 125.3, respectively. The particle size of the emulsions increases with the introduction of various crosslinked monomers, for the P5 emulsion, it is up to 184.7 nm due to the coalescence of emulsion particles. The TEM micrographs of MCPE particles before (a) and after (b) 15days of storage at 50 °C are illustrated in Fig. 4. The appearance of MCPE is no visible change, no precipitation and flocculation are found after 15days of storage at 50 °C (Fig. 4d), and its emulsion particles still
3.3. Performance of emulsion ink films on PVC film Adhesion ratio and alcohol scrub numbers of different emulsion ink films on PVC film are shown in Fig. 8. The adhesion ratio of P0, P1, P2, P3, P4 and P5 emulsion ink films on PVC film are 0%, 30%, 60%, 85%, 100% and 100%, respectively. It increases as crosslinkable monomers gradually introduces, which is because the carboxyl groups, hydroxymethyl and epoxy groups in MAA, NMA and BC2060 can interact with the functional groups on the surface of substrate on one hand, and one the other hand, the surface tension of the ink film decreases with A151 introduced. They are all benefit to improve the adhesion of ink film on PVC film, and the higher content of crosslinkable monomers is, the higher adhension ratio of ink film is. It also can be found from Fig. 8 that the number of alcohol scrubbing of P0, P1, P2, P3, P4 and P5 ink films on PVC film are 0, 0, 12, 20, 35 and 50, respectively. The pictures of emulsion film before alcohol scrubbing of P0, P1, P2, P3, P4 and P5, and after alcohol scrubbing of P0 (1 scrub), P1 (1 scrub), P2 (13 scrubs), P3 (21 scrubs), P4 (36 scrubs) and P5 (51 scrubs) are shown in Fig. 9. It can be seen from Fig. 9 that the performance of alcohol resistance from P0 to P5 are gradually better. It indicates that the increase of cross-linking density of the film
Fig. 3. DLS measurement on the size distribution of various emulsions. 4
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Fig. 4. The TEM micrographs of the MCPE particles before (a) and after (b) 15days of storage at 50 °C.
Fig. 5. Crosslinking density, tensile strength and elongation at break of P0, P1, P2, P3, P4 and P5 emulsion films.
Fig. 6. SEM morphologies of the fracture surface of the cured films for P0 (a), P1 (b), P2 (c), P3 (d), P4 (e) and P5 (f). 5
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Fig. 7. DSC curves of the films for P0 (a), P1 (b), P2 (c), P3 (d), P4 (e), P5 (f) and P-Core (g) emulsion.
Fig. 10. Effect of BA distribution ratio within core and shell on the surface drying time of the ink films.
the effect of BA distribution ratio within core and shell on the surface drying time of the ink films. It can be seen from Fig. 10 that when all of BA monomers are distributed within the shell of MCPE particles, the surface drying time of the ink film is the shortest. Conversely, the ink film with all of BA monomers within the core takes the longest time to reach the surface drying. This can be explained that when BA monomers are completely polymerized in the shell part, the MCPE particles form hard core-soft shell structures. It is benefit to decrease the MFT of the emulsion [27] and shorten the surface drying time of the ink film. When BA monomers are completely polymerized within the core part, the MCPE particles possess soft core-hard shell structures, which raise the MFT of the MCPE and increase the surface drying time of the ink film. 4. Conclusion MCPE with high content of crosslinkable monomers was successfully synthesized via core-shell emulsion polymerization. Specially, cross-linkable monomers MAA and NMA were designed as the core layer monomers, vinyl triethoxysilane (A151) and epoxy emulsion (BC2060) were designed as the shell layer monomers, and aziridine (XR-100) was added as crosslinker. The stability of MCPE was excellent, and the core-shell structures of MCPE particles were kept after stored for 15 days at 50 °C because several crosslinkable monomers were
Fig. 8. Adhesion ratio and number of alcohol scrubbing of different emulsion ink films on PVC film.
is propitious to improve their alcohol resistance. In general, in order to build film with core-shell emulsion at room temperature, it requires not only the low Tg of the shell polymer, but also enough shell polymer to form a continuous phase. Fig. 10 shows
Fig. 9. The pictures of emulsion film before alcohol scrubbing of P0, P1, P2, P3, P4 and P5, and after alcohol scrubbing of P0 (1 scrub), P1 (1 scrub), P2 (13 scrubs), P3 (21 scrubs), P4 (36 scrubs) and P5 (51 scrubs). 6
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introduced into different layers of emulsion particles. The MCPE film displayed excellent tensile properties that the tensile strength is 23.40 MPa and the elongation at break is 30%. The crosslinking density of MCPE film reached up to 93%. Tg of the MCPE film was about 38.6 °C, which was obviously higher than that of the pure polyacrylate emulsion film (12.5 °C). When the MCPE ink film was applied to PVC film, the surface drying time of the film was shortened to 6 s dramatically and the adhesion ratio of the ink film could reach 100%. After drying at room temperature for 7days, the film could resist 50 times of alcohol scrubbing. All the above facts prove that this emulsion has wide applications in the field of PVC film packaging and decoration.
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