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Preparation and characterization of self-crosslinking fluorinated polyacrylate latexes and their pressure sensitive adhesive applications
International Journal of Adhesion and Adhesives 95 (2019) 102417
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International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Preparation and characterization of self-crosslinking fluorinated polyacrylate latexes and their pressure sensitive adhesive applications
T
Cheng Fang*, Kai Zhu, Xinbao Zhu, Zhongxiang Lin College of Chemical Engineering, Nanjing Forestry University, Nanjing, 210037, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorinated polyacrylate Self-crosslinking Emulsion polymerization Surface property Pressure sensitive adhesive
Self-crosslinking fluorinated polyacrylate latexes based on butyl acrylate (BA), fluorine monomer octafluoropentyl methacrylate (OFPMA), self-crosslinking functional monomers acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA) were synthesized by a monomer-starved seeded semi-continuous emulsion polymerization process. The latexes and their corresponding films were characterized by laser particle size analyser, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), contact angle goniometer, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results indicated that the particle size of the latexes and the gel content of the films were both independent of the amount of OFPMA employed. On the other hand, the particle size of the latexes decreased and the gel content of the films increased with the incorporation of AA and HEA as expected. Glass transition temperature (Tg) and the thermal stability of the copolymer were both improved gradually as OFPMA content increased. XPS, AFM and water contact angle measurement indicated that the fluoroalkyl groups had a tendency to enrich on the surface of the films. However, this enrichment of fluorine on the film surface was reduced after the introduction of self-crosslinking functional monomers into the system. Finally, the adhesive property of the latexes was evaluated for application as a pressure sensitive adhesive (PSA).
1. Introduction Pressure sensitive adhesives (PSAs) are viscoelastic materials that allow an instantaneous adhesion to a variety of surfaces within a short contact time and low contact pressure without any phase transition or chemical reaction [1]. In particular, acrylate PSAs have many advantages, such as excellent aging characteristics, resistance to elevated temperatures and exceptional optical clarity. Therefore they are widely used in many applications such as tape, label, protection film and medical products [2,3]. PSAs can be produced via hot-melt, solution polymerization and emulsion polymerization techniques [4]. Recently, the latter has attracted much interest, due to its relatively more environmental friendly process. Hence, the focus of the current study is on latex-based PSAs. Acrylate latex PSAs prepared by emulsion polymerization generally have high surface energy and thus poor wettability. So, how to reduce the surface energy and improve wettability of acrylate emulsion pressure sensitive adhesive has attracted the interest of many researchers. An effective method is to modify the PSA by introducing hydrophobic monomers or low surface energy monomers in block or random copolymers. This will reduce the surface energy of the PSA, which in turn
*
Corresponding author. E-mail address: [email protected] (C. Fang).
https://doi.org/10.1016/j.ijadhadh.2019.102417 Received 23 January 2019; Accepted 23 July 2019 Available online 29 July 2019 0143-7496/ © 2019 Elsevier Ltd. All rights reserved.
leads to excellent wettability. Among low surface energy monomers, the fluorinated monomers and polymers are used widely in industry due to functional groups, such as CF, CF2 and CF3 [5–7]. The surface energy can be reduced to 10–13 mN/m when the main chain is attached by the fluorine-containing groups, especially the CF3 group [8,9]. Furthermore, fluorinated polymers also have other unique properties, for instance, oil repellency, water repellency, heat resistance and antifouling properties, which are attributed to the largest electronegativity and small atomic radius of fluorine [10,11]. Recently, the application of fluorinated monomers in acrylate emulsion polymerization has been extensively studied. For example, Machotová et al. [6] investigated the effects of crosslinking and ambient drying conditions on water sensitivity of fluorine-containing polyacrylate latex coatings. And they found that the highest level of hydrophobicity at the same amount of copolymerized 2,2,2-trifluoroethyl methacrylate could be achieved in the case of noncrosslinked latex films dried at elevated temperatures, whereas the highly crosslinked latexes combining precoalescence crosslinking and keto-hydrazide self-crosslinking provided the most water whiteningresistant coating films. Hao et al. [12] synthesized a series of fluorine and silicon acrylic latexes via emulsion polymerization. And they
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discovered that fluorine content at the surface of fluorosilicone acrylic film decreased as the film forming temperature was increased. Xu et al. [13] prepared fluorine-containing poly-styrene-acrylate latexes with core-shell structures by seeded semi-continuous emulsion polymerization using the fluorine monomer Actyflon-G04 and reactive emulsifier DNS-86. They concluded that the film of latex has excellent water repellency, thermal stability, and chemical resistance properties when the amount of fluorine monomer was only 8.0 wt%. However, as far as we know, there have been no studies involving octafluoropentyl methacrylate (OFPMA) for the preparation of fluorinated acrylate latex pressure sensitive adhesives. And the effects of self-crosslinking functional monomers on the comprehensive properties, especially surface properties (e.g. F content, morphology, etc) and adhesive properties (i.e. tack, peel and shear) of fluorinated latex PSA films have also been scarcely reported in the open literature. In this paper, the synthesis of fluorinated acrylate latex PSAs based on butyl acrylate (BA) and octafluoropentyl methacrylate (OFPMA), together with the self-crosslinking functional monomers acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA) prepared via a monomerstarved seeded semi-continuous emulsion polymerization process are discussed. The effects of OFPMA on the conversion of monomer and particle size of the latex, as well as on the gel content, thermostability (DSC, TG), surface properties (XPS, WCA, AFM) of the polymer films are discussed. In addition the influence of self-crosslinking functional monomers (AA, HEA) on the migration of F, hydrophobicity and morphology of the surface of the films are also considered. Finally, loop tack, shear strength and 180° peel strength of PSA tapes were investigated to reveal the adhesive properties of the resultant fluorinated PSAs.
Table 1 Recipes for self-crosslinking fluorinated polyacrylate emulsion. Sample
PAE-F0-1
PAE-F5-1
PAEF10-1
PAEF10-0
PAEF10-2
PAEF15-1
PAEF20-1
BA/g OFPMA/g AA/g HEA/g CO-436/g APS/g NaHCO3/g DI water/g
100 0 1 2 1.5 0.5 0.15 94
95 5 1 2 1.5 0.5 0.15 94
90 10 1 2 1.5 0.5 0.15 94
90 10 0 0 1.5 0.5 0.15 94
90 10 2 2 1.5 0.5 0.15 94
85 15 1 2 1.5 0.5 0.15 94
80 20 1 2 1.5 0.5 0.15 94
with a stirring rate of 270 rpm at 82–85 °C and kept still for 30 min. Then both the remaining pre-emulsion and APS aqueous solution (0.35 g of APS was dissolved in 24 g water) were added dropwise into the reacting mixture for 3.5 h. After the feed was completed, the reaction was allowed to proceed for an additional 1 h to increase monomer conversion. The latex was then cooled to room temperature and poured into a glass bottle and used for further characterization. Hereinafter, self-crosslinking fluorinated polyacrylate emulsions are denoted PAE-Fx-y in which “x” refers to the weight of OFPMA, and “y” stands for the weight of AA, Table 1. 2.3. Characterization 2.3.1. Particle size analysis The size of the latex particles was measured using a dynamic light scattering (DLS) instrument (Malvern NanoS Zetasizer). The analyses were carried out at 25 °C, and every result was an average of three parallel measurements. The latex was diluted until the solid content was about 1%. The reported diameter is an intensity-weighted average particle size.
2. Experimental 2.1. Materials Butyl acrylate (BA), acrylic Acid (AA), 2-Hydroxy ethyl acrylate (HEA), ammonium persulfate (APS) and sodium bicarbonate (NaHCO3) were purchased from Shanghai Lingfeng Chemical Co., Ltd. and used as received. Octafluoropentyl methacrylate (OFPMA) was purchased from Shanghai Aladdin Chemistry Co., Ltd. (Shanghai, China) and used as received. Ammonium nonyl phenol ethoxylate sulfate surfactant (Rhodapex CO-436) was obtained from Shanghai Honesty Fine Chemical Co., Ltd. and used as received. Ammonia (25 wt% in H2O) was obtained from Nanjing Chemical Reagent Co., Ltd. Distilled deionized water (DI-H2O) was used throughout the study. The molecular structure of OFPMA is shown in Scheme 1.
2.3.2. Gel content determination The gel content of the acrylic PSA polymers was measured via the solvent-extraction method. Three samples (around 0.2 g) of the dried latex film were weighed and sealed in a PTFE coated membrane pouch. Then the membrane pouch was put into a Soxhlet extractor with tetrahydrofuran (THF), and then refluxed for 24 h. After the extraction process, the membrane pouch was removed and first dried in a fume hood for 3 h and then in a vacuum oven at 70 °C until it reached a constant weight. The weight of the remaining dry gel was taken and the gel content was calculated using: Gel content = mass of the dry gel/mass of the initial dry polymer (1).
2.2. Synthesis of self-crosslinking fluorinated polyacrylate emulsion 2.3.3. FTIR analysis The latex was dried in a vacuum oven at 105 °C until it reached a constant weight. Fourier transform infrared (FTIR) spectra of the dried latex films were recorded with a Bruker VERTEX80 FTIR spectrometer (Germany) in the range 4000 to 400 cm−1.
The recipes for self-crosslinking fluorinated polyacrylate emulsions are described in Table 1. For a typical experiment, 90 g of BA, 10 g of OFPMA, 1 g of AA, 2 g of HEA, 1.5 g of CO-436 and 25 g of deionized water were mixed in a 500 mL four-neck round-bottom flask and stirred vigorously to form the pre-emulsion. Another 500 mL four-neck round-bottom flask with a reflux condenser, a thermometer and a mechanical stirrer was filled with 0.15 g of NaHCO3, 6 g pre-emulsion, 0.15 g of APS and 45 g of deionized water
2.3.4. Differential scanning calorimetry (DSC) The glass transition temperature (Tg) of the latex films was measured by a differential scanning calorimeter (DSC, Model 214 Polyma, NETZSCH Instruments, Germany). 5–15 mg of dry polymer was weighed into a standard DSC hermetic alumina crucible. In order to eliminate thermal history, two scanning cycles of heating-cooling were performed for each sample at a heating rate of 10 °C/min in the temperature range of −70 to 100 °C under a nitrogen atmosphere. The second heating run was used to determine the Tg. 2.3.5. Thermal gravimetric analysis (TGA) TGA was carried out to demonstrate the thermal stability of the
Scheme 1. The molecular structure of OFPMA. 2
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latex PSA polymers using a NETZSCH 209F1 analyser, Germany. The polymer films (around 5–10 mg) were heated from ambient temperature to 600 °C at a rate of 10 °C/min under 20 mL/min nitrogen flow.
Table 2 Some properties of self-crosslinking fluorinated polyacrylate emulsion.
2.3.6. Contact angle on the latex film Water contact angles on the latex films were determined by a contact angle goniometer (Attension Theta, Biolin Scientific, Sweden) at 20 °C using sessile drop method. The used injection volume was 4 μL and the final data of each sample was an average of at least ten readings.
Latex
Conversion (wt%)
Particle size (nm)
PDI
Gel content (wt%)
PAE-F0-1 PAE-F5-1 PAE-F10-1 PAE-F10-0 PAE-F10-2 PAE-F15-1 PAE-F20-1
97.76 98.08 97.36 92.53 97.28 96.57 96.94
234.0 231.4 226.3 242.3 217.8 228.5 225.0
0.038 0.016 0.040 0.008 0.003 0.024 0.034
56.55 55.42 56.08 51.89 63.26 55.25 56.35
± ± ± ± ± ± ±
0.188 0.819 0.613 0.520 1.983 0.782 3.230
3. Results and discussion 2.3.7. X-ray photoelectron spectroscopy (XPS) analysis The X-ray photoelectron spectra (XPS) analysis was performed on an AXIS UltraDLD (UK) with Al Kα (X-ray) lamp-house. The nominal photoelectron take-off angle was 30°, and the vacuum degree of analysis chamber was 7 × 10−8 Pa. In the XPS measurement, the deviation of the binding energy was corrected by the C1s photoelectron emission signal occurring at 284.8 eV due to hydrocarbon contamination on the sample surface.
3.1. Characterization of latexes The conversion of monomer and particle size of latex as a function of OFPMA are shown in Table 2. The results indicate that all the experiments were completed successfully with conversions exceeding 96% as expected, except for latex PAE-F10-0, which was prepared without the functional monomers AA and HEA. The five latexes {i.e. PAE-F(0-20)-1} with different amounts of OFPMA had similar z-average particle sizes around 230 ± 5 nm and narrow particle size distributions (PDI < 0.04). Besides, by comparing latex PAE-F10-0 to latexes PAEF10-1 and PAE-F10-2 in Table 2, it is evident that with the addition of self-crosslinking monomers AA and HEA, the latex particle size was reduced, while the monomer conversion was increased greatly, which was mainly due to the strong hydrophilicity and consequent homogeneous nucleation [16] in the system. Qualitatively, the ability of acid monomers to induce particle formation can be explained by the theory of Fitch and Tsai [17], Ugelstad and Hansen [18]. They clarified that persulfate radicals induce polymerization in the aqueous phase producing highly carboxylated oligomers which combine and nucleate to form new particles, thus increasing the reaction rate and decreasing the particle size of the latex. On the other hand, it can be observed from Table 2 that the fluorine monomer OFPMA has no significant effect on the gel content of the acrylate latex. However, by comparing latexes PAE-F10-0, PAE-F10-1 and PAE-F10-2 in Table 2, it is evident that the incorporation of self-crosslinking monomers would increase the gel content of the latex owing to the crosslinking reaction between -COOH and -OH when heated, as shown in Fig. 1. This result was consistent with the work of Qie [4], who found that carboxyl groups reacted with OH groups at high temperature (> 120 °C), thus slightly increasing the gel content.
2.3.8. AFM analysis Surface topographies of the latex films were examined by atomic force microscopy (AFM, Dimension Edge, Bruker, Germany) operated in tapping mode at room temperature. AFM images were taken continuously with a scan rate of 1.0 Hz. Each measurement was repeated five times. The roughness (Ra) of each polymer film was the average value.
2.3.9. PSA testing [14,15] The latex was coated with RK (Manchester, UK) KHC.10.5 wire-rod coater onto 30 μm thickness, 25 mm width poly (ethylene terephthalate) strip to give a film with a dry thickness of 20 μm and dried in a vacuum oven at 105 °C for 5 min. The PET strips with adhesive coated on one side were stuck on release paper for further tests. Before the tests, the PET strips were conditioned for 24 h at standard conditions of temperature and humidity (23 ± 2 °C and 50 ± 5% relative humidity). A universal BLD-100S electronic stripping tester was used to evaluate loop tack and peel strength. For the loop tack test, the strip was formed into a loop with the adhesive side facing outwards. Approximately 25 mm at both ends of the strip was inserted into the upper grip. The instrument moved the upper grip downward at a speed of 300 mm/min until an area of 25 mm2 came into contact with the stainless steel substrate mounted into the lower grip. Next, the tester moved the upper grip upwards at the same speed while recording the force needed to detach the loop from the substrate. The maximum force of detachment was reported as loop tack. For the 180° peel test, strips of the adhesive-coated films were laminated against the stainless steel substrate using a 2 kg rubber roller. The rubber roller was passed through the PET strip front to back three times. After a 20 min dwell, 180° peel from the substrate was done at 300 mm/min. The average force per 25 mm required to peel the strip from the substrate was recorded and reported as 180° peel strength. For shear holding power, the strips were laminated against stainless steel using a 2 kg rubber roller to make a contact area of 25 mm2. After a 20 min dwell, the sample was fixed into the tester vertically with a 1 kg load suspended in the other end. Automatic timers were placed below the weights to count the time of failure. The shear holding power was the time that had elapsed between the application of the load and the completed separation of the strip from the stainless steel. Every result was an average of five parallel measurements.
3.2. FTIR analysis The chemical structures of self-crosslinking fluorinated polyacrylates were characterized by Fourier transform infrared (FTIR) spectroscopy. FTIR spectra of PAE-F0-1, PAE-F10-1 and PAE-F20-1 are shown in Fig. 2. It could be seen that all FTIR spectra exhibit the characteristic stretching peaks of C–H (CH2) at 2958, 2934 and 2873 cm−1, stretching vibration of C]O at 1730 cm−1, and distortion vibration of CH2 at 1452 and 1396 cm−1, asymmetric and symmetric stretching vibration of C–O–C at 1242 and 1159 cm−1, as well as the absorption at 1063 and 941 cm−1 resulting from characteristic of BA. However, as compared with the spectrum of fluorine-free polyacrylate (PAE-F0-1), the spectra of fluorine-containing polyacrylates (PAE-F10-1 and PAE-F20-1) show wider peaks between 1117 and 1242 cm−1. This is because the strong absorption of C–O–C is overlapped by stretching vibrations of C–F [19,20]. Moreover, there is no adsorption peak at 1641 cm−1 which is attributed to the characteristic of C]C bonds of the monomers. All these results of FTIR analysis indicated that the fluorinated acrylate monomer OFPMA could be introduced into the latex particles as desired through emulsion polymerization. 3
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Fig. 1. Self-crosslinking reaction between AA and HEA.
Fig. 2. FTIR spectra of self-crosslinking polyacrylates containing various amounts of OFPMA.
3.3. DSC analysis Glass transition temperature (Tg) is one of the most important factors affecting PSA performance. A higher Tg commonly shows the higher rigidity of the PSA chains, which might manifest itself in better elasticity and cohesive behaviour. On the contrary, PSAs with lower Tg will demonstrate better fluidity or deformability, which makes for better tack but might cause cohesive failure. Generally, PSAs with Tg values ranging from −30 to −50 °C are favored, which will show good end-use performance. Fig. 3(a) shows the DSC curves of the three representative samples with 0, 10 wt% and 20 wt% OFPMA added. It is evident that all samples show similar curves with one characteristic endothermic peak indicating the presence of a homogeneous polymer. Furthermore, from Fig. 3(a), it is observed that with the inclusion of the OFPMA fraction in the copolymers, the Tgs of the PSAs are increased. This phenomenon is attributed to the Tg of the homopolymer of OFPMA (26 °C [21]), which is higher than that of the homopolymer of BA (−54 °C, soft monomer). On the other hand, the effect of the selfcrosslinking monomers (i.e. AA and HEA) on the Tg of the polymer films was also investigated and shown in Fig. 3(b). We can see that the incorporation of small amounts of AA and HEA in the system would help
Fig. 3. DSC graphs of the latex films with different content of (a) OFPMA and (b) self-crosslinking monomer.
4
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Fig. 4. TGA graphs of the latex films with different content of (a) OFPMA and (b) self-crosslinking monomer.
Fig. 5. XPS spectra of the film of self-crosslinking fluorinated polyacrylate emulsions with different contents of (a) OFPMA and (b) self-crosslinking monomer.
increase the Tg of the polymer films as a result of the self-crosslinking reaction between carboxyl and hydroxyl groups during film formation.
characteristic signal of F1s at 688 eV appeared in the survey spectra of PAE-F10-1 and PAE-F20-1 compared with that of PAE-F0-1. Furthermore, the intensity of the fluorine signal was much higher for PAE-F20-1 than for PAE-F10-1, indicating greater fluorine enrichment on the surface of PAE-F20-1. To further investigate these tendencies, the experimental values of elemental concentration percentages of C, O and F elements on the topmost surface calculated from the survey scan and the theoretical ones calculated from the chemical formula of the monomer for the bulk were compared and are shown in Table 3. It can be observed from Table 3 that when compared to PAE-F10-1, the experimental value
3.4. TG analysis The effect of the fluorine monomer OFPMA on the thermal stability of the latex film was investigated by comparing the TGA curves of the latex with different amounts of OFPMA, and the results are shown in Fig. 4(a). It can be seen that the thermal stability of the latex film was gradually improved as the OFPMA content was increased which can be ascribed to long-chain fluoroalkyl groups. OFPMA has fluoroalkyl chains containing C–F bonds with high bond energies, which are able to shield and protect the non-fluorinated segment beneath the fluorinated segment and thus improve thermal stability. Besides, the addition of small amounts of AA and HEA in the system would enhance the thermal stability of the polymer film as expected, as shown in Fig. 4(b).
Table 3 Atomic composition of surface of the PSA films measured by XPS. Sample
3.5. XPS analysis XPS analysis can give some insights into the chemical compositions of the surface of a self-crosslinking fluorinated polyacrylate latex film and the results are shown in Fig. 5. All the XPS survey spectra revealed the strong characteristic signals of C1s and O1s at 284 and 531 eV, respectively. It can also be observed from Fig. 5(a) that a new
PAE-F0-1 PAE-F10-0 PAE-F10-1 PAE-F10-2 PAE-F20-1
5
Theoretical atomic content (wt%)
Experimental atomic content (wt%)
C
O
F
C
O
F
71.89 68.64 68.24 68.08 64.63
28.11 25.81 26.38 26.59 24.68
0 5.54 5.38 5.33 10.69
74.26 70.41 71.40 73.14 59.37
25.74 23.55 22.79 24.41 24.58
0 6.04 5.81 2.45 16.05
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of fluorine content on the surface of PAE-F20-1 was much higher than the theoretical one in the bulk. That is, the more the amount of fluorine monomer is used in the recipe, the stronger the migration of F to the surface of the film. This can be understood by the fact that fluorine atoms have extremely low surface free energy and self-aggregating characteristics, which causes fluorinated segments to be preferentially oriented to the polymer surface during the film formation so as to decrease the surface free energy of the film [22,23]. On the other hand, when comparing the experimental and theoretical values of fluorine atomic content on the surface of PAE-F10-0, PAE-F10-1 and PAE-F10-2 in Table 3, it was found that the incorporation of self-crosslinking monomers (AA and HEA) limits the migration of F to the surface of the film. This is more obvious for PAE-F10-2 with higher gel contents aforementioned, with the experimental value of F on the surface (2.45%) being much lower than the theoretical value in the bulk (5.33%). This phenomenon can be attributed to the fixation of latex polymer by cross-linking, which hinders the orientation of higher amounts of fluoroalkyl groups towards the film-air interface during film formation [6]. It is expected that the surface compositions have significant influences on both the morphology and hydrophobic property of the films, which will be discussed in the following paragraphs. 3.6. AFM analysis Atomic force microscopy (AFM) was used to examine the surface morphology of polyacrylate film, fluorinated polyacrylate film, and selfcrosslinking fluorinated polyacrylate film, as shown in Fig. 6. It can be seen from Fig. 6 that the average surface roughness (Ra) for PAE-F0-1, PAE-F10-1 and PAE-F20-1 is 1.45 nm, 3.06 nm and 6.25 nm, respectively, suggesting the introduction of OFPMA increases the roughness of the film, and the greater the OFPMA content, the higher the roughness. This phenomenon can be explained by the fact that the organic fluorine segment tends to migrate towards and enrich the film surface during film formation [24], as confirmed by the XPS results aforementioned, and micro-phase separation between fluorinated and nonfluorinated components occurs for fluorinated polyacrylate film, thus enhancing the roughness of the latex film [7,25]. On the other hand, by comparing the Ra values of PAE-F10-0 (1.56 nm), PAE-F10-1 (3.06 nm) and PAEF10-2 (3.50 nm), it was found that the surface roughness of the film increased with the introduction of self-crosslinking functional monomers into the fluorinated polyacrylate latex PSA. However, an increase of surface roughness of the PSA film with further crosslinking was not obvious. This result is believed to originate from two factors. Firstly, the introduction of cross-linking will increase the surface roughness of the film, as reported in our previous work [26]. Secondly, the fluorine content on the surface of the film was reduced with further crosslinking, as shown in Table 3, thus decreasing the surface roughness. When the crosslinking density (i.e. gel content) was high, the first positive factor partly compensates for the second negative one, and hence the increase of surface roughness was not very obvious. 3.7. Water contact angle analysis The hydrophobic property of a polymeric material can be estimated in terms of contact angle measurement by depositing a water drop on the surface of a film and the value of contact angle depends on the chemical composition of the film surface [27]. Moreover, the stronger the hydrophobicity of the film surface, the higher the water contact angle (WCA) is. The results of WCA measurements are presented in Fig. 7. By comparing the WCA values of PAE-F0-1 (102°), PAE-F10-1 (116°) and PAE-F20-1 (133°) in Fig. 7, one can see that the WCA of the polymer films increase from 102 to 133° with OFPMA content increasing from 0 to 20 wt% in the recipe due to the increment of hydrophobic fluorine content. In addition, by comparing the WCA values of PAE-F10-0 (114°), PAE-F10-1 (116°) and PAE-F10-2 (106°), it is evident that the WCA of the polymer film was not improved with the
Fig. 6. AFM images of latex PSA films: PAE-F0-1, Ra = 1.45 nm; PAE-F10-1, Ra = 3.06 nm; PAE-F10-0, Ra = 1.56 nm; PAE-F10-2, Ra = 3.50 nm; PAE-F20-1, Ra = 6.25 nm. 6
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Fig. 7. Water contact angles of self-crosslinking fluorinated polyacrylate latex films.
incorporation of self-crosslinking monomers (AA, HEA) into the system. On the contrary, the more the self-crosslinking monomer is used, the higher the crosslinking density (i.e. gel content) and the lower the contact angle of the film, which was mainly attributed to the reduction of fluorine content on the surface as mentioned above in the XPS section.
3.8. Adhesive properties analyses The results of experimental measurements of loop tack, peel strength, and shear strength of the self-crosslinking fluorinated polyacrylate emulsion PSAs are presented in Fig. 8. It is well known that peel strength decreases with an increase in the Tg of a PSA [28,29], but at the same time it can be increased by lowering the surface energy of the PSA [20,30]. The effect of the low surface energy fluorine monomer OFPMA on the 180°peel strength of PSA in this study may be a combination of these two aspects. It can be observed from Fig. 8(a) that when the amount of OFPMA was below 10 g (i.e. PAE-F0~10-1), peel strength decreased with increasing OFPMA content which may be due to an increase of Tg as shown in Fig. 3(a), where the effect of Tg on peel strength is predominant here. Nevertheless, when the amount of OFPMA was above 10 g (i.e. PAE-F10~20-1), peel strength increased with OFPMA concentration which is mainly because of the lower surface energy of the PSA caused by greater fluorine atom enrichment on the film surface aforementioned. Thus the effect of surface free energy on peel strength is predominant. Taking into account that the change in loop tack with the increase of OFPMA, as shown in Fig. 8(b), is very similar to that in the peel strength, it is reasonable to hypothesize that the above explanation for the change in peel strength should also apply to the change in loop tack. Furthermore, as can be seen from Fig. 8(c), the shear strength of the PSA was increased gradually with increasing OFPMA content due to the elevated Tg, as expected. On the other hand, by comparing the adhesive properties of PAE-F10-0, PAE-F10-1 and PAEF10-2 in Fig. 8, it is evident that the introduction of self-crosslinking monomers (AA, HEA) in the system will significantly improve the shear strength of the PSA, while at the sacrifice of both loop tack and peel strength, which was mainly attributed to the increase of gel content, as we have discussed above. This result was in good agreement with the work of Qie [4], who found that, with an increase in gel content, tack and peel strength decreased, while shear strength increased. Moreover, for weakly cross-linked adhesives, failure occurs by creep, while for highly cross-linked adhesives, failure is caused by fracture [31].
Fig. 8. Adhesive properties of self-crosslinking fluorinated polyacrylate emulsion PSA: (a) peel strength, (b) loop tack and (c) shear strength.
4. Conclusions A series of self-crosslinking fluorinated polyacrylate latexes, consisting of butyl acrylate (BA), octafluoropentyl methacrylate (OFPMA), acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA), were successfully prepared via emulsion polymerization. The main conclusions derived were as follows: (1) FTIR analysis confirmed that OFPMA successfully participated in 7
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(2)
(3)
(4)
(5)
emulsion polymerization and monomers formed the fluorinated polyacrylate copolymer. The incorporation of OFPMA has no significant effects on the particle size of the latex and gel content of the film. However, the particle size of the latex decreased and the gel content of the film increased with the addition of AA and HEA, as expected. Glass transition temperature (Tg) and thermal stability of the PSA were both enhanced with augment of the OFPMA fraction in copolymer. XPS, AFM and water contact angle measurements indicated that the fluoroalkyl groups had the tendency to enrich on the surface of the film. However, this enrichment of fluorine on the film surface was reduced after the introduction of self-crosslinking functional monomers into the system. With increasing OFPMA content, shear strength increased gradually but loop tack and peel strength decreased first and then increased. The introduction of self-crosslinking monomers (AA, HEA) in the system significantly improved the shear strength of the PSA, while at the sacrifice of both loop tack and peel strength.
2015;331:504–11. [8] Munekata S. Fluoropolymers as coating material. Prog Org Coat 1988;16:113–34. [9] Malshe VC, Sangaj NS. Fluorinated acrylic copolymers: Part I: study of clear coatings. Prog Org Coat 2005;53:207–11. [10] Thomas RR, Anton DR, Graham WF, Darmon MJ, Stika KM. Films containing reactive mixtures of perfluoroalkylethyl methacrylate copolymers and fluorinated Isocyanates: synthesis and surface properties. Macromolecules 1998;31:4595–604. [11] Mazzola M, Frediani P, Bracci S, Salvini A. New strategies for the synthesis of partially fluorinated acrylic polymers as possible materials for the protection of stone monuments. Eur Polym J 2009;39:1995–2003. [12] Hao G, Zhu L, Yang W, Chen Y. Investigation on the film surface and bulk properties of fluorine and silicon contained polyacrylate. Prog Org Coat 2015;85:8–14. [13] Xu G, Deng L, Wen X, Pi P. Synthesis and characterization of fluorine-containing poly-styrene-acrylate latex with core-shell structure using a reactive surfactant. J Coat Technol Res 2011;8:401–7. [14] Pressure Sensitive Tape Council. Test methods for pressure sensitive adhesive tapes. Northbrook, Illinois: Pressure Sensitive Tape Council; 2004. [15] Fang C, Lin Z. Effect of propyleneimine external cross-linker on the properties of acrylate emulsion pressure sensitive adhesives. Int J Adhesion Adhes 2015;61:1–7. [16] Ismail H, Ahmad Z, Yew FW. Effect of monomer composition on adhesive performance of waterborne acrylic pressure sensitive adhesives. J Phys Sci 2011;22:51–63. [17] Fitch RM, Tsai CH. Fitch RM, editor. Polymer colloids. New York: Plenum Press; 1971. p. 73–116. [18] Ugelstad J, Hansen FK. Kinetics and mechanism of emulsion polymerization. Rubber Chem Technol 1976;49:536–609. [19] Lü T, Qi D, Zhang D, Liu Q, Zhao H. Fabrication of self-cross-linking fluorinated polyacrylate latex particles with core-shell structure and film properties. React Funct Polym 2016;104:9–14. [20] Wang Y, Long J, Bai Y, Zhang C, Cheng B, Shao L, Qi S. Preparation and characterization of fluorinated acrylic pressure sensitive adhesives for low surface energy substrates. J Fluorine Chem 2015;180:103–9. [21] Li S, Zhang M, He J, Ni P. Film properties of fluorinated polymethacrylate/polyisobutylene based block copolymers. Acta Polym Sin 2014;12:1648–58. [22] Dreher WR, Jarrett WL, Urban MW. Stable nonspherical fluorine-containing colloidal Dispersions: synthesis and film formation. Macromolecules 2005;38:2205–12. [23] Dreher WR, Singh A, Urban MW. Effect of perfluoroalkyl chain length on synthesis and film formation of fluorine-containing colloidal dispersions. Macromolecules 2005;38:4666–72. [24] Xu W, An QF, Hao LF, Zhang D, Zhang M. Synthesis and characterization of selfcrosslinking fluorinated polyacrylate soap-free lattices with core-shell structure. Appl Surf Sci 2013;268:373–80. [25] Xiao XY, Liu JF. Synthesis and characterization of fluorine-containing polyacrylate emulsion with core-shell structure. Chin J Chem Eng 2008;16:626–30. [26] Fang C, Jing Y, Zong Y, Lin Z. Effect of trifunctional cross linker triallyl isocyanurate TAIC on the surface morphology and viscoelasticity of the acrylic emulsion pressure. J Adhes Sci Technol 2017;31:858–73. [27] Cui X, Zhong S, Wang H. Emulsifier-free core-shell polyacrylate latex nanoparticles containing fluorine and silicon in shell. Polymer 2007;48:7241–8. [28] Fang C, Jing Y, Zong Y, Lin Z. Effect of N,N-dimethylacrylamide (DMA) on the comprehensive properties of acrylic latex pressure sensitive adhesives. Int J Adhesion Adhes 2016;71:105–11. [29] Guo J, Severtson SJ. Optimizing the monomer composition of acrylic water-based pressure-sensitive adhesives to minimize their impact on recycling operations. Ind Eng Chem Res 2007;46:2753–9. [30] Zosel A. Adhesion and tack of polymers: influence of mechanical properties and surface tensions. Colloid Polym Sci 1985;263:541–53. [31] Sosson F, Chateauminois A, Creton C. Investigation of shear failure mechanisms of pressure-sensitive adhesives. J Polym Sci, Part B: Polym Phys 2005;43:3316–30.
Acknowledgment We express our great gratitude to the Projects from National Key Research and Development Program of China (No. 2018YFD0600405), Fund for Scientific Research of High-level (Highly educated) Talents of Nanjing Forestry University (No. GXL2018038) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for their financial support. The test supports from the teachers of Advanced Analysis & Testing Center of Nanjing Forestry University are also greatly appreciated. References [1] Benedek I, Feldstein MM. Technology of pressure-sensitive adhesives and products. London, New York: CRC-Taylor & Francis; 2009. Boca Raton. [2] Benedek I. Development and manufacture of pressure sensitive products. New York: Marcel Dekker; 1999. [3] Fang C, Yan Q, Liu Z, Lu Y, Lin Z. The influence of monobutyl itaconate and βcarboxyethyl acrylate on acrylic latex pressure sensitive adhesives. Int J Adhesion Adhes 2018;84:387–93. [4] Qie L, Dubé M. The influence of butyl acrylate/methyl methacrylate/2-hydroxy ethyl methacrylate/acrylic acid latex properties on pressure sensitive adhesive performance. Int J Adhesion Adhes 2010;30:654–64. [5] Xu W, Zhao W, Hao L, Wang S, Pei M, Wang X. Synthesis and characterization of novel fluoroalkyl-terminated hyperbranched polyurethane latex. Appl Surf Sci 2018;436:1104–12. [6] Machotová J, Černošková E, Honzíček J, Šňupárek J. Water sensitivity of fluorinecontaining polyacrylate latex coatings: effects of crosslinking and ambient drying conditions. Prog Org Coat 2018;120:266–73. [7] Zhou J, Chen X, Duan H, Ma J, Ma Y. Synthesis and Characterization of nano-SiO2 modified fluorine-containing polyacrylate emulsifier-free emulsion. Appl Surf Sci
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Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Preparation and characterization of self-crosslinking fluorinated polyacrylate latexes and their pressure sensitive adhesive applications
T
Cheng Fang*, Kai Zhu, Xinbao Zhu, Zhongxiang Lin College of Chemical Engineering, Nanjing Forestry University, Nanjing, 210037, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Fluorinated polyacrylate Self-crosslinking Emulsion polymerization Surface property Pressure sensitive adhesive
Self-crosslinking fluorinated polyacrylate latexes based on butyl acrylate (BA), fluorine monomer octafluoropentyl methacrylate (OFPMA), self-crosslinking functional monomers acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA) were synthesized by a monomer-starved seeded semi-continuous emulsion polymerization process. The latexes and their corresponding films were characterized by laser particle size analyser, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), contact angle goniometer, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results indicated that the particle size of the latexes and the gel content of the films were both independent of the amount of OFPMA employed. On the other hand, the particle size of the latexes decreased and the gel content of the films increased with the incorporation of AA and HEA as expected. Glass transition temperature (Tg) and the thermal stability of the copolymer were both improved gradually as OFPMA content increased. XPS, AFM and water contact angle measurement indicated that the fluoroalkyl groups had a tendency to enrich on the surface of the films. However, this enrichment of fluorine on the film surface was reduced after the introduction of self-crosslinking functional monomers into the system. Finally, the adhesive property of the latexes was evaluated for application as a pressure sensitive adhesive (PSA).
1. Introduction Pressure sensitive adhesives (PSAs) are viscoelastic materials that allow an instantaneous adhesion to a variety of surfaces within a short contact time and low contact pressure without any phase transition or chemical reaction [1]. In particular, acrylate PSAs have many advantages, such as excellent aging characteristics, resistance to elevated temperatures and exceptional optical clarity. Therefore they are widely used in many applications such as tape, label, protection film and medical products [2,3]. PSAs can be produced via hot-melt, solution polymerization and emulsion polymerization techniques [4]. Recently, the latter has attracted much interest, due to its relatively more environmental friendly process. Hence, the focus of the current study is on latex-based PSAs. Acrylate latex PSAs prepared by emulsion polymerization generally have high surface energy and thus poor wettability. So, how to reduce the surface energy and improve wettability of acrylate emulsion pressure sensitive adhesive has attracted the interest of many researchers. An effective method is to modify the PSA by introducing hydrophobic monomers or low surface energy monomers in block or random copolymers. This will reduce the surface energy of the PSA, which in turn
*
Corresponding author. E-mail address: [email protected] (C. Fang).
https://doi.org/10.1016/j.ijadhadh.2019.102417 Received 23 January 2019; Accepted 23 July 2019 Available online 29 July 2019 0143-7496/ © 2019 Elsevier Ltd. All rights reserved.
leads to excellent wettability. Among low surface energy monomers, the fluorinated monomers and polymers are used widely in industry due to functional groups, such as CF, CF2 and CF3 [5–7]. The surface energy can be reduced to 10–13 mN/m when the main chain is attached by the fluorine-containing groups, especially the CF3 group [8,9]. Furthermore, fluorinated polymers also have other unique properties, for instance, oil repellency, water repellency, heat resistance and antifouling properties, which are attributed to the largest electronegativity and small atomic radius of fluorine [10,11]. Recently, the application of fluorinated monomers in acrylate emulsion polymerization has been extensively studied. For example, Machotová et al. [6] investigated the effects of crosslinking and ambient drying conditions on water sensitivity of fluorine-containing polyacrylate latex coatings. And they found that the highest level of hydrophobicity at the same amount of copolymerized 2,2,2-trifluoroethyl methacrylate could be achieved in the case of noncrosslinked latex films dried at elevated temperatures, whereas the highly crosslinked latexes combining precoalescence crosslinking and keto-hydrazide self-crosslinking provided the most water whiteningresistant coating films. Hao et al. [12] synthesized a series of fluorine and silicon acrylic latexes via emulsion polymerization. And they
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discovered that fluorine content at the surface of fluorosilicone acrylic film decreased as the film forming temperature was increased. Xu et al. [13] prepared fluorine-containing poly-styrene-acrylate latexes with core-shell structures by seeded semi-continuous emulsion polymerization using the fluorine monomer Actyflon-G04 and reactive emulsifier DNS-86. They concluded that the film of latex has excellent water repellency, thermal stability, and chemical resistance properties when the amount of fluorine monomer was only 8.0 wt%. However, as far as we know, there have been no studies involving octafluoropentyl methacrylate (OFPMA) for the preparation of fluorinated acrylate latex pressure sensitive adhesives. And the effects of self-crosslinking functional monomers on the comprehensive properties, especially surface properties (e.g. F content, morphology, etc) and adhesive properties (i.e. tack, peel and shear) of fluorinated latex PSA films have also been scarcely reported in the open literature. In this paper, the synthesis of fluorinated acrylate latex PSAs based on butyl acrylate (BA) and octafluoropentyl methacrylate (OFPMA), together with the self-crosslinking functional monomers acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA) prepared via a monomerstarved seeded semi-continuous emulsion polymerization process are discussed. The effects of OFPMA on the conversion of monomer and particle size of the latex, as well as on the gel content, thermostability (DSC, TG), surface properties (XPS, WCA, AFM) of the polymer films are discussed. In addition the influence of self-crosslinking functional monomers (AA, HEA) on the migration of F, hydrophobicity and morphology of the surface of the films are also considered. Finally, loop tack, shear strength and 180° peel strength of PSA tapes were investigated to reveal the adhesive properties of the resultant fluorinated PSAs.
Table 1 Recipes for self-crosslinking fluorinated polyacrylate emulsion. Sample
PAE-F0-1
PAE-F5-1
PAEF10-1
PAEF10-0
PAEF10-2
PAEF15-1
PAEF20-1
BA/g OFPMA/g AA/g HEA/g CO-436/g APS/g NaHCO3/g DI water/g
100 0 1 2 1.5 0.5 0.15 94
95 5 1 2 1.5 0.5 0.15 94
90 10 1 2 1.5 0.5 0.15 94
90 10 0 0 1.5 0.5 0.15 94
90 10 2 2 1.5 0.5 0.15 94
85 15 1 2 1.5 0.5 0.15 94
80 20 1 2 1.5 0.5 0.15 94
with a stirring rate of 270 rpm at 82–85 °C and kept still for 30 min. Then both the remaining pre-emulsion and APS aqueous solution (0.35 g of APS was dissolved in 24 g water) were added dropwise into the reacting mixture for 3.5 h. After the feed was completed, the reaction was allowed to proceed for an additional 1 h to increase monomer conversion. The latex was then cooled to room temperature and poured into a glass bottle and used for further characterization. Hereinafter, self-crosslinking fluorinated polyacrylate emulsions are denoted PAE-Fx-y in which “x” refers to the weight of OFPMA, and “y” stands for the weight of AA, Table 1. 2.3. Characterization 2.3.1. Particle size analysis The size of the latex particles was measured using a dynamic light scattering (DLS) instrument (Malvern NanoS Zetasizer). The analyses were carried out at 25 °C, and every result was an average of three parallel measurements. The latex was diluted until the solid content was about 1%. The reported diameter is an intensity-weighted average particle size.
2. Experimental 2.1. Materials Butyl acrylate (BA), acrylic Acid (AA), 2-Hydroxy ethyl acrylate (HEA), ammonium persulfate (APS) and sodium bicarbonate (NaHCO3) were purchased from Shanghai Lingfeng Chemical Co., Ltd. and used as received. Octafluoropentyl methacrylate (OFPMA) was purchased from Shanghai Aladdin Chemistry Co., Ltd. (Shanghai, China) and used as received. Ammonium nonyl phenol ethoxylate sulfate surfactant (Rhodapex CO-436) was obtained from Shanghai Honesty Fine Chemical Co., Ltd. and used as received. Ammonia (25 wt% in H2O) was obtained from Nanjing Chemical Reagent Co., Ltd. Distilled deionized water (DI-H2O) was used throughout the study. The molecular structure of OFPMA is shown in Scheme 1.
2.3.2. Gel content determination The gel content of the acrylic PSA polymers was measured via the solvent-extraction method. Three samples (around 0.2 g) of the dried latex film were weighed and sealed in a PTFE coated membrane pouch. Then the membrane pouch was put into a Soxhlet extractor with tetrahydrofuran (THF), and then refluxed for 24 h. After the extraction process, the membrane pouch was removed and first dried in a fume hood for 3 h and then in a vacuum oven at 70 °C until it reached a constant weight. The weight of the remaining dry gel was taken and the gel content was calculated using: Gel content = mass of the dry gel/mass of the initial dry polymer (1).
2.2. Synthesis of self-crosslinking fluorinated polyacrylate emulsion 2.3.3. FTIR analysis The latex was dried in a vacuum oven at 105 °C until it reached a constant weight. Fourier transform infrared (FTIR) spectra of the dried latex films were recorded with a Bruker VERTEX80 FTIR spectrometer (Germany) in the range 4000 to 400 cm−1.
The recipes for self-crosslinking fluorinated polyacrylate emulsions are described in Table 1. For a typical experiment, 90 g of BA, 10 g of OFPMA, 1 g of AA, 2 g of HEA, 1.5 g of CO-436 and 25 g of deionized water were mixed in a 500 mL four-neck round-bottom flask and stirred vigorously to form the pre-emulsion. Another 500 mL four-neck round-bottom flask with a reflux condenser, a thermometer and a mechanical stirrer was filled with 0.15 g of NaHCO3, 6 g pre-emulsion, 0.15 g of APS and 45 g of deionized water
2.3.4. Differential scanning calorimetry (DSC) The glass transition temperature (Tg) of the latex films was measured by a differential scanning calorimeter (DSC, Model 214 Polyma, NETZSCH Instruments, Germany). 5–15 mg of dry polymer was weighed into a standard DSC hermetic alumina crucible. In order to eliminate thermal history, two scanning cycles of heating-cooling were performed for each sample at a heating rate of 10 °C/min in the temperature range of −70 to 100 °C under a nitrogen atmosphere. The second heating run was used to determine the Tg. 2.3.5. Thermal gravimetric analysis (TGA) TGA was carried out to demonstrate the thermal stability of the
Scheme 1. The molecular structure of OFPMA. 2
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latex PSA polymers using a NETZSCH 209F1 analyser, Germany. The polymer films (around 5–10 mg) were heated from ambient temperature to 600 °C at a rate of 10 °C/min under 20 mL/min nitrogen flow.
Table 2 Some properties of self-crosslinking fluorinated polyacrylate emulsion.
2.3.6. Contact angle on the latex film Water contact angles on the latex films were determined by a contact angle goniometer (Attension Theta, Biolin Scientific, Sweden) at 20 °C using sessile drop method. The used injection volume was 4 μL and the final data of each sample was an average of at least ten readings.
Latex
Conversion (wt%)
Particle size (nm)
PDI
Gel content (wt%)
PAE-F0-1 PAE-F5-1 PAE-F10-1 PAE-F10-0 PAE-F10-2 PAE-F15-1 PAE-F20-1
97.76 98.08 97.36 92.53 97.28 96.57 96.94
234.0 231.4 226.3 242.3 217.8 228.5 225.0
0.038 0.016 0.040 0.008 0.003 0.024 0.034
56.55 55.42 56.08 51.89 63.26 55.25 56.35
± ± ± ± ± ± ±
0.188 0.819 0.613 0.520 1.983 0.782 3.230
3. Results and discussion 2.3.7. X-ray photoelectron spectroscopy (XPS) analysis The X-ray photoelectron spectra (XPS) analysis was performed on an AXIS UltraDLD (UK) with Al Kα (X-ray) lamp-house. The nominal photoelectron take-off angle was 30°, and the vacuum degree of analysis chamber was 7 × 10−8 Pa. In the XPS measurement, the deviation of the binding energy was corrected by the C1s photoelectron emission signal occurring at 284.8 eV due to hydrocarbon contamination on the sample surface.
3.1. Characterization of latexes The conversion of monomer and particle size of latex as a function of OFPMA are shown in Table 2. The results indicate that all the experiments were completed successfully with conversions exceeding 96% as expected, except for latex PAE-F10-0, which was prepared without the functional monomers AA and HEA. The five latexes {i.e. PAE-F(0-20)-1} with different amounts of OFPMA had similar z-average particle sizes around 230 ± 5 nm and narrow particle size distributions (PDI < 0.04). Besides, by comparing latex PAE-F10-0 to latexes PAEF10-1 and PAE-F10-2 in Table 2, it is evident that with the addition of self-crosslinking monomers AA and HEA, the latex particle size was reduced, while the monomer conversion was increased greatly, which was mainly due to the strong hydrophilicity and consequent homogeneous nucleation [16] in the system. Qualitatively, the ability of acid monomers to induce particle formation can be explained by the theory of Fitch and Tsai [17], Ugelstad and Hansen [18]. They clarified that persulfate radicals induce polymerization in the aqueous phase producing highly carboxylated oligomers which combine and nucleate to form new particles, thus increasing the reaction rate and decreasing the particle size of the latex. On the other hand, it can be observed from Table 2 that the fluorine monomer OFPMA has no significant effect on the gel content of the acrylate latex. However, by comparing latexes PAE-F10-0, PAE-F10-1 and PAE-F10-2 in Table 2, it is evident that the incorporation of self-crosslinking monomers would increase the gel content of the latex owing to the crosslinking reaction between -COOH and -OH when heated, as shown in Fig. 1. This result was consistent with the work of Qie [4], who found that carboxyl groups reacted with OH groups at high temperature (> 120 °C), thus slightly increasing the gel content.
2.3.8. AFM analysis Surface topographies of the latex films were examined by atomic force microscopy (AFM, Dimension Edge, Bruker, Germany) operated in tapping mode at room temperature. AFM images were taken continuously with a scan rate of 1.0 Hz. Each measurement was repeated five times. The roughness (Ra) of each polymer film was the average value.
2.3.9. PSA testing [14,15] The latex was coated with RK (Manchester, UK) KHC.10.5 wire-rod coater onto 30 μm thickness, 25 mm width poly (ethylene terephthalate) strip to give a film with a dry thickness of 20 μm and dried in a vacuum oven at 105 °C for 5 min. The PET strips with adhesive coated on one side were stuck on release paper for further tests. Before the tests, the PET strips were conditioned for 24 h at standard conditions of temperature and humidity (23 ± 2 °C and 50 ± 5% relative humidity). A universal BLD-100S electronic stripping tester was used to evaluate loop tack and peel strength. For the loop tack test, the strip was formed into a loop with the adhesive side facing outwards. Approximately 25 mm at both ends of the strip was inserted into the upper grip. The instrument moved the upper grip downward at a speed of 300 mm/min until an area of 25 mm2 came into contact with the stainless steel substrate mounted into the lower grip. Next, the tester moved the upper grip upwards at the same speed while recording the force needed to detach the loop from the substrate. The maximum force of detachment was reported as loop tack. For the 180° peel test, strips of the adhesive-coated films were laminated against the stainless steel substrate using a 2 kg rubber roller. The rubber roller was passed through the PET strip front to back three times. After a 20 min dwell, 180° peel from the substrate was done at 300 mm/min. The average force per 25 mm required to peel the strip from the substrate was recorded and reported as 180° peel strength. For shear holding power, the strips were laminated against stainless steel using a 2 kg rubber roller to make a contact area of 25 mm2. After a 20 min dwell, the sample was fixed into the tester vertically with a 1 kg load suspended in the other end. Automatic timers were placed below the weights to count the time of failure. The shear holding power was the time that had elapsed between the application of the load and the completed separation of the strip from the stainless steel. Every result was an average of five parallel measurements.
3.2. FTIR analysis The chemical structures of self-crosslinking fluorinated polyacrylates were characterized by Fourier transform infrared (FTIR) spectroscopy. FTIR spectra of PAE-F0-1, PAE-F10-1 and PAE-F20-1 are shown in Fig. 2. It could be seen that all FTIR spectra exhibit the characteristic stretching peaks of C–H (CH2) at 2958, 2934 and 2873 cm−1, stretching vibration of C]O at 1730 cm−1, and distortion vibration of CH2 at 1452 and 1396 cm−1, asymmetric and symmetric stretching vibration of C–O–C at 1242 and 1159 cm−1, as well as the absorption at 1063 and 941 cm−1 resulting from characteristic of BA. However, as compared with the spectrum of fluorine-free polyacrylate (PAE-F0-1), the spectra of fluorine-containing polyacrylates (PAE-F10-1 and PAE-F20-1) show wider peaks between 1117 and 1242 cm−1. This is because the strong absorption of C–O–C is overlapped by stretching vibrations of C–F [19,20]. Moreover, there is no adsorption peak at 1641 cm−1 which is attributed to the characteristic of C]C bonds of the monomers. All these results of FTIR analysis indicated that the fluorinated acrylate monomer OFPMA could be introduced into the latex particles as desired through emulsion polymerization. 3
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Fig. 1. Self-crosslinking reaction between AA and HEA.
Fig. 2. FTIR spectra of self-crosslinking polyacrylates containing various amounts of OFPMA.
3.3. DSC analysis Glass transition temperature (Tg) is one of the most important factors affecting PSA performance. A higher Tg commonly shows the higher rigidity of the PSA chains, which might manifest itself in better elasticity and cohesive behaviour. On the contrary, PSAs with lower Tg will demonstrate better fluidity or deformability, which makes for better tack but might cause cohesive failure. Generally, PSAs with Tg values ranging from −30 to −50 °C are favored, which will show good end-use performance. Fig. 3(a) shows the DSC curves of the three representative samples with 0, 10 wt% and 20 wt% OFPMA added. It is evident that all samples show similar curves with one characteristic endothermic peak indicating the presence of a homogeneous polymer. Furthermore, from Fig. 3(a), it is observed that with the inclusion of the OFPMA fraction in the copolymers, the Tgs of the PSAs are increased. This phenomenon is attributed to the Tg of the homopolymer of OFPMA (26 °C [21]), which is higher than that of the homopolymer of BA (−54 °C, soft monomer). On the other hand, the effect of the selfcrosslinking monomers (i.e. AA and HEA) on the Tg of the polymer films was also investigated and shown in Fig. 3(b). We can see that the incorporation of small amounts of AA and HEA in the system would help
Fig. 3. DSC graphs of the latex films with different content of (a) OFPMA and (b) self-crosslinking monomer.
4
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Fig. 4. TGA graphs of the latex films with different content of (a) OFPMA and (b) self-crosslinking monomer.
Fig. 5. XPS spectra of the film of self-crosslinking fluorinated polyacrylate emulsions with different contents of (a) OFPMA and (b) self-crosslinking monomer.
increase the Tg of the polymer films as a result of the self-crosslinking reaction between carboxyl and hydroxyl groups during film formation.
characteristic signal of F1s at 688 eV appeared in the survey spectra of PAE-F10-1 and PAE-F20-1 compared with that of PAE-F0-1. Furthermore, the intensity of the fluorine signal was much higher for PAE-F20-1 than for PAE-F10-1, indicating greater fluorine enrichment on the surface of PAE-F20-1. To further investigate these tendencies, the experimental values of elemental concentration percentages of C, O and F elements on the topmost surface calculated from the survey scan and the theoretical ones calculated from the chemical formula of the monomer for the bulk were compared and are shown in Table 3. It can be observed from Table 3 that when compared to PAE-F10-1, the experimental value
3.4. TG analysis The effect of the fluorine monomer OFPMA on the thermal stability of the latex film was investigated by comparing the TGA curves of the latex with different amounts of OFPMA, and the results are shown in Fig. 4(a). It can be seen that the thermal stability of the latex film was gradually improved as the OFPMA content was increased which can be ascribed to long-chain fluoroalkyl groups. OFPMA has fluoroalkyl chains containing C–F bonds with high bond energies, which are able to shield and protect the non-fluorinated segment beneath the fluorinated segment and thus improve thermal stability. Besides, the addition of small amounts of AA and HEA in the system would enhance the thermal stability of the polymer film as expected, as shown in Fig. 4(b).
Table 3 Atomic composition of surface of the PSA films measured by XPS. Sample
3.5. XPS analysis XPS analysis can give some insights into the chemical compositions of the surface of a self-crosslinking fluorinated polyacrylate latex film and the results are shown in Fig. 5. All the XPS survey spectra revealed the strong characteristic signals of C1s and O1s at 284 and 531 eV, respectively. It can also be observed from Fig. 5(a) that a new
PAE-F0-1 PAE-F10-0 PAE-F10-1 PAE-F10-2 PAE-F20-1
5
Theoretical atomic content (wt%)
Experimental atomic content (wt%)
C
O
F
C
O
F
71.89 68.64 68.24 68.08 64.63
28.11 25.81 26.38 26.59 24.68
0 5.54 5.38 5.33 10.69
74.26 70.41 71.40 73.14 59.37
25.74 23.55 22.79 24.41 24.58
0 6.04 5.81 2.45 16.05
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of fluorine content on the surface of PAE-F20-1 was much higher than the theoretical one in the bulk. That is, the more the amount of fluorine monomer is used in the recipe, the stronger the migration of F to the surface of the film. This can be understood by the fact that fluorine atoms have extremely low surface free energy and self-aggregating characteristics, which causes fluorinated segments to be preferentially oriented to the polymer surface during the film formation so as to decrease the surface free energy of the film [22,23]. On the other hand, when comparing the experimental and theoretical values of fluorine atomic content on the surface of PAE-F10-0, PAE-F10-1 and PAE-F10-2 in Table 3, it was found that the incorporation of self-crosslinking monomers (AA and HEA) limits the migration of F to the surface of the film. This is more obvious for PAE-F10-2 with higher gel contents aforementioned, with the experimental value of F on the surface (2.45%) being much lower than the theoretical value in the bulk (5.33%). This phenomenon can be attributed to the fixation of latex polymer by cross-linking, which hinders the orientation of higher amounts of fluoroalkyl groups towards the film-air interface during film formation [6]. It is expected that the surface compositions have significant influences on both the morphology and hydrophobic property of the films, which will be discussed in the following paragraphs. 3.6. AFM analysis Atomic force microscopy (AFM) was used to examine the surface morphology of polyacrylate film, fluorinated polyacrylate film, and selfcrosslinking fluorinated polyacrylate film, as shown in Fig. 6. It can be seen from Fig. 6 that the average surface roughness (Ra) for PAE-F0-1, PAE-F10-1 and PAE-F20-1 is 1.45 nm, 3.06 nm and 6.25 nm, respectively, suggesting the introduction of OFPMA increases the roughness of the film, and the greater the OFPMA content, the higher the roughness. This phenomenon can be explained by the fact that the organic fluorine segment tends to migrate towards and enrich the film surface during film formation [24], as confirmed by the XPS results aforementioned, and micro-phase separation between fluorinated and nonfluorinated components occurs for fluorinated polyacrylate film, thus enhancing the roughness of the latex film [7,25]. On the other hand, by comparing the Ra values of PAE-F10-0 (1.56 nm), PAE-F10-1 (3.06 nm) and PAEF10-2 (3.50 nm), it was found that the surface roughness of the film increased with the introduction of self-crosslinking functional monomers into the fluorinated polyacrylate latex PSA. However, an increase of surface roughness of the PSA film with further crosslinking was not obvious. This result is believed to originate from two factors. Firstly, the introduction of cross-linking will increase the surface roughness of the film, as reported in our previous work [26]. Secondly, the fluorine content on the surface of the film was reduced with further crosslinking, as shown in Table 3, thus decreasing the surface roughness. When the crosslinking density (i.e. gel content) was high, the first positive factor partly compensates for the second negative one, and hence the increase of surface roughness was not very obvious. 3.7. Water contact angle analysis The hydrophobic property of a polymeric material can be estimated in terms of contact angle measurement by depositing a water drop on the surface of a film and the value of contact angle depends on the chemical composition of the film surface [27]. Moreover, the stronger the hydrophobicity of the film surface, the higher the water contact angle (WCA) is. The results of WCA measurements are presented in Fig. 7. By comparing the WCA values of PAE-F0-1 (102°), PAE-F10-1 (116°) and PAE-F20-1 (133°) in Fig. 7, one can see that the WCA of the polymer films increase from 102 to 133° with OFPMA content increasing from 0 to 20 wt% in the recipe due to the increment of hydrophobic fluorine content. In addition, by comparing the WCA values of PAE-F10-0 (114°), PAE-F10-1 (116°) and PAE-F10-2 (106°), it is evident that the WCA of the polymer film was not improved with the
Fig. 6. AFM images of latex PSA films: PAE-F0-1, Ra = 1.45 nm; PAE-F10-1, Ra = 3.06 nm; PAE-F10-0, Ra = 1.56 nm; PAE-F10-2, Ra = 3.50 nm; PAE-F20-1, Ra = 6.25 nm. 6
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Fig. 7. Water contact angles of self-crosslinking fluorinated polyacrylate latex films.
incorporation of self-crosslinking monomers (AA, HEA) into the system. On the contrary, the more the self-crosslinking monomer is used, the higher the crosslinking density (i.e. gel content) and the lower the contact angle of the film, which was mainly attributed to the reduction of fluorine content on the surface as mentioned above in the XPS section.
3.8. Adhesive properties analyses The results of experimental measurements of loop tack, peel strength, and shear strength of the self-crosslinking fluorinated polyacrylate emulsion PSAs are presented in Fig. 8. It is well known that peel strength decreases with an increase in the Tg of a PSA [28,29], but at the same time it can be increased by lowering the surface energy of the PSA [20,30]. The effect of the low surface energy fluorine monomer OFPMA on the 180°peel strength of PSA in this study may be a combination of these two aspects. It can be observed from Fig. 8(a) that when the amount of OFPMA was below 10 g (i.e. PAE-F0~10-1), peel strength decreased with increasing OFPMA content which may be due to an increase of Tg as shown in Fig. 3(a), where the effect of Tg on peel strength is predominant here. Nevertheless, when the amount of OFPMA was above 10 g (i.e. PAE-F10~20-1), peel strength increased with OFPMA concentration which is mainly because of the lower surface energy of the PSA caused by greater fluorine atom enrichment on the film surface aforementioned. Thus the effect of surface free energy on peel strength is predominant. Taking into account that the change in loop tack with the increase of OFPMA, as shown in Fig. 8(b), is very similar to that in the peel strength, it is reasonable to hypothesize that the above explanation for the change in peel strength should also apply to the change in loop tack. Furthermore, as can be seen from Fig. 8(c), the shear strength of the PSA was increased gradually with increasing OFPMA content due to the elevated Tg, as expected. On the other hand, by comparing the adhesive properties of PAE-F10-0, PAE-F10-1 and PAEF10-2 in Fig. 8, it is evident that the introduction of self-crosslinking monomers (AA, HEA) in the system will significantly improve the shear strength of the PSA, while at the sacrifice of both loop tack and peel strength, which was mainly attributed to the increase of gel content, as we have discussed above. This result was in good agreement with the work of Qie [4], who found that, with an increase in gel content, tack and peel strength decreased, while shear strength increased. Moreover, for weakly cross-linked adhesives, failure occurs by creep, while for highly cross-linked adhesives, failure is caused by fracture [31].
Fig. 8. Adhesive properties of self-crosslinking fluorinated polyacrylate emulsion PSA: (a) peel strength, (b) loop tack and (c) shear strength.
4. Conclusions A series of self-crosslinking fluorinated polyacrylate latexes, consisting of butyl acrylate (BA), octafluoropentyl methacrylate (OFPMA), acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA), were successfully prepared via emulsion polymerization. The main conclusions derived were as follows: (1) FTIR analysis confirmed that OFPMA successfully participated in 7
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(2)
(3)
(4)
(5)
emulsion polymerization and monomers formed the fluorinated polyacrylate copolymer. The incorporation of OFPMA has no significant effects on the particle size of the latex and gel content of the film. However, the particle size of the latex decreased and the gel content of the film increased with the addition of AA and HEA, as expected. Glass transition temperature (Tg) and thermal stability of the PSA were both enhanced with augment of the OFPMA fraction in copolymer. XPS, AFM and water contact angle measurements indicated that the fluoroalkyl groups had the tendency to enrich on the surface of the film. However, this enrichment of fluorine on the film surface was reduced after the introduction of self-crosslinking functional monomers into the system. With increasing OFPMA content, shear strength increased gradually but loop tack and peel strength decreased first and then increased. The introduction of self-crosslinking monomers (AA, HEA) in the system significantly improved the shear strength of the PSA, while at the sacrifice of both loop tack and peel strength.
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Acknowledgment We express our great gratitude to the Projects from National Key Research and Development Program of China (No. 2018YFD0600405), Fund for Scientific Research of High-level (Highly educated) Talents of Nanjing Forestry University (No. GXL2018038) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for their financial support. The test supports from the teachers of Advanced Analysis & Testing Center of Nanjing Forestry University are also greatly appreciated. References [1] Benedek I, Feldstein MM. Technology of pressure-sensitive adhesives and products. London, New York: CRC-Taylor & Francis; 2009. Boca Raton. [2] Benedek I. Development and manufacture of pressure sensitive products. New York: Marcel Dekker; 1999. [3] Fang C, Yan Q, Liu Z, Lu Y, Lin Z. The influence of monobutyl itaconate and βcarboxyethyl acrylate on acrylic latex pressure sensitive adhesives. Int J Adhesion Adhes 2018;84:387–93. [4] Qie L, Dubé M. The influence of butyl acrylate/methyl methacrylate/2-hydroxy ethyl methacrylate/acrylic acid latex properties on pressure sensitive adhesive performance. Int J Adhesion Adhes 2010;30:654–64. [5] Xu W, Zhao W, Hao L, Wang S, Pei M, Wang X. Synthesis and characterization of novel fluoroalkyl-terminated hyperbranched polyurethane latex. Appl Surf Sci 2018;436:1104–12. [6] Machotová J, Černošková E, Honzíček J, Šňupárek J. Water sensitivity of fluorinecontaining polyacrylate latex coatings: effects of crosslinking and ambient drying conditions. Prog Org Coat 2018;120:266–73. [7] Zhou J, Chen X, Duan H, Ma J, Ma Y. Synthesis and Characterization of nano-SiO2 modified fluorine-containing polyacrylate emulsifier-free emulsion. Appl Surf Sci
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