Influence of alcohols on stability and mechanical properties of polyacrylate coating material.
Acrylic resins have been widely used in coatings, (1-5) leather adhesives, (6,7) biomaterials, (8,9) and pressure-sensitive adhesives (10,11) because of their attractive properties such as weatherfastness, aging resistance, and inoxidizability. The waterborne polyacrylate prepared via conventional emulsion polymerization exhibits the disadvantages of its linear structure. In order to improve the properties of polyacrylate emulsion, several groups have modified it by the introduction of new materials including crosslinking monomers and polymerizable emulsifiers, core-shell structure, and new polymerization process. For example, Cheng and Wang (12) synthesized a novel fluorinated acrylate monomer containing hydrophilic hydroxyl and hydrophobic perfluoroalkyl group to prepare core-shell-structured polyacrylate without any organic solvents or fluorine-containing surfactants for remarkable results of surface tensions and water contact angles.
Moreover, modifying polyacrylate with nanomaterials has become one of most common methods to improve their properties. Nanomaterials such as Si[O.sub.2], (13) ZnO, (14) and Ti[O.sub.2] (15) are used in polymer-based composites by blending, in situ polymerization, or other methods. The incorporation of nanoparticles into polymer matrix has led to impressive enhancement of the basic properties and has improved photoelectricity, (16) photocatalysis, (17) thermal behavior, (18) etc. Solvents, especially acetone or alcohols, have been widely used in the preparation process of polymer-based nanocomposites such as the preparation of nanoparticles and solvation of polymers. Most studies are mainly focused on the influences of nanomaterials on the polymer, (19,20) not on the effect of solvents on the polymer.
In our previous reports, we principally focused on the effect of Si[O.sub.2.sup.6] and ZnO on the polyacrylate matrix. When we blended the sol of nanoparticles with the polyacrylate latex, we found that the solvents contained in the sol could also have a significant impact on the properties of the latex. The aim of this article is to present the effects of alcohols on the properties of polyacrylate latex. We have synthesized the polyacrylate latex and have investigated the influence of different alcohols at a certain pH on its stability and mechanical properties. The experiments demonstrated that the alcohols have great effects on latex particle size, hence, on the mechanical properties. Consequently, the polyacrylate could be strengthened or toughened only by mixing a certain alcohol instead of using other more complex methods.
Methyl methacrylate (MMA, = 99.0%), n-butyl acrylate (BA, = 99.0%), acrylic acid (AA, = 99.5%), sodium dodecyl sulfate (SDS, analytical pure), polyethylene glycol-400 (PEG-400, number molecular weight 380420 g/mol), ammonia water (25-28%, aq.), ammonium persulfate (APS, = 98.0%), and all necessary alcohols, i.e., ethanol (= 99.7%), isopropanol (= 99.7%), n-propanol (= 99.0%), and ethylene glycol (= 99.0%) from Sinopharm Chemical Reagent Co., Ltd. were used without any additional treatment. Deionized water was used throughout the experiment.
Synthesis of polyacrylate latex
The synthesis process of polyacrylate latex (PA) was as follows: 2.5 g of emulsifier (comprising 2.0 g of SDS and 0.5 g of PEG-400), 142 g of deionized water, and 1/3 mixture monomers of MMA, BA, and AA (6:12:1 by weight) were placed in a 500 mL four-necked flask with a mechanical stirrer, a thermometer, and a condenser. The flask was placed in a water bath at 50°C for 10 min, and then heated to 75°C and kept for 30 min. The residual mixture of monomers (63 g) and 60 g of APS solution (1.6%) were added dropwise into the flask within 2 h. After 2 h, the emulsion was cooled naturally.
Stability of PA incorporated with alcohols
Alcohol (2 g) and 20 g of PA were mixed with a magnetic stirring apparatus after adjusting the pH value of PA accurately from 2.50 to 9.00 by adding ammonia water with the help of a pH meter. Then the latex was used for stability investigation with the help of a centrifuge at the speed of 3000 rpm. PA in which ethanol, n-propanol, isopropanol, or ethylene glycol was incorporated will be referred to in this article as PA-ethanol, PA-propanol (for PA-n-propanol to avoid misunderstandings), PA-isopropanol and PA-ethylene glycol, respectively.
Alcohol was added into 20 g of poly (MMA-BA-AA) latex which was adjusted to the proper pH by the following procedure for film preparation. The feed ratios of alcohol were taken to be 2%, 4%, 6%, 8%, and 10% to study the influence of the amounts of alcohol on film properties.
Films with the same amount of PA were naturally dried in covered petri dishes to prevent contamination. All the dried films were placed in a vacuum oven for 24 h at 60°C before the next procedure in order to eliminate the influence of residual alcohols.
For mechanical tests, the films were cut by a dumbbell-shaped sampler into dumbbell shapes of 80 x 15 [mm.sup.2] size. The bench markers used to measure elongation were 30 mm in length, 5 mm in width, and 1 mm in thickness. The precise sizes of the specimen were measured before carrying out each test. The break strength and elongation at break were obtained using a Gotech AI-3000 Servo control tensile testing machine. To evaluate the reason for stability change with different alcohols yet further, the time-dependent behavior of the latex was investigated by the transmission profile measurements using a Turbiscan LAB (Formulaction, Toulouse, France) every minute for 30 min at 30°C. The results were given in the form of diagrams of intensity of transmission as a function of time. (21,22) A Malvern Zetasizer Nano-ZS was used to measure the average diameter of the latex particles. The film formation process of PA, PA-ethanol, PA-propanol, PA-isopropanol, and PA-ethylene glycol with the thickness of 500 µm was monitored by a Horus Film Formation Analyzer from Formulaction on glass, and the measurements were carried out at 27°C and an RH of 22%. Film morphology was inspected with a field emission scanning electron microscope (FESEM, S4800, Hitachi) operating at 20 kV.
Results and discussion
Stability of PA latex
Stability is one of the most important indexes of PA latex. In order to study the influence of the alcohols on polyacrylate latex, first the stability must be investigated. The stability of polyacrylate latex means that there is no demulsification or delamination. Data presented in Table 1 show that emulsion stability depends on alcohol type and pH value. The pure PA and PA-ethylene glycol were both stable within the pH range of 2.50-9.00. Ethanol, n-propanol, and isopropanol keep the latex steady, for pH values were higher than 3.00, 6.00, and 3.00, respectively. Further study of the latex was carried out at a constant pH value of 7.00.
The changes of particle size in time as a measure of the latex stability were determined with Turbiscan LAB equipment. Data are presented in Fig. 1 as the variation of the transmission profile ?T vs the height of the samples as measured every minute for 30 min. The interpretation of the transmission profiles was based on the change in the light transmission caused by the changes of the size of particles occurring over sample cells. When changes take place in the latex, the transmission profiles vary with the height of the sample and with time. Note that the transmission intensity profiles are meaningful only for heights higher than 1 mm (the red line in Fig. 1), which is the thickness of the container bottom. We did not notice any sedimentation on the bottom of all latex as it is proved by the uniform increase of ?T. The blue framed boxes on Fig. 1 indicate that clarification appears to a different extent on the top of all latices. In order to directly quantify the effect of alcohols on the latex stability, we used the stability coefficient (TSI--Turbiscan Stability Index) introduced by Chibowski: (23)
TSI = [square root of [?.sup.n.sub.i=1] [([x.sub.i] - [x.sub.BS]).sup.2]/n - 1],
where [x.sub.i] is the backscattering for each minute of measurement, [x.sub.Bs] is the average [x.sub.i], and n is the number of scans.
The TSI value shown in Fig. 2 suggests that alcohol incorporation reduces the stability of latex in various degrees. The PA latex was found to be the most stable. N-Propanol slightly labilizes the latex, whereas ethylene glycol significantly deteriorates the latex stability. Ethanol and isopropanol share the similar changes. Latex particle size is really important for the latex stability because it is connected with the particle ordering and particle deformation. Increase or decrease of a latex particle leads to swell or collapse which results in the change in stability. Latex particles and the aqueous medium with alcohols can be viewed as an osmotic system. Initially, there was no alcohol inside the particles, so the particles were in a hypertonic solution and would be extruded. According to the Morse equation, osmotic pressure has an inverse relationship with molar mass. That is normal because the penetration process of alcohols into latex particles is influenced by the molecular structure. Figure 3 shows the distribution of particle size for polyacrylate/alcohol composite latex with a uniform particle distribution. The average particle sizes of PA latex, PA-ethanol, PA-propanol, PA-isopropanol, and PA-ethylene glycol are 186, 174, 184, 172, and 170 nm, respectively. The addition of n-butyl alcohol and glycerol result in demulsification, so the particle size could be regarded as of infinite size. DLS dates are in good agreement with TSI measurements. The change in particle size correlates directly to the influence on stability (TSI value). For example, the effect of ethylene glycol on latex stability is most obvious and the particle size of the PA-ethylene glycol undergoes the largest change, specifically, from 186 to 170 nm. In the case of PA-ethanol and PA-isopropanol, the TSI curves are close to each other and have similar particle sizes, 174 and 172 nm, respectively. Both stability and particle size of latex can be attributed to the carbon chain length and hydroxy amount. Ethanol, isopropanol, and ethylene glycol have the same backbone length, and the particle sizes of the corresponding composite latex are all around 172 nm. Glycerin containing three OH groups leads to latex demulsification. Polyacrylate latex particles are squeezed, probably by the strong water absorption of glycerin and the destruction of surfactant stabilization. The particle sizes of PA latex, PA-ethanol, and PAethylene glycol composite latex are 186, 174, and 170 nm, respectively, which indicates that the particle size decreases with the increase of the amount of hydroxyl groups. It is assumed that the alcohols would embed in the surface and swell into the latex particles to make the particle shrink, as shown in Fig. 4. Alcohols and surfactants have a similar structure (hydrophobic end and hydrophilic end), so a certain amount of alcohols would insert among the surfactant. Alcohols could change the osmotic pressure of the latex particle surface as they work as cosurfactant in miniemulsion. The change in the osmotic pressure will result in the deformation of the latex particle surface, which leads to changes in the particle size and affects latex stability. In addition, alcohols that swelled into the latex particle will make the conformation of the PA chains stretched or gathered like solvents.
Effect of alcohol on physical properties of latex films
The alcohol types and their amounts in the latex have a significant effect on the break strength and elongation at break as shown in Figs. 5a and 5b, respectively. Ethanol, isopropanol, and n-propanol strengthen the films. For example, break strength shows a maximum for 2 wt% addition of n-propanol but decreases for further increase of the n-propanol amount. Isopropanol behaves similarly but its influence on mechanical properties is slighter than that in the latter case. Ethylene glycol weakens the film capability to maintain a certain degree of break strength (Fig. 5a).
The elongation at break is shown in Fig. 5b. Obviously, the elongation at break shows a trend of leveling off for isopropanol, whereas n-propanol shows a slight ascent followed by a decrease at large content. Ethylene glycol increases the elongation at break to at least 680%, for 8 wt% ethylene glycol, and rises up to 1658% for higher content, while that of the pure latex shows an elongation at break of 483%. The lowest elongation at break, i.e., around 350%, is displayed by ethanol. To summarize, every alcohol has different effects on mechanical properties. Ethylene glycol enhances the elongation at break but weakens the break strength. Ethanol displays opposite effects. Isopropanol and n-propanol seem to improve both the break strength and elongation at break.
There are tight connections between the film-forming process of latex and boiling points and hydrophilia of alcohols. For example, if the boiling point of the alcohol is low, the latex film will be formed easily. If the alcohol is too hydrophilic, it makes the latex film hard to get dry. The kinetics of the latex film-forming process is shown in Fig. 6. To explain the different variations observed on the kinetics, we divided the duration into four stages necessary for the sample to turn to dry (see Table 2): open time, dust free, set to touch, and touch dry, as obtained automatically. Actually, after Brun, the four stages represent the evaporation, particle ordering, particle deformation, and interdiffusion of latex. Compared with the PA, the first three stages of PA-ethanol were slightly longer. However, the last stage lasted much longer than previous ones, which indicates that the latex particles had sufficient time to interact with each other to form a compact film (see Fig. 7b). The evaporation and particle ordering time of PA-propanol was shortened by half, but the particle deformation and interdiffusion time was doubled. That means the introduction of n-propanol made the latex particles reorganize to form close-packed arrays quickly and kept the arrays firmly within plenty of time (see the "layered structure" on Fig. 7c). In the case of PA-isopropanol, the first two stages (evaporation and particle ordering) were shortened, leading to the formation of a slightly "layered structure" (Fig. 7d). The length of the third stage cost of PA-isopropanol was almost the same as that of PA, but the last stage was greatly increased. Therefore, the film had enough time to decrease the roughness of the cross-section. According to the data in Table 2, the first two stages were slightly longer than that of PA, because ethylene glycol has two hydrophilic hydroxyl groups and a high boiling point. The stage of particle deformation (set to touch) was shorter and the stage of interdiffusion (touch dry) was much longer, which means that ethylene glycol gives the polyacrylate latex particles not enough time for deformation as the basis of compact structure. Figure 7e also shows that the film indeed presents a loose structure with small pores. This special construction resulted in low break strength and high elongation at break.
The polyacrylate/alcohol composite latex was prepared by mixing the alcohols into the latex directly at a suitable pH. The composite can only be steady at a certain range of pH. The pure latex and polyacrylate/ ethylene glycol latex were both stable within the pH range of 2.50-9.00. Ethanol, n-propanol, and isopropanol keep the latex steady when the pH was higher than 3.00, 6.00, and 3.00, respectively. The latex particle size is influenced by the molecular structure of alcohols. Alcohols affect the process of film forming and lead to the variation of film structure and properties. Ethylene glycol enhances the elongation at break and weakens the break strength, while ethanol is just the reverse. Isopropanol and n-propanol improve both the break strength and elongation at break. Isopropanol increases the water absorption rate because of the surfactant on the surface. All these results indicate that the polyacrylate latex has strong responsiveness to alcohols. The effects of mixed alcohols or other solvents on the properties of polyacrylate latex need to be further investigated.
D. Gao, W. Zhang, J. Ma ([mail]), C. Li
College of Resources and Environment, Shaanxi University of Science and Technology, Xi'an 710021, Shaanxi Province, PR China
Culture and Communications School, Shaanxi University of Science and Technology, Xi'an 710021, Shaanxi Province, PR China
Acknowledgments This work was supported by the Science and Technology Research and Development Program of Shaanxi Province (2012KJXX-31), the Scientific Research Program of Shaanxi University of Science & Technology (TD12-03), and the Graduate Innovation Fund of Shaanxi University of Science and Technology (YC0608).
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Table 1: PA latex stability for various alcohols as measured at different pH values PA-ethylene pH PA PA-ethanol PA-propanol PA-isopropanol glycol 2.50 + - - - + 3.00 + + - + + 3.50 + + - + + 4.00 + + - + + 4.50 + + - + + 5.00 + + - + + 5.50 + + - + + 6.00 + + + + + 6.50 + + + + + 7.00 + + + + + 7.50 + + + + + 8.00 + + + + + 8.50 + + + + + 9.00 + + + + + + Stable, - Demulsificated Table 2: The duration of stages that the samples turn to dry Sample Open time Dust free Set to touch Touch dry PA 48" 332" 160" 145" PA-ethanol 57" 393" 129" 661" PA-propanol 18" 192" 461" 399" PA-isopropanol 28" 192" 124" 690" PA-ethylene glycol 70" 350" 120" 436"
Please note: Some tables or figures were omitted from this article.
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|Author:||Gao, Dangge; Zhang, Wenbo; Ma, Jianzhong; Li, Congmin; Zhang, Jing|
|Publication:||Journal of Coatings Technology and Research|
|Date:||Nov 1, 2015|
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