Molecular design and application of divinyl monomers to synthesis of UV-curable latex.
Keyword Divinyl momomer, UV-curable
In recent years, increasing social and political awareness, coupled with the tightening of worldwide environmental legislation, has forced coating industries to decrease levels of pollutant substances released into the atmosphere. (1) Photo polymerizable resins are now being increasingly used in various applications to replace conventional thermally cured solvent-based coatings and adhesives, (2) However, these resins are combustible, therefore reduction of the amount of polluting solvents and monomers in radiation curable coatings may be achieved by waterborne UV-curable technology, which combines the advantages of water-borne (3-5) and UV (6-8) technologies.
The UV-curable polymers that had been reported were prepared by (1) conjugation of vinyl monomers with polymers containing pendant functional groups, (9-11) (2) polymerization of supramolecular complexes, (12) and (3) copolymerization with crosslinkers containing multiple unsaturated groups differing in reactivity. (13), (14) The first approach is limited, however, by intricacy of technical process while the latter is limited by the choice of monomers and crosslinkers.
Therefore, the goal of our work was to investigate a new method of simplifying the preparation of unsaturated latex. For this purpose, four divinyl monomers whose double bonds had different polymerization reactivity were designed, and then properties were calculated based on density functional theory (DFT) using standard B3LYP/6 - 31g(d), B3LYP/6 - 31 + g(d,p), and B3Iyp/6-311 + g(3df,2pd) method and basis set combinations. One of the monomers was selected to be the only one whose active double bond is prone to polymerization, while the inactive double bond would be left for further UV-curing. The inactive double bond could also be activated by forming a charge transfer complex (CTC) polymerization system with electron-donating comonomers, and its activity would be in direct proportion to the electron-donating ability of comonomers. Therefore, the degree of resin saturation can be easily controlled by comonomers. In addition, the UV-curing performance of the unsaturated latex membrances was studied.
To investigate a new method for simplifying the preparation of UV-curable latex, four divinyl monomers were designed as shown in Fig. 1. These four divinyl monomers are (Z)-4-(2-(aeryloyloxy)ethoxy)-4-oxobut-2-enoic acid (d,s-AEOEA), (Z)-4-(2-(methacryloyloxy)ethoxy)-4-oxobut-2-cnoic acid (cis-MAJEOEA). (E)-4-(2-(acryloyloxy)ethoxy)-4-oxobut-2-enoic acid (trans-AEOEA), and (E)-4-(2-(methacryloyloxy)ethoxy)-4-oxobut-2-enoic acid (trans-MAEOEA). whose C = C double bonds are different in polymerization reactivity. The active C3 = C4 is prone to polymerization, while the inactive C1 = C2 is left for further UV-curing. In addition, the inactive C1 = C2 double bond can be activated by forming CTC polymerization systems with electron-donating comonomers, and its activity is in direct proportion to the electron-donating ability of comonomers. Therefore, the unsaturation degree of resins can be controlled easily by comonomers.
[FIGURE 1 OMITTED]
The designed divinyl momomers should not form self-CTC polymerization systems, otherwise the inactive C1 = C2 doubled bond will be activated by the divinyl monomer for further UV-curing. Therefore, the divinyl monomers will be selected based on their molecular properties, and the effects of selected divinyl monomers on the preparation of UV-curable latex were investigated in this work.
Styrene (St), methyl methacrylate (MMA), n-butyl acrylate (BA), and n-butyl methacrylate (BMA) were purchased from Beijing Chemical Factory, and were purified via vacuum distillation. Cis-AEOEA was synthesized by esterification of maleic anhydride with hydroxyethyl acrylate (HEA), and used without further purification. Ammonium persulphate (APS), sodium hydrosulphite (SHS), and sodium dodecylsulfate (SDS) were purchased from Longxi Fine Chemical, and were used without further purification. All other chemicals were used as received. Distilled and deionized water was used throughout the work.
Synthesis of cis-AEOEA
Hydroxyethyl acrylate (116 g) and maleic anhydride (98 g) were mixed with continuous stirring in 100 mL of CHC13 in a 500 mL three-neck, round-bottom flask, with a condenser attached to the flask. The flask was closed with rubber septa, purged with N2, and kept in a water bath to keep the temperature at 40[degrees]C. The final mixture was then allowed to react for 30 days. After completion of the reaction, the mixture was evaporated using a rotary evaporator to remove CTIC13 at room temperature. The residue was then extracted with water and the organic layer was separated and extracted more than two times to remove the unreacted hydroxyethyl acrylate and maleic anhydride. The final product was dried in a vacuum oven at 35[degrees]C for two days.
Preparation of UV-curable latex using cis-AEOEA
The UV-curable latex was prepared by emulsion polymerization using APS and SHS as the initiators and SDS as the surfactant. A solution of water (263 g) and SDS (0.5 g) was placed in the reaction flask, and the flask was closed with rubber septa attached with a reflux condenser and equipped with a steel paddle-type stirrer. The solution was kept in a water bath so the temperature could be adjusted. The monomer mixture (50 g) and initiator solution (APS and SHS combined in 10 g of water separately) were fed continuously through separate valves to the reaction flask in 2 h. After that, the latex was kept at polymerization temperature for an additional two hours to increase the monomer conversion. The latex was then cooled to room temperature and filtered. All these processes were repeatedly degassed and purged with [N.sub.2].
A photoinitiator (Darocur 1173) was added to the latex (calculated on solids content). The mixture was allowed to stand overnight. Films were prepared from the latexes by casting onto clean glass plates and air drying at room temperatures. The films were cured in air and at room temperature by exposure to UV light from an RW-UV.2BP curing unit. The unit contained one medium pressure mercury lamp operating at 120 watts/cm. The distance between the latex membrane and the UV lamp is about 15 cm.
Gel content measurement
The prepared UV-Curable latex was dried at room temperature in a vacuum for 7 days, and the polymer membranes were cut into 2 cm x cm (weight = [W.sub.0]) and then put into the Soxhlet extractor with acetone as the solvent for 72 h. The rest of membranes were dried for 72 h at 25[degree]C and 1-2 mm Hg atmosphere (weight = W). The gel content was then calculated according to the following formula (15).
FTIR spectra were recorded on a Bruker Tensor 37 FTIR spectrometer using pressed KBr plates.
Pendulum hardness measurements
The measurements were run using a pendulum hardness tester (QBY-II, Tianjin Building Instrument & Testing Machine Company, China) with films of 100 [micro]m thickness on glass. The average time of oscillations was obtained at least five replicated measurements. (16)
Viscosities were determined using an automatic viscosimeter DV-1 (Shanghai Qunchang Science instrument Ltd.) with a 0# rotor.
Particle size and polydispersity index
Particle size and the polydispersity index (PdI) were determined using Malvern Nano-ZS Zeta Sizer equipment.
Functional calculations of quantum chemical density were carried out with the 03 version of a Gaussian suite of programs (17) using the B3LYP functions (18), (19) combined with the standard 6-31g(d), 6-31 + g(d,p), and 6-311 + g(3df,2pd) basis seta. The vibrational frequencies were computed at optimized geometry and used to characterize stationary points as stable states (number of imaginary frequencies is 0). All the calculations refer to isolated molecules in a vacuum.
Results and discussion
Molecular properties of designed divinyl monomers
Polymer architecture governs its properties and performance (20) and polymer architecture is controlled by monomer structure. Therefore, the geometries of divinyl monomers designed in this paper were optimized, and their C=C bond length and frontier orbital energies were used for the selection of designed divinyl monomers.
To prepare the unsaturated latex, the designed divinyl monomers could not form self-CTC polymerization systems, otherwise the inactive C1 = C2 double bond would be activated by divine monomers and could not be left in polymer form for further UV-curing. Therefore, the designed monomer should have lower electron-donating and higher electron-accepting ability. One method of estimating electron donating and accepting abilities is to compute them as energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The effects of basis sets on calculated frontier molecular orbital energies are given in Table 1.
Table 1: Frontier orbital energies of designed divinyl monomers Compounds Orbital 6-31G(d) 6-31 + G(d.p) 6-311 + G(3df,2pd) cis-AEOEA HOMO -0.27623 -0.29104 -0.29279 LUMO -0.11116 -0.12685 -0.12387 cis-MAEOEA HOMO -0.27247 -0.28506 -0.28690 LUMO -0.11021 -0.12461 -0.12166 trans-AEOEA HOMO -0.26217 -0.28043 -0.28194 LUMO -0.08416 -0.0999 -0.09790 trans-MAEOEA HOME -0.27730 -0.28724 -0.28892 LUMO -0.09360 -0.11013 -0.10835 All values are in hartrees
It can be seen that, at the B3LYP/6-31G(d)calculation level, the lowest energies of HOMO and LUMO were found with trans-MAEOEA and cis-AEOEA, separately. With the increase of calculation precision, the lowest energies of HOMO and LUMO were both found with cis-AEOEA. Therefore, cis-AEOEA should not form self-CTC polymerization systems in these four designed divinyl monomers.
In addition, the surfaces of the HOMO and LUMO calculated at the B3LYP/6-311 + G(3df,2pd) level are plotted in Fig. 2. There is obviously a difference between both plots. The HOMO of cis-AEOEA is mainly a C3 = C4 double bond and oxygen atoms in an acrylate segment. While the LUMO is principally on the maleic acid segment.
[FIGURE 2 OMITTED]
Therefore, the inactive C1 = C2 double bond will be activated by forming CTC copolymerization systems with electron-donating comonomers for the electron-accepting capability of maleic acid segments, and its activity will be in direct proportion to the electron-donating ability of comonomers.
For the purpose of curving the UV-curable latex, the reserved C1 = C2 double bond should be curved under UV irradiation. Therefore, the reserved C1 = C2 double bonds with higher reactivity are suitable for further UV-curving. One method of evaluating the polymerization activity of double bonds with the same structure is to compute them in bond length, and the polymerization activity is enhanced with the increase in bond length. The effect of basis sets on calculated C = C bond lengths are given in Table 2.
Table 2: Bond length of carbon-carbon double bonds Compounds Bond type 6-31G(d) 6-31 + G(d,p) 6-311 + G(3df,2pd) cis-AEOEA C1=C2 1.34801 1.34904 1.34019 C3=C4 1.33599 1.33787 1.32851 cis-MAEOEA C1=C2 1.34825 1.34901 1.34015 C3=C4 1.33892 1.34057 1.33115 trans-AEOEA C1=C2 1.33977 1.34106 1.33171 C3=C4 1.33674 1.33835 1.32910 trans-MAEOEA C1=C2 1.33815 1.33941 1.33033 C3=C4 1.33883 1.34097 1.33190 All values are in [Angstrom]
From these, is it clear that the bond length of C = C was influenced by conformation and the CH3 - group bonded to C3 = C4. At all computational levels, the longer bond length of C1 = C2 was found with the cis conformation, and longer bond length of C3 = C4 was found within the designed monomers whose C3 = C4 double bond was attached to a CH3-group. Therefore, the cis conformation divinyl monomers are suitable for the curing of UV-curable latex due to their longer C1 = C2 bond length.
Therefore, cis-AEOEA was selected as the only designed divinyl monomer whose double bonds are different in terms of polymerization activity. The C3 = C4 double bond is prone to polymerization, while C1 = C2 is left in polymer form. The polymerization activity of C1 = C2 is improved when copolymerized with electron-donating comonomers. Moreover, the reserved C1 = C2 double bonds can be cured under UV irradiation.
Orthogonal experimental design
An orthogonal experimental design was used to prove the result mentioned above, as well as evaluate the effects of polymerization temperature and dosage of initiator. All the factors and levels are shown in Table 3.
Table 3: Factors and levels of orthogonal experiment L[9.sub.3.sup.4] Factors Level 1 Level 2 Level 3 1 Comonomer BA BMA St 2 Ratio of comonomers and cis-AEOEA 0/1 2/1 4/1 3 Polymerization temperature ([degree]C) 60 70 80 4 Initiator dosage (%) 1 2 3 The electron-donating ability of comonomers is in increasing order
If the C1=C2 is activated, the reserved C=C double bonds will decrease and the crosslinking density of the copolymer will increase. Therefore, the gel comparing the polymerization activity of C1=C2. The gel contents of each treatment are shown in Table 4, and the average value of levels and range of factors were calculated. From the intuitive analysis, the effects of factors and levels on the polymerization activity of C1-C2 double bond were assessed.
Table 4: Experiment results and intuitive analysis of orthogonal experiment [L9.sub.3.sup.4] Factor 1 Factor 2 Factor 3 Factor 4 Gel contents (%) Experiment 1 1 1 1 1 63.05 Experiment 2 1 2 2 2 87.72 Experiment 3 1 3 3 3 85.33 Experiment 4 2 1 2 3 68.39 Experiment 5 2 2 3 1 91.03 Experiment 6 2 3 1 2 93.19 Experiment 7 3 1 3 2 64.47 Experiment 8 3 2 1 3 98.38 Experiment 9 3 3 2 1 98.57 Average value 1 78.70% 65.30% 84.87% 84.22% / Average value 2 84.20% 92.38% 84.89% 81.79% / Average value 3 87.14% 92.36% 80.28% 84.03% / Range 8.44% 27.07% 4.62% 2.42% /
The ratio of comonomer and cis-AEOEA was the factor with the most impact on the gel content. The comonomer type was the next most important factor in the gel content. The effect of the other factors is slight. Furthermore, all the lower gel contents were found for treatments whose ratio of comonomer and cis-AEOEA was 0/1. and their average value is the lowest. It is clear that gel content increased with the improvement of the electron-donating ability of comonomers. Therefore, the C3=C4 double bond in ci-AEOEA is active while the C1=C2 is inactive, and the activity of C1=C2 would be improved with an increase in the electron-donating ability of comonomers.
It is said that cis-AEOEA can be used for preparing unsaturated polymers for UV curing, and the unsaturated degree of resin can be controlled by comonomers. In addition, the UV-curing performance of cis-AEOEA latex will be discussed in the following sections.
IR spectroscopy of UV-curable latex membranes
The FTIR spectra of latex membranes before and after UV irradiation are presented in Fig. 3.
[FIGURE 3 OMITTED]
As shown in Fig. 3, the characteristic C=C vibration at 1639 [cm.sup.1] (see peak 1) decreased and disappeared with UV irradiation. Therefore, the FTIR analyses support the fact that cis-AEOEA can be used for preparing unsaturated latex, and that the latex membrane can be cured under UV irradiation.
Fig. 3: FTIR spectra of latex membranes
Effect of photoinitiator content on the gel content of UV-cured latex membranes
For UV-curable systems, the photoinitiator concentration substantially influences the polymerization rate and the final conversion, as well as the properties of latex membranes. The effect of photoinitiator concentration on the gel content of UV-cured latex membranes is illustrated in Fig. 4. The irradiation time is 30 s.
[FIGURE 4 OMITTED]
When the dosage of the photoinitiator was increased from 0 to 2.3 wt%, the gel content increased and then decreased. The largest gel content was found with a photoinitiator content of 1.2%.
The effect of cis-AEOEA content
Both C=C double bonds and COOH unit are contained in the designed cis-AEOEA. One of the two double bonds C14-C14 is used for polymerizing, while the other C1=C2 is reserved in polymer form for further UV-curing. On the other hand, COOH content has a significant influence on the stability of latex. Therefore, the concentration of cis-AEOEA substantially influences not only the degree of saturation and the final material properties, but also the stability of latex. The effect of cis-AEOEA content on the particle size and PdI of latexes is illustrated in Fig. 5.
[FIGURE 5 OMITTED]
Figure 5 shows the relationship between the cis-AEOEA content and latex particle size. It was found that when increasing the dosage of cis-AEOEA from 5 to 26 wt%. the latex particle size increased and then leveled off. In addition, latex PdI increased with the increasing of cis-AEOEA content. Therefore, the dosage of cis-AEOEA should not go beyond 20%.
[FIGURE 6 OMITTED]
Moreover, cis-AEOEA content influences the degree of saturation and the final material properties. Therefore, the effect of cis-AEOEA content on the hardness of UV-cured latex membranes is shown in Fig. 6.
Figure 6 shows the relationship between the membranes hardness and irradiation time. It was found that hardness increased with irradiation. The hardness of membranes also increased with an increase of cis-AEOEA content. This was attributed to the increased degree of unsaturation with the increase in cis-AEOEA.
A new method for the preparation of UV-curable latex suitable for coating applications was investigated. Four divinyl monomers were designed, and cis=AEOEA was selected as the only nonelectron-donating divinyl monomer whose C3=C4 double bond was prone to polymerization while the C1=C2 double bond was left in polymer form for further UV-curing, therefore, the preparation of UV-curable latex was simplified and the properties of UV-cured latex membranes were excellent.
Acknowledgment We thank Professor Wenchuan Wang for the guidance of quantum chemistry calculation.
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S. Chen, X.Y.Li ()
College of Materials Science and Engineering Beijing University of Chemical Technology, No. 15, Beisanhuan East Road, Beijing 100029, people's Republic of China e-mail: Lixy@mail.buct.edu.cn
Division of Molecular and Materials Simulation. The Key Laboratory for Nanomaterials, Ministry of Education, No. 15, Beisanhuan East Road, Beijing 100029. people's Republic of China
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|Author:||Chen, Song; Li, Xiao Yn|
|Date:||Dec 1, 2008|
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