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Preparation, properties and application of waterborne hydroxyl-functional polyurethane/acrylic emulsions in two-component coatings.

Abstract A serious of waterborne hydroxyl-functional polyurethane/acrylic (PUA) emulsions for two-component coatings was prepared by copolymerizing different weight ratios of hydroxyethyl acrylate (HEA)/butyl acrylate (BA) with methyl methacrylate and acrylic-terminated polyurethane dispersion via an emulsion process. The results showed that increasing the weight ratio of HEA/BA led to a decrease in the average particle sizes and a narrowing in the particle size distribution, but an increase in the viscosity of their emulsions. Meanwhile, their films showed increased thermal resistance, adhesion on substrate, tensile strength, and hardness, but decreased water resistance and enhanced wettability. When the emulsions were applied in two-component coatings, their water resistance was greatly improved and the wet-tability was reduced; in addition, their glass transition temperature, thermal stability, adhesion on substrate, tensile strength, and hardness were further increased. It was suggested that hydroxyl groups of PUA can not only engage in the intramolecular and/or intermolecular hydrogen bonding, but also form covalently crosslinked networks with polyisocyanate in their films.

Keywords Polyurethane/acrylic emulsion, Hybrid material, Two-component coatings

Introduction

Polyurethanes (PU) dispersion is an important class of binders for coating applications due to its film with excellent properties, such as adhesion on substrates, elasticity, and abrasion resistance. However, PU dispersion has been in general expensive, which limits its application in some cases. In order to get the best balance of properties at a reasonable cost, the combination of PU with a low cost material is now a common routine in the coating market. (1) Acrylic polymer (AP) is also a useful binder in aqueous coatings due to its prominent outdoor stability and low cost. Therefore, the combination of PU and AP to prepare waterborne polyurethane/acrylic (PUA) hybrid emulsions has attracted a great deal of interest both in academia and in industry. (2-7)

Physical blending of aqueous PU and AP emulsions is an easily handled approach to prepare PUA emulsions. However, the incompatibility between the two polymers causes their phase separation in many cases, which results in films with poor qualities. (2,3) To achieve a forced compatibility between PU and AP, the use of hydroxyethyl acrylate (HEA) to build up chemical bonds has become a common practice. In this process, acrylicterminated PU is first synthesized by the reaction of HEA with isocyanic (NCO)-terminated PU prepolymer (polycondensation of diisocyanate, dimethylolpropionic acid, hydroxyl-terminated polyether or polyester) and then the carboxyl group is neutralized with triethylamine. After dispersing with water, the double bond in PU remains active and serves to copolymerize with subsequently added acrylic monomers to prepare PUA hybrid emulsions by batch, semi-batch, or continuous emulsion polymerization process. (3-7) Obviously, the chemical structure of PUA is the PU linkage grafting with the AP segment and this material shows better properties than the blending. As to the PUA system, AP contributes to the hard segment domains which are relatively readily miscible with the hard segments of PU through chemical bond and hydrogen bonding. (7)

To achieve the films with super appearance, generally, a small amount of polyisocyanate hardener (PIH) has been used as a crosslinker in PU or AP dispersions to prepare two-component (2-K) coatings. (1,8-10) In the case of this system, the formed film with high crosslinking density can be obtained due to the reaction between PIH and the active hydrogen (hydroxyl group, amine group, or carboxyl acid) in the polymers. PUA bears carboxyl acid, but the film-forming temperature is very high. (9) Therefore, the preparation of hydroxyl-functional PUA is necessary for this application.

HEA has always been used to prepare hydroxylfunctional polymers. (11-13) The hydroxyl group of HEA not only reacts with PIH to form covalently crosslinking networks, but also engages in intramolecular and/or intermolecular hydrogen bonding with carbonyl, ether, or other electronegative acceptor functionality in the polymeric states. In this work, hydroxyl-functional PUA emulsions for 2-K coatings were prepared by copolymerizing HEA with butyl acrylate (BA), methyl methacrylate (MM A), and aery lie-terminated PU in the presence of an initiator via the emulsion process. To keep a fixed weight ratio of PU/AP based on constant content MMA and PU, HEA was matched with BA and varying weight ratios of HEA/BA were used in the preparations. The effects of HEA on the properties of PUA emulsions and their films cured with and without PIH were investigated. In addition, the intermolecular hydrogen bonding and covalently crosslinked networks in the films are also discussed.

Experimental

Materials

Polypropylene glycol (PPG, [M.sub.n] = 2000 g/mol) was obtained from Shandong Dongda Chemical Co. Ltd., China. Isophorone diisocyanate (IPDI) was supplied by Rongrong Chemical Co., China. Polyisocyanate hardener (PIH, trimer of hexamethylene diisocyanate, -NCO content was 21.8 [+ or -] 0.3%) was bought from Haoyi Chemical Co., China; dimethylolpropionic acid (DMPA), triethylamine (TEA), dibutyltin dilaurate (DBTDL), and N,N-dimethylformamide (DMF) were bought from Tianjin Chemical Regent Co. Ltd., China. MMA, BA, and HEA were provided by Nanjing Wairui Chemical Co. Ltd., China. All of the acrylates were dehydrated by immersion in 4A molecular sieves for more than 2 days. Potassium persulfate ([K.sub.2][S.sub.2][O.sub.8]) was used as received. Deionized water was used throughout the work.

Preparation of PUA hybrid emulsions

The preparation of PUA emulsions was carried out according to the procedure shown in Fig. 1.

Acrylic-terminated PU dispersion

For two hours, 117.0 g (0.06 mol) of PPG was dried at 110[degrees]C to remove the moisture in a 1000 mL four-necked glass reactor equipped with a mechanical stirrer, a thermometer, a nitrogen inlet, and a condenser. After the temperature was reduced to 80[degrees]C, 42.0 g (0.19 mol) of IPDI was added and reacted with PPG in the presence of 0.1 g of DBTDL for 2 h under stirring at nitrogen atmosphere. As a chain extender with the carboxyl acid group, 11.4 g (0.09 mol) of DMPA was predissolved in 11 mL of DMF and then added into the system to obtain NCO-terminated PU. This reaction proceeded at 85[degrees]C until the NCO reached the theoretical value which was determined by the dibutylamine back titration. (3) To synthesize the acrylic-terminated PU, 13.6 g (0.12 mol) of HEA was added to react with NCO-terminated PU for 4 h at the maintained temperature. After the system was cooled to 50[degrees]C, 49.0 g of MMA was used to reduce the viscosity and 8.6 g (0.09 mol) of TEA was added to neutralize the carboxylic acid in PU macromolecules. 450 mL of deionized water was added slowly under vigorous stirring (ca. 1000 rpm) to prepare PU dispersion with a theoretical solid content of 26 wt% (note: MMA was not included in the solid weight).

PUA hybrid emulsions

The weight ratio of PU/PA was kept at 60:40 for the preparation of all the PUA hybrid emulsions. Therefore, varying HEA/BA weight ratio from 0/10 to 10/0 (total weight of HEA and BA was 57 g) was used to copolymerize with a fixed amount of the above acrylicterminated PU dispersion in the presence of [K.sub.2][S.sub.2][O.sub.8]. The mixture of BA and HEA was added into the above PU dispersion and stirred for 24 h at room temperature to swell the PU colloidal particles. After the temperature was raised to about 75[degrees]C, 40 g of 1.5 wt% aqueous [K.sub.2][S.sub.2][O.sub.8] solution was added with the feeding rate of 13 mL/h under stirring and then the reaction continued for another 2 h at 85[degrees]C. Adjusted with water, PUA hybrid emulsions with about 31 wt% solid content were obtained.

Application in two-component coatings and preparation of their films

In a 500 mL flask, 100 g of PUA emulsion was mixed with 2.8 g of PIH under high speed stirring for 10 min to prepare 2-K coatings. For these coatings, the mole ratios of NCO/OH ranged from 1/90 to 1/450.

The mixture was cast on a standard PTFE mold at wet thicknesses of 1 mm and then dried at 50[degrees]C for 5 h and 80[degrees]C for 4 h. After demolding, the films were kept in a desiccator to be used for various characterizations. In a similar process, the black PUA films were prepared to compare their properties with the above films.

Measurements and characterization

The average particle sizes, average specific surface area of particle, and particle size distribution index (PDI) of the PUA emulsions were measured by a Berttersize-2000 laser particle size distribution analyzer (PADA, China) at room temperature.

The viscosity of hybrid emulsions was tested by an NDJ-79 rotation viscometer (China) at 25[degrees]C with a spindle speed of 100 rpm.

The FTIR spectrum of the sample was determined by a Thermo Nicolet-360 Fourier transform infrared spectrometer (USA). The liquid sample was coated on KBr discs and the cured film was directly used for analysis. Each sample was scanned 32 times with resolution 2 [cm.sup.-1] within the range 400-4000 [cm.sup.-1].

Thermal behavior of the film was analyzed by differential scanning calorimetry (DSC, Perkin-Elmer DSC 8500, USA). The DSC analysis of the sample was carried out within a temperature range of -50 to 200[degrees]C at a heating rate of 20[degrees]C/min in the atmosphere of nitrogen. The sample was first heated to 150 at 30[degrees]C/ min and then cooled down at 30[degrees]C/min before scanning to erase its thermal history.

The thermal stability of the film was preformed on an HCT-1 thermal analyzer (Hnven Scientific Instrument, China). The thermogravimetric analysis (TGA) data were collected at a heating rate of 10[degrees]C/min under [N.sub.2] in a heating range from 25 to 800[degrees]C. The differential thermal analysis (DTA) data were obtained simultaneously with the TGA data.

The water contact angle of the film was measured by a sessile drop method using a DSA20X contact angle goniometer (Germany). Drops of purified water were deposited onto the surfaces of the film and the direct microscopic measurement of the water contact angle was done with software for drop shape analysis.

The water resistance of the film was examined by measuring its water absorption content. Each film was weighed after thorough drying ([W.sub.0]) and immersed in purified water. After 48 h, the film was taken out of the water, wiped dry with tissue paper, and weighed again immediately (W). Water absorption was determined as follows:

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

The cross cut test was carried out to evaluate the adhesion of the coating on tinplate according to the ISO2409:2007 standard. Each emulsion was coated on the substrate, and then dried to obtain dry film with a thickness of 60-80 [micro]m. After the coating was scratched and tape tested, the scratched coating was inspected under a magnifying glass. The number of squares on the substrate was quantified before ([N.sub.0]) and after ([N.sub.1]) tape testing. The coating retention (%) was calculated according to the following equation:

Coating retention (%) = [N.sub.1]/[N.sub.0] x 100

The tensile strength and elongation at the break of the film with a thickness of 60-80 [micro]m were measured at room temperature with an LCD electronic tensile meter (China) according to ASTM D 638 specifications. A crosshead speed of 50 mm/min was used throughout this investigation to determine the ultimate tensile strength and elongation at break for each sample. The quoted value was the average of five measurements.

Shore A hardness of the film was measured using an HT-6510A indentation hardness tester (China) according to ASTM D 2240-75.

Results and discussion

The performance of PUA hybrid emulsions

The emulsion properties like particle size, particle size distribution index (PDI), and viscosity are important parameters in deciding the application of PUA emulsions. The determined results are given in Table 1.

When the HEA/BA weight ratio increased from 0/ 10 to 6/4, the average particle size of PUA emulsions decreased significantly from 325 to 230 nm, and their distribution index was commensurately reduced from 1.97 to 1.59; whereas the average specific surface area of particles increased from 7.13 to 9.42 [m.sup.2]/g. After this point, these parameters remained almost unchanged. From the formation mechanism of the emulsions, (8,14-17) it may be suggested that the hydrophilic segments from HEA and acrylic-terminated PU tend to locate at the surface of the colloidal particles, whereas the hydrophobic segments formed from BA and MMA are preferably packaged inside the core of the particles. While increasing the weight ratio of HEA/BA, the core volume of particles decreases accordingly, and a larger quantity of hydrophilic groups available for stabilizing PUA particles may lead to smaller particles, with higher specific surface area and narrower size distribution. However, when the weight ratio of HEA/BA reaches a certain value, the hydrophilic segment may also be trapped in the inside of particles, so the average particle size and size distribution first decrease, and then remain unchanged. (18-21)

It is noteworthy that the viscosity of the emulsions increased indistinctively from 5 to 54 mPa.s when the weight ratio of HEA/BA was increased from 0/10 to 6/ 4, and then their viscosity increased dramatically from about 54 to 295 mPa s. It is well known that the viscosity of emulsion arises from the interplay between the particle-particle interactions (repulsive force between the similar charges surrounding the particles or hydrogen bonding interaction between the hydroxyl-functional particles) and hydrodynamic interaction. (15-18) When the weight ratio of HEA/BA is increased, all the particles have similar charge density due to a fixed content of CO[O.sup.-] N[H.sup.+] [([C.sub.2][H.sub.5]).sub.3] (neutralized DMPA) in the preparation, but the hydroxyl content is increased. As a result, hydrogen bonding interaction between particles may be enhanced accordingly, and the viscosity of the emulsion is also increased. When the concentration of hydroxyl groups reaches a certain value, a spatial network may possibly form due to this interaction, which leads to a significant increase in their viscosity. (19)

The FTIR and thermal behaviors of the films

Figure 2 gives the FTIR spectra of PIH, and the typical PUA film cured with and without PIH. In the FTIR spectrum of PIH (Fig. 3c), the observed characteristic absorption peak at 2278 [cm.sup.-1] has been ascribed to the asymmetric stretching vibrations of the -NCO group. However, after it cured with PUA, this peak disappeared completely (Fig. 3b), confirming the complete utilization of the -NCO groups with active hydrogen in PUA. In addition, there were no significant changes between the films cured with and without PIH. The intense peak at 3367 [cm.sup.-1] was attributed to the stretching vibration of O-H and N-H bonds in their macromolecules, and the consecutive peak in the range of 2851-2944 [cm.sup.-1] corresponded to the asymmetric and symmetric stretching of aliphatic C-H bonds. The spectra showed a strong peak at about 1731 [cm.sup.-1] owing to the stretching vibration of C=0 groups. The remaining important peaks in 1470 and 1113 [cm.sup.-1] were assigned to the stretching vibrations of C-N and C-O in the urethane and ester groups, respectively.

The DSC curves of typical PUA films cured with and without PIH are shown in Fig. 3. There was only one clear glass transition temperature ([T.sub.g]) at 68.1[degrees]C, and one melting temperature ([T.sub.m]) at 168[degrees]C, in the curve of H0/10. Once cured with PIH, the [T.sub.g] shifted to 69.2[degrees]C, but the [T.sub.g] cannot be observed in the range of test temperatures. This indicates that PIH can increase the [T.sub.g] of PUA because it provides a hard segment in the polymer. (10) According to Fox's law, it is expected that increasing the weight ratio of HEA/BA can also increase the [T.sub.g] of the PUA film because the [T.sub.g] of HEA is higher than that of BA (-15[degrees]C for HEA and -56[degrees]C for BA). Unfortunately, the lack of clear [T.sub.g] and [T.sub.m] in their curves makes this determination impossible. Analogous phenomena have been attributed to homogenous behavior of polymer at the molecular level by previous studies. (2,17,20) It has been previously suggested that the homogenous structure of copolymer is attained in response to crosslinking and phase uniformity due to secondary interaction between hard and soft segments of PU and AP. (2,20) In this study, the hydroxyl groups introduced into PUA macromolecules possibly facilitate in the establishment of this homogeneous structure due to the hydrogen bonding interaction formed from the hydroxyl groups with carbonyl, amide, ether, and other electronegative acceptors in the PUA.

Figure 4 illustrates the thermal resistance of the typical PUA films analyzed by TG-DTA in the temperature range from 25 to 800[degrees]C under a nitrogen atmosphere. It was clearly observed that the degradation of all the samples had two stages of weight loss. In the curves of H0/10, the first weight loss of about 38 wt% was possibly due to loss of polyurethane linkage, and the corresponding endothermic peak was located in the range from 180 to 385[degrees]C. The second stage took place between 380 and 460[degrees]C, corresponding with the decomposition of the PA chains. (20) Increasing the weight ratio of HEA/BA led to a slight increase in both their first and second decomposition temperatures. After being cured with PIH, their films showed a further increased thermal decomposition temperature. It is well known that the polymer with higher crosslinking density yields higher thermal resistance. (3,20) For the PUA cured films without PIH, the increased decomposition temperature is possible because the added hydroxyl group of HEA produces enhanced hydrogen bonding interaction in the copolymers, which yields high crosslinking density. (3) However, the decomposition temperature of film cured with PIH is higher than that of the corresponding film because the covalently crosslinking density is increased by the reaction between the hydroxyl group of PUA and the NCO group of PIH. (8-10)

The water wettability and water absorption of the films

The water contact angle is a measure of the surface wettability, and the results determined are shown in Fig. 5. The water contact angles on the surface of WPAU films were observed to decrease from 68.5 to 64.3[degrees]C while increasing the weight ratio of HEA/BA from 0/10 to 6/4, and then the water contact angle remains almost unchanged after this point. When the films were cured with PIH, their water contact angle was only changed from 70 to 69[degrees]C at the same time, which was much higher than that of PUA films. This indicates that the introduction of HEA increases the wettability of the polymeric surface, but PIH can eliminate this influence. The water contact angle is defined as the angle formed by the intersection of the water/air interface and the water/solid interface, which relates to the content of hydrophilic groups on the solid surface. (15-17) Increasing the HEA content leads to an increase in the hydrophilic groups on the surface of PUA films. When the content of the hydroxyl groups reaches a certain point, as discussed above, the hydrophilic chains may be buried inside particles, resulting in little additional influence on the water contact angle. When the films are cured with PIH, some hydroxyl group of PUA may be consumed by PIH, and some hydrophilic groups may be shielded by the covalently crosslinked networks, resulting in the reduction of their surface wettability.

Figure 6 shows the effect of HEA on the water resistance of PUA films cured with and without PIH. When the weight ratio of HEA/BA increased from 0/ 10 to 10/0, the water absorption of the PUA films increased slowly from 6.1% to 45.0%. After cured with PIH, the water absorption of H0/10 was 5.5%, but the water absorption of H10/0 was notably reduced to 6.8%. The higher the weight ratio of HEA/BA, the greater was the improvement of the water resistance of the 2-K coatings achieved. Water resistance of polymers is greatly dependent on their chemical and physical structure. Hydroxyl and carboxyl acid groups of PUA not only increase hydrogen bonding interaction in the films, but also are sensitive to water. (15,16) When the films are immersed in water, these groups form strong hydrogen bonds with the water molecules to worsen the water resistance of PUA. The higher the content of HEA, the higher is the water absorption of the PUA films. However, when the films are cured with PIH, the chemical reaction between hydroxyl groups of PUA and NCO groups of PIH not only forms covalently crosslinked networks, but also consumes the hydroxyl group in PUA. When the weight ratio of HEA/BA is increased, the covalently crosslinking density is enhanced. As a result, water cannot easily enter into the crosslinked macromolecules and their water resistance is improved.

The adhesion of the coatings on substrate

The adhesion properties of the coatings on tinplate were evaluated by a cross cut test, and Fig. 7 shows the coating retentions after the tape test. While increasing the HEA/BA weight ratio from 0/10 to 10/0, the coating retentions of PUA on tinplate increased from 12% to 81%, but the retentions of 2-K coatings further increased from 52% to 91%. This indicates that both increasing the HEA content and introducing PIH can increase the adhesion of the coatings on the substrate. The adhesion of coatings to substrates mainly depends on the following factors: the ability of wetting the surface of substrates; the small particle size, allowing them to permeate porous substrates; and the hydrogen bonds or covalent bonds with substrates. (5,14,16) Increasing the weight ratio of HEA/BA results in the formation of small particle size in emulsions and enhanced hydrogen bonding interaction in PUA coatings, so the adhesion of PUA on the substrate is increased. When PIH is introduced, their adhesion is further enhanced because the NCO can also provide introduced chemical-physical interaction with the substrates. (8-10)

The mechanical properties and hardness of the films

The mechanical properties of the films are also strongly affected by the HEA content and PIH, and the determined results are illustrated in Fig. 8. While increasing the HEA/BA weight ratio from 0/10 to 10/ 0, the tensile strength of PUA films increased from 5.1 to 11.8 MPa, whereas their elongation at break decreased simultaneously from 450% to 312%. After curing with PIH, their films showed higher tensile strength and lower elongation at break than PUA films. Meanwhile, their tensile strength also rose and elongation at break went down with the increasing weight ratio of HEA/BA. As discussed above, increasing the weight ratio of HEA/BA leads to an increase in the glass transition temperature and hydrogen bonding interaction, but the introduction of PIH yields films with a high glass transition temperature and a covalently crosslinked network. As a result, the temperature and crosslinking density of glass are increased, resulting in enhanced stiffness of the films. (8-10,16)

As shown in Fig. 9, the Shore A hardness of the films was expected to be increased by increasing the HEA/BA weight ratios and/or introducing PIH. This result also supports the above conclusion that HEA increases the hydrogen bonding interaction and PIH increases the covalently crosslinked networks in the films. Both of them endow PUA films with high hardness.

Conclusions

To prepare hydroxyl-functional PUA emulsions for two-component coating, PUA hybrid emulsions were synthesized by copolymerization, varying weight ratio of HEA/BA with a constant of MMA and acrylateterminated PU in the presence of [K.sub.2][S.sub.2][O.sub.8] via the emulsion process. It was found that the performances of PUA emulsions, films, and two-component coatings were affected by the weight ratio of HEA/BA.

While increasing the HEA/BA weight ratio from 0/10 to 6/4, the average particle sizes of PUA hybrid emulsions decreased and their distribution narrowed progressively; whereas the average special surface area of the particles and the viscosity of the emulsions increased. After this point, the viscosity dramatically increased, but the other parameters were unchanged.

Increasing the weight ratio of HEA/BA yields PUA films with homogenous structure, high thermal resistance, and hydrogen bonding interaction. As a result, their thermal resistance, adhesion on substrates, tensile strength, and hardness were increased, but the wettability was also increased and the water resistance worsened.

After being applied in two-component coatings, PIH reacted with the hydroxyl group of PUA to form a covalently crosslinking network in their films. Therefore, the cured films exhibited increased glass transition temperature and thermal resistance, improved water resistance, and reduced wettability. Meanwhile, their adhesion on substrates, tensile strength, and hardness were further enhanced.

10.1007/s11998-014-9647-y

G. Ma ([mail]), T. Guan. C. Hou, J. Wu, G. Wang, X. Ji Shanxi Research Institute of Applied Chemistry, Taiyuan 030027 Shanxi, China

e-mail: maguozhang@hotmail.com

T. Guan. J. Wu. G. Wang, B. Wang Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024 Shanxi, China

Acknowledgments This research is financially supported by Shanxi Scholarship Council of China (No. 2012-8) and Scientific Research Foundation of Shanxi Province, China (No. 20111101059).

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Table 1: The properties of PUA hybrid emulsions

Sample        Weight ratio of    Average       Average specific
designation       HEA/BA        particle        surface area of
                                size (nm)   particles ([m.sup.2]/g)

H0/10              0/10            325               7.13
H2/8               2/8             301               7.28
H4/6               4/6             279               8.05
H6/4               6/4             232               9.42
H8/2               8/2             234               9.43
H10/0              10/0            233               9.43

Sample          Particle size      Viscosity
designation   distribution index    (mPa s)

H0/10                1.97               5
H2/8                 1.76               8
H4/6                 1.65              10
H6/4                 1.59              54
H8/2                 1.58             110
H10/0                1.58             295


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Author:Ma, Guozhang; Guan, Taotao; Hou, Caiying; Wu, Jianbing; Wang, Gang; Ji, Xiian; Wang, Baojnn
Publication:Journal of Coatings Technology and Research
Geographic Code:9CHIN
Date:May 1, 2015
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