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Aziridine cure of acrylic colloidal unimolecular polymers (CUPs).

Abstract Polymers were synthesized with a 1:7 or 1:8 ratio of acrylic acid to acrylate monomers to produce an acid-rich resin. The polymers were water-reduced and solvent-stripped to produce colloidal unimolecular polymers (CUPs). These particles are typically 3-9 nm in diameter, depending upon the molecular weight, and have different rheological behavior from micelles, polyelectrolytes, fullerenes, and latex particles, due to their charged surface and large surface areas. They were then formulated into ambient cure clearcoatings with aziridine crosslinking. These aziridine-cured acrylic CUPs were either solvent-free or very low VOC. The coatings were evaluated for their MEK resistance, adhesion, hardness, gloss, flexibility, wet adhesion, and abrasion and impact resistance properties.

Keywords Colloidal unimolecular polymer, CUP, Aziridine, Water reduction, Zero VOC, Acrylic polymer

Introduction

The promise of nanotechnology to deliver breakthrough coating performance in areas such as scratch resistance, hardness, barrier properties, mechanical properties, etc., has been the main reason for the high level of interest in the scientific community. Researchers of nanotechnology have promoted the idea that "smaller is better." (1) Although early work on applications of nanomaterials in coating has been scattered, there are many coatings scientists, formulators, and researchers around the world today who are investigating, different aspects of nanodispersions in coatings. (2-4) As a result, the conditions are set for breakthroughs in this area over the next several years.

Numerous methods have been developed for the water reduction of polymers to give resins for coatings. One such method describes that when multiple chain polymers containing blocks of both hydrophilic and hydrophobic regions are placed in an aqueous environment at an appropriate pH, the hydrophilic polyether ester portions of the chains orient into the water phase such that they leave the hydrophobic region in the interior domain, forming macromolecular polymeric micelles with an average diameter of 50-120 nm; larger than typical micelles of 2-10 nm or roughly twice the diameter of the hydrocarbon chain. (5) In another water reduction study by Morishima, the micelle behavior of a single polyelectrolyte chain was observed to be "self-assembled" in a poor solvent when the chains collapsed into unimolecular micelles of a diameter of approximately 5.5 nm. (6) Multiple chain polymer collapse has also been observed in waterborne urethane resins synthesized by reaction of isocyanate by Reichhold with subsequent removal of the acetone solvent from the resin water blend, causing the chains to collapse into aggregates with a diameter of approximately 25 nm. (7), (8) Water-reducible resins containing ionizable carboxylic acid groups neutralized with amines were synthesized in another study and dissolved in high boiling, water miscible solvents. Water was then introduced into the system, until the solvent blend became a less than theta solvent condition which caused the entangled polymer chains to collapse. (9)

The term colloidal unimolecular polymer (CUP) (10) has been introduced to describe the solid spherical single molecule polymer particles suspended in the continuous aqueous phase. CUPs contain a hydrophobic backbone and hydrophilic groups such as carboxylic acid salts. The process by which these are formed is basically water reduction with subsequent removal of a volatile water-loving solvent. Therefore, the CUP solution can be VOC free. This research article explores the synthesis of acrylic CUPs in the true nanoscale range (less than 10 nm) with particular emphasis on coating performance enhancements offered by the aziridine curing of the polymeric films.

In this study, polymers of CUPs were synthesized in THF using acrylic monomers by free radical polymerization. Four polymers were investigated: two of low [T.sub.g], below room temperature, and two of high [T.sub.g], above room temperature. For both the polymers, two molecular weights were chosen: one high ~50,000 and the other a lower molecular weight ~20,000. The low molecular weight would require crosslinking to obtain any respectable physical properties, whereas the higher molecular weight would have marginal lacquer performance. THF was selected as the primary solvent due to its good solvency for acrylics, miscibility with water, and low boiling point allowing it to be easily stripped off after water reduction without loss of a significant amount of water. The hydrophilic/lipophilic balance indicated that the acid monomer to the acrylate used have a ratio of about 1:7 or 1:8. This ratio yields a monomolecular reduction to the CUP particles. If less acid groups were incorporated, some aggregation was observed. Triethylamine was added to neutralize the carboxylic acid groups on the synthesized polymers during water reduction. Bases like NaOH and KOH cannot be used for neutralization, as they form salts that do not leave the film during drying, and thus will not crosslink with the aziridine. Ammonium hydroxide can be used for neutralization, but triethylamine was chosen in this study. Water is added slowly during water reduction to avoid a large regional solvent composition change which causes the formation of more coagulum and leads to visible cloudiness indicating large aggregates. A modest stirring rate is essential for avoiding any regional solvent composition change. The synthesized acrylic CUP resins will give the performance of a lacquer without a crosslinker, and therefore, to enhance the performance, a commercially available aziridine (CX-100 from DSM NeoResins Inc.) with a functionality of 3 was chosen. The low [T.sub.g] CUP resin developed is a zero VOC system, except for the amine. For high [T.sub.g] polymers cured by means of aziridine. Texanol (Eastman Chemical Co.) was used as a coalescing aid to ease the process of film formation. The coalescent aid was, the only VOC in the system, other than the base, making the high [T.sub.g] resin a low VOC system.

Figure 1 illustrates the size comparison of two conventional small coating resin particles represented by a latex and a typical waterborne urethane resin. The third particle in the series is the CUP (10) particle, which is the topic of this report. The CUPs are single polymer chain particles which are collapsed and suspended in water. Their small size and high charge density coupled with the Brownian motion of the solvent molecules allows them to be thermodynamically stable in water."

[FIGURE 1 OMITTED]

Experimental

Materials

Methacrylic acid (MAA), butyl methacrylate (BMA), ethyl acrylate (EA), ethyl methacrylate (EMA), 2-ethylhexyl methacrylate (2-EHMA), 2.2'-azobis (2-methylpropionitrile) (AIBN), and 1-dodecanethiol were obtained from Aldrich. MAA was purified by distillation with copper (I) bromide under vacuum. All other monomers were purified by washing with a 10% (w/w) solution of aqueous sodium bicarbonate, followed by rinsing with de-ionized water and brine, after which the solution was filtered after drying over sodium sulfate and purified by distillation under nitrogen with copper(I) bromide as an inhibitor. The solvent THF was dried and distilled before use. The initiator AIBN was recrystallized before use from methanol while 1-dodecanethiol was used as received.

Polymer syntheses

All polymers were synthesized by free radical polymerization in tetrahydrofuran (THF). The monomer composition of polymers J-31 and J-32 was in molar ratios--MAA: EMA: BMA = 1:2.5:5.5 with the acid:acrylate ratio 1:8 while the monomer composition of polymers J-51 and J-52 was in molar ratios--MAA:EA:2-EHMA: BMA = 1:1.5:1.5:4 with the acid: acrylate ratio 1:7. The monomer ratios were chosen such that the glass transition temperature of two of those polymers would be above and two below room temperature for adequate evaluation of the polyacrylic resins synthesized. The molar ratio of dodecanethiol was varied to produce a low and a high range of molecular weight. The solvent THF was added in the amount of 2.5 times the total weight of monomers. It should be noted that all the polymers were insoluble in water as well as at an alkaline pH.

Synthesis for polymer J-31

The monomers. BMA (0.477 mol, 67.79 g), EMA (0.217 mol, 24.74 g), and MAA (0.0865 mol, 7.47 g), in a 1000 mL three-neck round bottom flask with the initiator AIBN (0.781 mol, 0.094 g), along with chain transfer agent dodecanethiol (0.0033 mol. (0.658 g) and THF (250 g), were stirred. The flask was fitted with a nitrogen line, condenser, and gas outlet adapter connected to an oil bubbler to allow a positive pressure of nitrogen throughout the polymerization process. The flask was heated slowly to reflux and allowed to react for 24 h. The polymer solution was then cooled to room temperature, and precipitated in cold de-ionized water under high shear, then dried in vacuo. Polymers J-32, J-51, and J-52 were also synthesized as per the above-mentioned protocol.

Characterization of polymers synthesized

The [.sup.1]H NMRs were recorded on the synthesized polymers using a Varian 400 MHz FT/NMR spectrometer in a 5 mm outer diameter thin-walled glass tube with sample concentrations around 30 mg/mL in CD[Cl.sub.3]. All spectra were consistent with proposed polymer structures. Absolute number average molecular weights ([M.sub.n]) were measured by gel permeation chromatography (GPC) in THF at 25[degrees]C on a Viscotek GPCmax from Malvern Instruments coupled with a triple detector array TDA305 (static light scattering, differential refractometer, and intrinsic viscosity). Acid values (AV--reported in mg of KOH/g of polymer sample) for all polymers were measured by titration method ASTM D-974, which was modified by using potassium hydrogen phthalate (KHP) in place of hydrochloric acid and phenolphthalein as an indicator in place of methyl orange. Glass transition temperature ([T.sub.g]) was measured on TA Instruments Q2000 by means of modulated-differential scanning calorimeter (DSC) method at a scan rate of 10[degrees]C/min.

Water reduction of polymers to form CUPs

Polymers were dissolved in a low boiling water miscible solvent, THF (20% w/w) and stirred overnight. The acid groups were neutralized with triethyl amine; de-ionized water was added by a peristaltic pump at the rate of 1.24 g/min and the pH of solution was maintained between 8.3 and 8.7 using triethylamine. After the water was added, the THF was stripped off under vacuum, giving CUPs in VOC-free aqueous solution, except for the added base, at the desired concentration. It should be noted that ammonium hydroxide works equally well and further reduces the VOC.

Water reduction process for the polymer J-31

Polymer J-31 (0.0174 mol, 20 g) was dissolved in THF (80 g) to make a 20% w/w solution; the acid groups were neutralized with triethylamine (0.006 mol, 0.61 g); and de-ionized water (80 g) was added by means of a peristaltic pump, after which the THF was stripped off under vacuum to give a 20% solution of CUPs in water. The CUP solutions were then filtered through a 0.45 [micro]m Millipore membrane to remove any foreign materials that were typically measured to be less than 0.05% by weight. Figure 2 depicts the process of CUP particles formation. It was found that the ratio of water to THF used was critical since the collapse from a random coil into a hard sphere can create a poly-chain particle instead of a CUP particle due to chain--chain entanglement if the collapse occurs at a high concentration.

[FIGURE 2 OMITTED]

Characterization of CUPs

After the water reduction process, viscosity measurements were done by Ubbelohde viscometer method at 25 and 30[degrees]C for use in measuring the particle size and reported in the units of centiStokes. The viscosity of 10% CUP solution in water was measured at 25[degrees]C on a Brookfield Rheometer model DV-III at a shear rate of 112.5 [s.sup.-1] and reported in centipoise. Particle sizes were measured by dynamic light scattering on a Nanotrac 250 particle size analyzer from Microtrac with a laser diode of 780 nm wavelength, and 180[degrees] measuring angle. The principle for the particle size measurement was that the particles in solution were constantly moving due to collisions by the solvent molecules, which is called Brownian motion. If the particles or molecules are illuminated with a laser, the intensity of the back-scattered light that strikes the detector was Doppler-shifted, and was dependent upon the size of the particles. The [.sup.1]H NMRs were recorded on a Varian 400 MHz FT/NMR spectrometer in a 5-mm outer diameter thin-walled glass tube with aqueous sample with added [D.sub.2]O and no THF peak was observed. Minimum film formation temperature (MFFT) was measured on a Rhopoint WP-Bar90 as per the method described in ASTM D-2354.

CUP coatings

The CUPs were prepared at 20% solids in water and cured by means of an aziridine for evaluating the coating characteristics of the clearcoat CUPs. The crosslinker was used in a 1:1 ratio of the acid equivalent of the resin. The aziridine used to cure CUP clearcoats was CX-100, obtained from DSM Resins with a functionality of 3 (Fig. 3). The aziridine was added just prior to application and used after 30 min to simulate use conditions. The coated samples were dried for 24 h at ambient conditions. As the MFFT of J-31 and J-32 was higher than room temperature, Texanol obtained from Eastman Chemicals was used as a coalescing aid to lower the MFFT for the aziridine cure. After addition of Texanol at 3% w/w of the total formulation, the MFFT of J-31 and J-32 was found to be lowered to 18.6[degrees]C.

[FIGURE 3 OMITTED]

CUP coatings front J-31

For 100 g of water-reduced resin, 2.80 g (0.006 mol) of CX-100 was added along with 3 g of Texanol (3% w/w on total formulation).

Testing of the CUP clearcoats

Aluminum panels A-36 mill finish and iron phosphated steel panels R-36 dull matte finish from Q-Pancl were used to test CUP clearcoats. The aziridine-cured coatings were ambient cured for 24 h and aged at 50[degrees]C for 24 h to accelerate aging. The CUP clearcoats were tested for their mechanical dry time, appropriate ratio of aziridine to acid for effective curing of CUPs, pot life at constant temperature, MEK resistance, adhesion, hardness, gloss, flexibility, and abrasion and impact resistance properties. The controls used for the testing protocol were the CUP clearcoats cast on panels without any crosslinker. Mechanical dry time was measured as per ASTM D-5895 by means of a Gardener dry time recorder. The ratio of aziridine:acid, based on the equivalent weight, was varied from 0.5:1, 0.75:1, 1:1, 1.25:1, and 1.5:1 to find out the appropriate ratio required for effective cure of CUPs. To determine the pot life, samples were stirred for 30 min and kept in a constant-temperature water bath at 25[degrees]C, after which drawdowns were made every 15 min for the next 2 h. The results of MEK double rubs and pencil hardness were evaluated to determine the pot life and the effective ratio of aziridine:acid for total cure of CUPs. All other CUP coatings were formulated at aziridine:acid ratio of 1:1. Gloss was measured on aluminum panels by a Byk-Gardener microgloss meter, and an average of three readings with standard deviation less than 1 were recorded at three angles: 20[degrees], 60[degrees], and 85[degrees]. MEK double rub tests were performed on aluminum panels by employing a lint-free cloth as per ASTM method D-4752 and an average of two readings was reported. Pencil hardness tests were performed on aluminum panels as per the ASTM method D-3363 by using pencils of varying hardness in the range of 9B-9H, and an average of three readings was reported. Film thickness was measured on aluminum panels by a coating thickness gauge by Elcometer-6000 Positector, and an average of three readings was reported in mil. Impact testing was done on iron phosphate steel Q-Panels as per ASTM D-2794 using Gardner Impact Tester with a 5/8-inch ball indenter of 4-lb weight, and results were reported in units of inch-lbs. Flexibility was tested on aluminum Q-Panels by mandrel test method as per ASTM D-522. Adhesion testing was done as per the ASTM D-4541 on iron phosphate steel Q-Panels by prepping the coatings with sandpaper # 320, wiping with isopropanol, air-drying for 1 h and gluing the grit-blasted and MEK-cleaned pucks onto the coating with a Locktite Quick Set 2-ton epoxy, and allowed to cure for 48 h, after which a torque wrench ComputorQ-11 was used instead of a pull-off tester to record the failure type and the torque value. The torque displayed in inch-pound units was recorded in PSI units by appropriate conversion and an average of four readings was reported. Wet adhesion testing was done by immersing one-third part of the aluminum panels in deionized water for 1 h and then inspecting the panels for delamination, change in clarity/transparency, etc. Pencil hardness was also done after exposure on those panels. Abrasion resistance testing was performed on 4" x 4" iron phosphate steel panels R-44 dull matte finish from Q-Panel by using a Taber Abraser 5150 with a load weight of 1000 g for 100 cycles, utilizing H-10 wheels as per the ASTM D-4060.

Results and discussion

Polymer synthesis and characterization

The initial study investigated four polymers, two of low [T.sub.g], below room temperature, and two of high [T.sub.g], above room temperature. For both the polymers, two molecular weights were chosen: one high ~50,000 and the other a lower molecular weight ~20,000. The molecular weights chosen here are only examples. The monomer composition can also be varied. Polymers with molecular weights ranging from 6000 to 130,000 have been successfully reduced to form CUPs, Table 1.
Table 1: Monomer composition, MAA:acrylate ratio, [T.sub.g],
and mol. wt. of the synthesized polymers

Polymer      Monomer          MAA:       Glass  Molecular
synthesized  composition  acrylate  transition     weight
                             ratio       temp.  [M.sub.n]
                                     [T.sub.g]
                                     ([agrees]
                                            C)

J-31         MAA: BMA:         1:8          55     19,000
             EMA

J-32         MAA: BMA:         1:8          55     50,000
             EMA

J-51         MAA: BMA:         1:7          21     21,000
             EA: 2-EHMA

J-52         MAA: BMA:         1:7          21     51,000
             EA: 2-EHMA


The monomers MAA, BMA, EA, EMA, and 2-EHMA were chosen in the particular composition specified to yield polymers with specified [T.sub.g]. The actual acid value of the synthesized polymers was found to be slightly higher than the theoretical acid value, as expected, because a part of the monomer MMA was lost with nitrogen purging through evaporation with solvent during polymer synthesis (Table 2). Good yields were observed for all the polymers synthesized.
Table 2: Polymer characterization: % yield, [T.sub.g], mol.
wt., acid value, and Mark-Houwink parameters in THF

Polymer          %   Acid         Mark-Houwink  Mark-Houwink
synthesized  Yield  value                  "a"       "log K"

                    Theo.  Expt.

J-31            89   48.7   48.8          0.65         -3.69

J-32            93   48.7   48.7          0.68         -3.88

J-51            91   50.9   52.5          0.70         -3.97

J-52            93   50.9   51.7          0.64         -4.37


For latex coatings composed of polar polymers, the MFFT is often below the [T.sub.g] (l2) rather than the typical MFFT greater than the [T.sub.g]. (13) The MFFT of the synthesized polymers were found to be lower than the [T.sub.g] which has been observed for waterborne resins (Table 3). The viscosity of water-reduced CUPs was water-like and the actual/measured particle sizes were close to the theoretical particle size, indicating the true nanoscale characteristics of the synthesized CUPs. As seen from Fig. 4, the viscosity profile of CUPs, as measured by a Brookfield rheometer, was linear for the sq. rt. of shear rate vs sq. rt. of shear stress with a slope of 0.1456. Therefore, zero point viscosity at 25[degrees]C was 2.12 cP. This corroborates with the results of kinematic viscosity, measured by an Ubbelohde viscometer, that the viscosity of water-reduced CUPs were water-like at this concentration: however, at higher concentrations the charged particles began to exhibit non-Newtonian behavior due in part to the charge--charge repulsion.

[FIGURE 4 OMITTED]
Table 3: Viscosity, particle size, [T.sub.g], and MFFT of the
water-reduced CUPS

Polymer      Kinematic    Vise.   Particle            Avg. MFFT
synthesized  viscosity       at  size (nm)         ([degrees]C)
                          shear
                        rate of
                          112.5
                           (cP)

                                     Theo.  Expt.

J-31              2.19     2.31        3.7    4.0          45.2

J-32              2.51     3.96        5.1    4.5          45.5

J-51              2.45     2.57        3.8    3.1           2.5

J-52              3.13     3.82        5.1    4.7           2.4


The four synthesized polymers were analyzed for comparison between the theoretical particle size calculated from the GPC fractions at different molecular weights and the actual/experimental particle size as found via DLS. As seen from Fig. 5, there was a good agreement between the distribution calculated from the molecular weights of fractions from the GPC and the particle diameters from DLS, assuming the density of the bulk polymer was the same as that of CUPs. The presence of THF, if not stripped off completely, influences the measured diameter of the CUPs, as it can migrate into the interior of the CUP particles and give a larger diameter than expected due to swelling. However, NMR was employed in this research to verify the removal of THF. It should be noted that water must be added in a slow gradient during reduction to avoid regional large solvent compositional changes. If they occur, coagulum may be formed, resulting in a cloudy solution due to large aggregates. The water must also be free of polyvalent cations like calcium or magnesium, which can bind to the carboxylates and cause gelling. If performed correctly, the solution appears water-clear, as evident from Fig. 6.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Latex particles are usually 100 nm in diameter, whereas dispersions can be as small as 25 nm. In the latex particles, only a small percentage of the acid groups or the reactive groups are on or near the surface, while the remainder is inside the particle. These reactive groups have to diffuse to the surface to react or the crosslinker will need to diffuse deep into the latex particle to react. The catalyst will be in the water phase until the water leaves and also must migrate into the resin to crosslink a latex or dispersion. These factors make the latex or dispersed larger particle resin less efficient at forming crosslinks with waterborne crosslinkers.

If we calculate the functionality as carboxylates on CUP J-32, the surface area occupied per carboxylate is 0.67 [nm.sup.2]. If a latex particle has the same functionality and is of a 100-nm diameter, the percentage of acid groups on the surface is approximately 9% of the total number of acid groups. Thus, only a fraction of total functionality is available for crosslinking for latexes. In case of small diameter dispersions and CUPs, most of the reactive groups are on the surface and can react without polymer diffusion. Again, as the crosslink density increases for a latex particle, the diffusion slows and will stop short of completion. In CUPs the crosslink efficacy is high since all the groups are at or near the surface for full reaction without the need for slow diffusion. The ester groups of the latex can also react, but this reaction is slow as compared to the acid groups. The need for the esters to react is not as significant in issue for CUPs as it would be for a latex resin.

Aziridine-cured CUP coatings

A mechanical dry time study was conducted to determine the drying stages of film formation for the CUP clearcoats. As evident from Table 4, the process of drying of the CUPs was complete around 3 h and gave an idea of when a freshly painted surface could be put hack to use. The low molecular weight polymers are expected to have a slightly faster drying time as compared to the higher molecular weight polymers owing to their faster reptational motion hut no such demarcation was observed due to the aziridine crosslinking of the CUP clearcoats. It should be noted that full aziridine cure will require additional time past through-dry. For comparison, four commercial clear floor coatings, waterborne urethane dispersions, were crosslinked with aziridine and dry times measured. They produced set to touch values of 50-57 min and hard dry times of 81-150 min. These values are similar to that of the CUP system and should be expected since the crosslinker and mechanism of hardening are the same.
Table 4: Mechanical dry time (min) of aziridine-cured CUPs
as per ASTM D-5895

Polymer      Cure type  Set-to-touch  Tack-free   Dry      Dry
synthesized                     time       time  hard  through
                                                 time     time

J-31         Control              53        105   145      190
             Aziridine            55        110   145      195

J-32         Control              55        105   145      190
             Aziridine            57        110   145      195

J-51         Control              53        100   150      195
             Aziridine            50        100   140      180

J-52         Control              51        100   150      195
             Aziridine            51        100   140      180


The ratio of aziridine:acid based on the equivalent weight was varied from 0.5:1, 0.75:1, 1:1, 1.25:1, and 1.5:1 to find out the appropriate ratio required for effective cure of CUPs. The performances of the samples were evaluated on the basis of their pencil hardness and MEK double rubs. As seen from Table 5 and Fig. 7, it was evident that the optimum ratio of aziridine:acid for the effective cure of acrylic CUPs was about 1.25:1. An aziridine with a functionality of 3 was used for the study, but a small percentage of the molecules might be bi-functional, If the aziridine:acid ratio is greater than 1.25:1, some of the excess aziridine crosslinker will only be able to link one of its 3 aziridine groups with a carboxylate group on the polymer chains which decreases the crosslink density. Again, the excess aziridine reacts with water to form an aminoalcohol. These two factors can substantially lower the performance of the CUP clearcoats. At the aziridine:acid ratio of 1.25:1, the slight excess of aziridine ensures that even if one of the three aziridine functionalities was hydrolyzed, the other two will be available for crosslinking, thus giving a highly cross-linked coating with excellent performance.

[FIGURE 7 OMITTED]
Table 5: Effect of aziridine:acid ratio on pencil hardness
and MEK double rubs

Polymer synthesized  Aziridine:acid ratio  Pencil hardness

J-31                                0.5:1  B
                                   0.75:1  HB
                                   1.00:1  F
                                   1.25:1  H
                                    1.5:1  F

J-32                                0.5:1  B
                                   0.75:1  B
                                   1.00:1  B
                                   1.25:1  H
                                    1.5:1  H

J-51                                0.5:1  B
                                   0.75:1  F
                                   1.00:1  F
                                   1.25:1  H
                                    1.5:1  HB

J-52                                0.5:1  B
                                   0.75:1  HB
                                   1.00:1  F
                                   1.25:1  H
                                    1.5:1  F


It is important to know the time frame in which the CUP coatings can be used after they are mixed with the aziridine crosslinker. The pot life study gives an estimate of the time after which the crosslinker becomes less effective. It is evident from Table 6 that the pot life of aziridine-cured acrylic CUPs was at least 120 min, after which the samples start to have lower performance. Even though aziridines are reactive compounds, the conditions for the nucleophile to attack on the aziridine ring are not conducive until the water leaves, and therefore it takes some time for the crosslink density to gain optimal strength which was reflected in the performance of the coatings during MEK double rubs and pencil hardness tests. One possible rationale for the increase in performance during the first 120 min may be a pre-equilibrium partitioning of the aziridines between the CUP interior and the water phase. Further investigations would be required to fully validate this possibility. The data clearly shows that the usable pot life was over 2 h with only about a 20% loss of performance in 2.5 h. The peak performance appears to be after a 90 min pot life, after which performance decreases. This pot life is consistent with typical commercial aziridine crosslinked waterborne finishes, specifically the waterborne urethane floor finishes.
Table 6: Pot-life study for aziridine cure of acrylic CUPs

Polymer      Pot-life time  Pencil    MEK double
synthesized          (min)  hardness        rubs

J-31                    30  HB                47
                        45  HB                48
                        60  F                 46
                        75  F                 48
                        90  H                 56
                       105  H                 49
                       120  H                 47
                       135  H                 47
                       150  HB                39

J-32                    30  HB                72
                        45  F                 70
                        80  F                 73
                        75  H                 71
                        90  H                 75
                       105  H                 69
                       120  H                 68
                       135  H                 65
                       150  HB                60

J-51                    30  HB                42
                        45  HB                44
                        60  F                 43
                        75  H                 44
                        90  H                 56
                       105  H                 50
                       120  H                 50
                       135  H                 40
                       150  F                 39

J-52                    30  F                 37
                        45  F                 39
                        60  H                 47
                        75  H                 47
                        90  H                 56
                       105  H                 45
                       120  H                 46
                       135  H                 41
                       150  F                 38


Pencil hardness is a measure of the hardness of the coating, while the MEK double rub test is a measure of the solvent resistance and an estimate of the crosslink density of a coating. It was observed that the CUP clearcoat made from high [T.sub.g] polymer (J-31 and J-32) gave good gloss, while the CUP clearcoat made from low [T.sub.g] polymer (J-51 and J-52) gave high gloss (Table 7). Coalescing aid was required in the high [T.sub.g], polymer to lower its [T.sub.g] and ease the process of film formation, but its presence can be the reason for the lower gloss of the high [T.sub.g] polymers as compared to the high gloss of low [T.sub.g] polymers. The lack of any crosslinker would render the coating a lacquer, thus resulting in lower hardness and solvent resistance as seen from the results of the control. It was observed that the MEK double rubs were moderate for all of the aziridine-cured CUP clearcoats, but higher compared to the controls, indicating that crosslinking with aziridine gave better performance.

As is evident from Table 7, crosslinking the CUPs gave a significant boost to the performance characteristics of the resin, as measured by the MEK double rubs and pencil hardness. In the CUP system, the carboxylates are on the surface of the particle and thus are all easily accessible to the aziridine for crosslinking. Thus, the CUP particles do not require extensive reptational motion to access the aziridine, nor to coalesce, as would a latex particle or dispersion such as a urethane.
Table 7: Film thickness, gloss, MEK double rubs, and pencil
hardness results of the CUP clearcoats

Polymer      Cure            Film         Gloss     MEK  Pencil

synthesized  type       thickness  20[degrees]/  double  hardness
                            (mil)  60[degrees]/    rubs
                                    85[degrees]

J-31         Control          0.5     81/83/ 90       5  B
             Aziridine        0.6     81/82/ 89      48  H

J-32         Control          0.5     87/91/ 93       7  B
             Aziridine        0.5     86/90/ 92      71  H

J-51         Control          0.7     90/93/ 97       5  B
             Aziridine        0.8     90/94/ 98      44  H

J-52         Control          0.5     90/94/ 97       7  B
             Aziridine        0.5     89/91/ 95      47  H


Mandrel flexibility and impact resistance results of the clearcoats formulated from CUPs

Both impact and flexibility require elongation of the polymer. If the polymer has a low elongation to break or has low tensile strength, then it will fail the flexibility and the impact testing. Similarly, if the coating is highly crosslinked, it may be brittle and fail. Typical polyesters have mol. wt. between 200 and 700 per functional group while CUPs J-31/J-32 have a molecular weight of 1166 and J-51/J-52 have a mol. wt. of 1115 per functional group. This higher molecular weight per functional group results in larger separation between the adjacent functional groups and greater distance between the crosslinked chains, which gives flexibility to CUPs. It was observed that all the formulated CUP clearcoats (the control as well as the aziridine crosslinked CUPs) passed the 1/8-in. Mandrel flexibility and the forward/reverse impact rating of 160+ in.-lb. This excellent flexibility and impact resistance can be also be attributed to the true nanoscale of CUP dispersions to form clearcoats with ready access to the crosslinking agent without having to penetrate into a large particle which requires diffusion time. A contributing factor to its performance is the carboxylates improving adhesion in the control coatings and the amines improving it in the aziridine-cured films.

Figure 8 is evidence of the high flexibility and impact resistance of the resin developed. It was observed that the aziridine-cured CUP clearcoats had both coating and epoxy-amine adhesive failure, indicating good adhesion of the polymeric film to the substrate (Table 8).

[FIGURE 8 OMITTED]
Table 8: Adhesion testing results of the clearcoats
formulated from CUPs

Polymer      Cure type    Avg.  Failure     Failure         %
synthesized             torque  of          type      Failure
                         (PSI)

J-31         Control       810  Coating to  Adhesive      100
                                substrate
             Aziridine    1055  Coating +   Adhesive    90:10
                                Epoxy

J-32         Control      1021  Coating to  Adhesive      100
                                substrate
             Aziridine     984  Coating +   Adhesive    90:10
                                Epoxy

J-51         Control       838  Coating to  Adhesive      100
                                substrate
             Aziridine     859  Coating +   Adhesive    90:10
                                Epoxy

J-52         Control      1065  Coating to  Adhesive      100
                                substrate
             Aziridine     927  Coating +   Adhesive    90:10
                                Epoxy


Wet adhesion test results of the clearcoats formulated from CUPs

One-third of the CUP coated panels were immersed in deionized water for 1 h, followed by 1 h of air-drying for the wet adhesion testing. No significant visible change was observed on any of the polymeric films nor was there any hazing or change in pencil hardness. It was anticipated that the affinity of the carboxylic acid groups to water might render water sensitivity; however, this was not observed. Though the CUPs are acid-rich, the true nanosize of CUPs increases the crosslink efficiency of the carboxylic acids to aziridine, which results in negligible free carboxylic acid groups present in the crosslinked resin. This is observable by low water sensitivity of the CUP clearcoatings.

It was observed that the wear index of control was usually high, indicating low abrasion resistance, as expected for a lacquer. The aziridine crosslinked coatings all showed significant improvement in the wear (Table 9). This corroborates that crosslinking the CUPs with aziridine enhances its performance as a coating. The wear also compares well to what is found with waterborne aziridine-cured floor coatings. They typically show 3-10 mg/100 cycle loss with the less aggressive CS-17 wheels.
Table 9: Abrasion resistance test results for the CUP clearcoats

Polymer           Cure  Mg lost/100   Wear
synthesized       type       cycles  index

J-3T           Control           39    390
             Aziridine           17    170

J-32           Control           27    270
             Aziridine           10    100

J-51           Control           22    220
             Aziridine           14    140

J-52           Control           23    230
             Aziridine           13    130


Conclusion

The true nanoscale nature of CUPs can result in a well crosslinked acrylic clearcoat. The coalescing mechanism for CUPs was similar to latex and the effect of coalescent aid was found to be analogous. Aziridine-cured CUP coatings produced well-crosslinked films. This work illustrates the utility of CUPs and their low viscosity. These near-zero VOC systems (if ammonia is used) offer a potentially high performance technology option for future coatings for both OEM and architectural applications.

Over the past few years, the development of acrylic CUPs has moved from the realm of laboratory investigation to the point today at which they can be tested and developed commercially in numerous applications. Thus, the utilization of aziridine curing agents for water-reduced acrylic CUPs has illustrated their usefulness and potential in such applications as clear floor finishes or clear topcoats.

Acknowledgments The authors would like to acknowledge the Missouri S&T Coatings Institute for the financial support and the following for CUP allied work: Cynthia Riddles, Minghang Chen, Sagar Gade, Ameya Natu and Catherine Hancock.

[c] American Coatings Association & Oil and Colour Chemists' Association 2013

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(10.) Riddles, CJ. Zhoa, W. Hu. H-J. Van De Mark, MR, "Colloid Unimolecular Polymers (CU Ps) Synthesized by Random Copolymerization of MMA/MAA." Polym. Preprints, 52 (2) 232-233 (2011)

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J. K. Mistry, M. R. Van De Mark ([??])

Department of Chemistry, Missouri S&T Coatings Institute, Missouri University of Science & Technology, Rolla, MO 65409, USA

e-mail: mvandema@mst.edu

DOI 10.1007/s11998-013-9489-z
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Author:Mistry, Jigar K.; Van De Mark, Michael R.
Publication:JCT Research
Date:Jul 1, 2013
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