Research on formulation design and application of "wet-on-wet" water-based engineering machinery coatings

In recent years, a series of environmental protection policies have been introduced in China, water-based coatings in the metal coating market share continues to improve, especially in the field of construction machinery, Sany Heavy Industry, Zhonglian Heavy Industry and other engineering machinery giants have been on-line water-based coatings. Engineering machinery coating is generally "epoxy primer and polyurethane paint" two coating system, based on accelerated production beat, energy saving and emission reduction, reduce manpower and other considerations, more and more construction machinery coating line will be the traditional two spray two baking (2C2B) changed to two spray one baking (2C1B): when the epoxy primer is not fully dry spray polyurethane paint, and then baking, so-called "wet wet". The "wet and wet
" process of mechanical coatings for water-based painting presents a great challenge to the application of water-based paints

. Due to the volatile latent heat of water, water-based epoxy primers generally slow dry, sprayed polyurethane paint on the "wet" epoxy, on the one hand, the bubbles in the epoxy paint did not escape on the cover paint, the bubbles inside the primer top the surface dry layer, The paint is prone to tweezers, as shown in Figure 1, on the other hand, the curing agent (fatamine) of epoxy primer will give priority to the curing agent (isocyanate) of polyurethane paint, so that the primer and finish can not be fully cured, performance is poor.
figure 1 "wet touch wet" surface painted tweezers
This paper with solidified chemical fast-drying epoxy emulsion Banco2092, fast-drying epoxy curing agent 920, hydroxypropyl dispersion Uacryl2150, isocyanate curing agent Uhard3596T as the base material, developed 15min "wet wet" appearance and performance are good water engineering mechanical substrate and paint coating.
experimental part
training raw materials
Table 1 Experimental raw materials table
coating preparation
will be deionized water and magnesium polysilicate aluminum high-speed stirring 20min, and then dispersants, moisturizers, deblisters in turn added dispersion In the cylinder, stir well, add the filler while stirring, spread over a medium speed for 20 minutes, grind to a fineness of less than 25 m, transfer to the paint tank, while stirring to add resin, film aid, moisturizer, and finally adjust the viscosity with thickener. Recipes are below Tables 2 and 3.
Table 2 Water-based epoxy primer formula
Table 3 water-based polyurethane paint formula
results and discussion of
water-based epoxy resin selection
"wet touch wet" requires epoxy primer at room temperature under the rapid table drying, which on the one hand is conducive to the rapid escape of air bubbles in the primer, on the other hand, is also conducive to the initial "anchoring" of fatty amine molecules, reduce and isocyanate curing agent reaction. Therefore, the selection of E20 molecular epoxy emulsified products: Banco2092 and competition 1, 2, 3. Compare the contact time and perform a 15min "wet touch wet" test. Wet touch wet process see Figure 2. Temperature and humidity control at 23 degrees C 50% RH, primer paint dry film thickness of 40-50um, the results can be seen in Table 4.
2 "Wet Wet" Process
Table 4 Banco2092 and Race 1, 2, 3 "Wet Wet" Test Comparison
by Table 4, Race 1 and Banco209 2 table dry the fastest, 15min "wet touch wet" behind the paint gloss high, no tweezers, but race 1 dry too fast, film-forming poor, spraying easy to crack at the edge of the plate, see Figure 3, thus affecting the surface of the paint. And competition 2, 3 table dry too slow, 15min "wet touch wet", paint a lot of tweezers, and the gloss is not high.
3 Competition 1 spray cracking phenomenon
resin is the main film-forming substance of the coating, which determines the main properties of the coating film. Banco2092 uses a reactive emulsion in the process of emulsion synthesis. As shown in Figure 4, reactive emulsion molecules bond to the surface of latex grains by co-price bonds, and their emulsions are more stable and film-forming faster than conventional emulsions. And because the reactive emulsifier after film formation will not migrate to the interface, in the air-coated interface and substrate-coated interface will not appear continuous hydrophobic layer, coating film denser degree, hydrophobic and adhesion is stronger.
4 Banco2092 Structural Diagram
The selection and optimization of
epoxy curing agent types are critical to crosslinking reaction speed, affecting the drying time of the coating and the establishment of initial hardness. In general, in order to extend the opening hours of water-based polyurethane paint, so that NCO and water-reflected CO2 fully escape, reduce the risk of tweezers, paint will be added to a large number of slow-drying solvents. And in the "wet touch wet", if the initial hardness of epoxy primer is not high, the paint's slow dry solvent is easy to "bite" the primer, affecting the appearance of the paint. Here, two types of water dispersion type curing agent are selected to make a wet-to-wet comparison, the results can be seen in Table 5. It can be known that Banco920 and Competition 1 drying speed is faster, "wet touch wet" after the paint no tweezers, also do not bite the bottom. This is because the dispersed type of amine usually hydrophobic oil, add epoxy emulsion, due to pro-oil, curing agent will quickly "drill" into the latex particles cross-linking reaction, thus having a faster drying speed. In addition, banco920 contains benzene rings and has a high initial hardness of the curing agent, reducing the risk of "bite" of paint solvents.
Table 5 Banco920 and Race 1, 2, 3 "wet touch wet" test comparison
In order to improve the drying speed, in the oily epoxy, usually mixed in the curing agent a certain amount of seramine promoter K54. However, K54 is usually insoluble in a water-curing agent. To this end, Banco 920 and Race 1 were selected for the comparison of the drying speed and storage test of K54, the results of which can be found in Table 6. It can be known that the Banco 920 and K54 compatible is better, long-term storage without hard and dull. Adding K54 can further shorten the table drying time, the larger the addition, the faster the drying speed. The addition of K54 to the Banco920 means that the drying speed of the curing agent can be effectively adjusted.
Table 6 Banco920 and Race 1 add K54 test comparison
hydroxypropyl dispersion selection
"wet touch wet" easy to produce tweezers, the choice of bubble-free film thick high hydroxypropyl dispersion is the basic principle. Here, Uacryl 2150, Race 1, Race 2 comparison, test its flat 10min, 60c baking 30min process under the bubble-free film thickness and "wet wet" effect, the results can be seen in Table 7. It can be known that Uacryl2150's bubble-free film thickness is the largest, "wet touch wet" is not easy to pick up tweezers.
table 7 Uacryl2150 and competition 1, 2, 3 "wet touch wet" test comparison
Figure 5 schematic, hydroxypropyl dispersion is generally a nuclear shell structure, resin body is wrapped in auxiliary solvents, and Uacryl2150 is much more than ordinary dispersion from Resin core outwards of the bit resistance group, these large molecules of the bit resistance group can be completely dry before the coating, leaving a certain micro-channel, so that the remaining solvents and water have sufficient time to escape, so that Uacryl2150 thick coating is not easy to lift tweezers, paint to get better leveling and surface effect.
Figure 5 Uacryl2150 structural schematic
formulation optimization and process adjustment
engineering machinery is mostly sand blasting steel parts, its substrate is uneven, the coating on top is difficult to achieve flat, so usually pay more attention to the paint 60 degrees angle gloss. However, as customers demand more and more appearance, especially in some flat steel parts, such as tank cars, sanitation vehicles, opposite paint 20 degrees gloss also put forward a high demand. From the point of view of the "wet touch wet" process, the primer does not polish on the top of the paint, the flatness of the primer and the surface condition is bound to be opposite the appearance of the paint, especially the 20 degrees gloss impact.
by the above resin and curing agent screening, "wet touch wet" optimal solution determined: Banco2092 plus Banco920 for fast-dry epoxy primer, Uacryl2150 plus Uahard3596T for polyurethane paint. To improve primer flatness, add a small amount of the strong leveling agent Addsil3076 to the primer formulation.
a large proportion of fillers in primer formulations, and the type of fillers has a great impact on the surface condition and resistance of primers. To this end, screening different fillers, to observe the primer fillers on the "wet touch wet" paint gloss effect, the results can be seen in Table 8. It can be known that the greater the amount of oil absorbed by primer fillers, the lower the gloss of the paint 20 degrees, the face paint in the primer containing filler 1 has a "foggy" appearance, which may be related to the primer surface is rougher, filler opposite the paint "absorption". Considering that the flaky structure of filler 1 has a physical shielding effect on corrosive media, can increase the performance of salt spray resistance, filler 1 and filler 3 for compounding, oil absorption is reduced, "wet touch wet" paint 20 degrees gloss can reach 83, and finally "wet touch wet" after the surface effect see Figure 6, paint gloss and surface condition basic and single-layer paint is no different.
Table 8 primer filler on the "wet touch wet" paint gloss effect
Figure 6 wet wet final effect map
In order to further improve the drying time of the primer, the curing agent Banco920 with ethanol thinning, it was found that the primer sprayed out with orange peel effect, so that the paint also become orange, see Figure 7. This may be because ethanol is drying too fast, when sprayed will be rapidly volatile, affecting the smoothness of the primer, so in the surface effect requirements of the occasion, it is not desirable to use ethanol to open thin curing agent.
Figure 7 Painted Orange Peel Photo
Performance Test
Performance Test for Banco2092 plus Banco920 Fast Dry Epoxy Single Layer Primer, as well as its composite layer after "wet and wet" with Uacryl 2150 plus Uahard3596T polyurethane paint, as shown in Tables 9 and 10, salt mist-resistant photos. It can be seen that the bottom surface matching can meet the performance requirements of most construction machinery coatings.
Table 9 Banco2092 plus Banco920 Fast Dry Epoxy Primer Performance Test
Table 10 Banco2092 and Banco920/Uacryl2150 plus Uahard3596T Wet Touch Performance Test
Figure 9 Salt Mist Test Photo
Knots
Selected Co-solid Chemistry's Quick Dry Epoxy resin and solids The chemical agent Banco2092 plus Banco920, bubble-free film thick, fresh and good serotonin dispersion Uacryl2150 plus Uahard3596T, mixed with epoxy serotonin promoter K54, screening fillers, optimized formula, we have developed a suitable 15min "wet wet" process, excellent appearance, good performance of water-based epoxy background and polyurethane paint formulation. "Wet and wet" composite layer 20 degrees gloss, 80, 60 degrees gloss, cold-rolled steel plate salt spray 720h, sand blasting steel plate salt spray 1000h, good heat resistance oil resistance, better temperature resistance. This scheme is a better choice for construction machinery coating customers with water-based epoxy-based polyurethane surface "wet and wet" demand.
References
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