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There are three main trends in the coatings market: (l) reducing volatile organic compounds (VOC), (ll) low cost in use and

(lll) high performance. To achieve low VOC targets, we may rely on waterborne (WB) technology, solventborne (SB) high solids or powder coatings. As for decreasing cost in use, there are also many routes, such as reducing coating layers, reducing film thickness and shortening process sections or time.

Transportation coatings are more diversified than Auto OEM because they have different levels of requirements. For example, the performance requirements of train and metro coatings are higher than trucks and lorries. Part of them use three-layer technology and some just two layers or even one layer (monocoat). Monocoat is very convenient to apply and low in cost. Thus, it has become popular in truck manufacturing lines.

For the refinish market, end-users want quick dry products and want their cars to be repaired as fast as possible. Garage owners and repairing technicians do not want to spend much time and money on establishing a thermal curing process for the refinish.

In this article, we will demonstrate how BASF’s low VOC and cost-efficient technologies meet customer needs (l) waterborne monocoat solutions for trucks based on an acrylic dispersion (Joncryl A) and melamine cross-linker (Luwipal B) and (ll) fast dry refinish clear coat, which can be buffed in around 30min just at room temperature (25°C), based on acrylic polyol (Joncryl C and Joncryl D).

nn EVALUATION OF WB MONOCOAT AND FAST DRY REFINISH CLEAR COAT

Key and important properties, such as solid content, viscosity, pot-life, coating stability, DOI (Distinctness of image), LW (Longwave), SW (Shortwave), gloss, hardness, adhesion, water immersion resistance, anti-scratch, cupping, chemical resistance and weathering resistance were used to evaluate the performance of the two technologies described in this article. For a fast dry solution, we used a benchmark product from the market to illustrate the key differences.

nn MONOCOAT SOLUTION

Formulation design: White is a popular colour in the market and it is easy to find out the

weathering performance via a QUV test, so we chose TiO2 (R706 from DuPont) as the pigment. As a classic white pigment paste package solution from BASF, Dispex Ultra PX 4575 (dispersing agent), Foamstar SI 2280 (defoamer) and Hydropalat WE 3650 (wetting agent) were used in the mill base. Main binder Joncryl A, which is produced, based on special rheology control (RC) technology from BASF, has a very small particle size (<100nm), which can provide high gloss and a transparent film surface and there is no surfactant used in RC dispersion, so the weathering property is also expected to be better. Luwipal B, partially methylated amino resin, was incorporated as a crosslinking binder to improve the physical properties, such as hardness, impact resistance and chemical resistance. One step needs to be mentioned – Luwipal B should be blended with butyl glycol before adding into the coating mixtures for better incorporation.

Futao Cheng, Jing He, Yaju Guo, Bin Zhao and Jiliang Ma, BASF, discuss solutions for efficient and productive applications: waterborne monocoat formulations for transportation and fast dry clear coats for refinish

Meeting customer needs with low-VOC and cost-efficient technologies

Ingredient Supplier Monocoat formulation (MF)

MF with DW type UV-HALS package (MF-DW)

Dispex Ultra PX 4575 BASF 1.56 1.56

R 706 DuPont 19.46 19.46

Foamstar SI 2280 BASF 0.14 0.14

DI water 6.58 6.58

Hydropalat WE 3650 BASF 0.06 0.06

Joncryl A BASF 50.6 50.6

BG 2.2 2.2

DPnB 6.3 6.3

DI water 2.6 2.6

Rheovis PU 1191 BASF 0.25 0.25

Rheovis PU 1291 BASF 0.65 0.65

Foamstar SI 2280 BASF 0.4 0.4

Tinuvin 1130 BASF 0.75 0

Tinuvin 292 BASF 0.75 0

Tinuvin 99DW BASF 0 2.5

Tinuvin 123DW BASF 0 2.5

Luwipal B BASF 7.2 7.2

BG 2 2

Total 101.5 105

Table 1. Formulation for WB monocoat solution

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Two types of thickeners were used in the formula. One suitable for low shear rate (for storage stability), the other for mid shear rate (mixing and application). The combined effect enables the product to have good application performance and sagging resistance.

Tinuvin 99DW and Tinuvin 123DW, which are BASF’s unique products for waterborne application were used as UV-HALS package to improve the outdoor durability and we also checked the traditional type UV-HALS package (Tinuvin 1130 and Tinuvin 292) as a reference to see the difference. All the ingredients and information in the formulation are summarised in Table 1.

nn PERFORMANCE EVALUATION

With the help of effective dispersing agent Dispex Ultra PX 4575 and wetting agent Hydropalat WE 3650, the fineness of the mill base can be easily decreased to less than 10μm. And then binders, other additives and some co-solvents are added to let down the pigment paste. The final pigment to binder ratio is 0.68, solid content is 48%. Initial viscosity is 1650cp but after diluting with 15% water, it can be cut down to 20s (DIN-4 cup, 23°C) (Relevant data shown in Table 2).

Due to the fine particle size and its low dispersity index, the gloss of the coating film can reach 96 (60°), also pencil

hardness is about F and still have enough film flexibility (cupping value is 8mm and impact performance is >50kg). The polarity and chemical structure of Joncryl A and Luwipal B balance very well so has very good adhesion on the substrate with electrophoretic coating layer. Due to proper crosslinking density and Tg, chemical resistance of the dry coating film is quite good against NaOH, H2SO4 and gasoline. Dipping in these chemicals after 24hr, the film surface shows no visual change.

QUVA, humidity resistance and a salt spray test were used to evaluate the durability performance of the product. After a 10-day humidity test, adhesion was still very good and there is no visual change on the film surface. Salt spray test also shows the corrosion area didn’t spread very much and was just under the acceptable range (width <3mm, Figure 1).

The 1000hr QUVA test shows the weathering property of the film is still good and there is a big difference between traditional UV-HALS package and DW product package in delta E. Detailed data are listed in Table 1. From the data, we can see after the 1000hr QUVA test, the gloss retention of the two formulae is nearly the same but the colour change (delta E) has

Figure 1. Salt spray test result of monocoat solution

Figure 2. Below and right. QUV test result of monocoat solution with different UV-HALS packages

Figure 3. Preparation process of DW UV-HALS products

Table 2. Test result summary of the monocoat solution

Performance summary of monocoat

Basic performance

pH 8.95

Fineness >10μm

P/B ratio 68%

Solid content 48%

Viscosity Initial 1650cp

15% water dilution 20s (DIN-4)

Physical property

Gloss 60°C 96

Pencil hardness F

Cupping 8mm

Impact 1Kg >50cm

Adhesion 5B

Chemical resistance (24hr, dipping) (5:best)

0.1N NaOH 5

0.1N H2SO4 5

93# gasoline 5

Durability QUVA (1000hr) delta E Traditional UV-HALS package 2.4

DW products package 1.4

Gloss retention

Traditional UV-HALS package 88.10%

DW products package 88.80%

Humidity resistance (100%RH* 240hr)

Adhesion 5B

No visual changeAppearance

Salt spray test (1000hr) OK

delta E Gloss retention (20°)

2.5% 99DW + 2.5% 123DW

0.75% 1130 + 0.75% 292

2.5% 99DW + 2.5% 123DW

0.75% 1130 + 0.75% 292

200hr 0.2 0.95 97% 100%

600hr 1.35 2.25 94.70% 96%

1000hr 1.4 2.4 88.80% 88.10%

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a one unit gap (Figure 2). To explain the result, we need to understand the structure of theDW product, which is produced via NEAT (Novel Encapsulation Additive Technology) from BASF. Traditional UVA and HALS particles were wrapped in the acrylic polymer colloidal, which is formed by mini-emulsion polymerisation (Figure 3). The acrylic polymer has very good water compatibility, stability in weathering and other physical performance that will hinder the UV-HALS particles from washing off from the surface of the film and provide a persistent protecting effect. On the other hand, DW products can be easily incorporated into waterborne coatings, so it is very convenient for production and application.

nn FAST DRY SOLUTION

Formulation design: A clear coat formula usually includes a main binder, which provides basic physical properties, solvent, different additives, which can improve the performance from liquid coating application to dry film. We chose Joncryl C as the main binder because it is a stable product, based on SGO (solid grade oligomer) technology, which is the best one to produce acrylic polyols with low viscosity and narrow PDI (Polydispersity index) (Figure 4). Narrow PDI means lower amount of high molecular part, which may contribute a lot to the final performance of the coatings, such as better flow and levelling, lower crater tendency, less thermal sagging and so on. To improve the levelling and appearance, we also use some Joncryl D, which has lower Mw, Tg and can provide better flow and gloss (Table 3). Basonat HI 100AP, which is aliphatic polyisocyanate with excellent weather and chemical resistance, is used to get a non-yellowing clear coat layer. NCO (Isocyanate) percentage is around 22%. One percent Dibutyltin Dilaurate (DBTDL) xylene solution was used as a catalyst for the OH/NCO reaction.

For a fast dry clear coat system, the solvent should have better solvency and

low viscosity. Quick dry is an important requirement, so we chose n-butyl acetate and xylene as the main solvents. Butyl acetate has good solvency and xylene’s evaporation is slower than n-butyl acetate, which is better for levelling and has low pinhole risk for the fast dry system. Three percent PMA is used to improve solvency and decrease evaporation speed for better levelling.

Fast dry means 10min touch free and 30min drying for polish, so after spraying on the substrate the NCO/OH reaction starts very quickly and there is less time for the solvent and air bubbles trapped in the wet film to escape and less time for levelling, which means the levelling agent and defoamer are critical for the formula.

After screening we found Efka FL 3770 combined with Efka SI 2722 shows a better appearance. Efka FL 3770 is a fluorocarbon modified polyacrylate, which has low Mw and wide compatibility and low bubble stabilisation effect, which is suitable for a 2K PU system. Efka SI 2722 is a silicon modified polymer that has a little lower compatibility with our binder system (need to use high shear force to incorporate into binder) but strong defoaming properties. However, we cannot use Efka SI 2722 too much because it has crater risk if used excessively. For the formula, 0.3% is recommended. The drier’s level is also very important because it can determine the pot-life and hardness development. We tried several levels, 100ppm, 200ppm and 300ppm. At 100ppm, drying speed is a little low and at 300ppm, its pot-life decreases. So we chose 200ppm level. Tinuvin 5151 is used (a combination of UV absorber and basic hindered amine light stabiliser) as an economic UV-HALS package to obtain good weathering performance. The UVA used in Tinuvin 5151 has broad

UV absorbance, especially in 300nm and 340nm. A very economical dosage FW (Formula Weight) is 0.5% and if the weathering performance requirement is high it can be enhanced to 1-2%FW to get a clear coat formulation by combining the materials described in Table 4.

nn PERFORMANCE EVALUATION

We took a popular benchmark product from the market to do a performance comparison. Let’s call it Benchmark. We tested the liquid coating and dry film. Detailed results are summarised in Table 5. Solid content of the two products is in the same range, both around 45%. Double viscosity pot-life of fast dry (FD) is 95min, which is 26% higher than Benchmark (75min). This may be caused by a higher drier level in the formula to obtain the quick dry performance but the pot-life balance is not so good. Also, the levelling and pinhole resistance are affected. The two products were dried at room temperature and film thickness for both was around 45-50μm. Touch free time and dry through time were in the same range, which means both products can be polished within 30min of air drying.

For hardness development, we can see Benchmark is a little better; after seven days FD’s pendulum hardness is 36, while Benchmark is 40 but their pencil hardness is the same (HB). Considering they have the same drying performance, we assume their hardness is the same.

Gloss and appearance results show FD is a little better than Benchmark because the secondary binder of FD is Joncryl D, which has low Mw and narrow PDI and owns excellent molecular level mobility, so its leading to better self-levelling is better. Benchmark and FD both have excellent chemical resistance. A humidity test shows that after 10 days, 100% RH test gloss

Figure 4. Mw (molecular weight) dispersity of SGO polyol and conventional polyol

Table 3. Summary of key parameters for Joncryl RPD 950-B and Joncryl RPD 980-B

Table 4. Formulation for fast dry solution

Joncryl C Joncryl D

Resin type Acrylic polyol Acrylic polyol

Solid content 95-61% 79%-81.5%

Solvent n-butyl acetate

n-butyl acetate

Viscosity (23°C, Pa•s)

4-5 3.6-8

OH% on solid 3.30% 4.20%

Tg (°C) 27 -7

Colour, hazen <50 <50

Fast cure modification

Yes Yes

Component wt%

Joncryl C 36.5

Joncryl D 7.2

Butyl acetate 25.8

Xylene 12.3

Propylene glycol monomethyl ether acetate (PMA)

3

Efka FL 3770 0.2

Efka SI 2722 0.2

TINUVIN 5151 1

1% DBTDL 2

HI 100 11.8

Total 100

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retention of Benchmark is lower than FD by 8% because the binder of FD has very good water resistance performance.

A cupping test shows FD is better than benchmark. Chemical resistance and cupping performance basically relate to the ‘toughness’ and ‘compactness’ (or crosslinking density) of the binders. Based on these data shown in Table 5, we can see FD has good balance in Tg and crosslinking density.

One key difference between the two products is the pinhole resistance. During application, it is much easier for

Benchmark to get pinhole problems when the film’s thickness is a little higher (>60μm) or temperature is higher (35°C) (Figure 5). However, FD formulation shows a very broad application window and pinhole problem was not observed even during test in extreme conditions. Because the drying speed of the coating is very high, there is less time for the air bubble trapped in the wet film to escape (Figure 5), so also an anti-foaming agent is quite important in the formulation. Overall, FD sample exhibits better performance than Benchmark in most key items, such as

pot-life, appearance, humidity and pinhole resistance (Figure 6).

nn SUMMARY

Efficiency and productivity are important requirements for coating end-users. Especially in the Asia Pacific region, many customers look for convenient, stable and economical coating solutions. Therefore, we have designed 1K waterborne monocoat solutions using a unique RC dispersion technology. Similarly, we have developed a fast dry refinish clear coat, using polyols designed by unique high temperature polymerisation (SGO technology). These technologies deliver outstanding physical properties and durability that can provide excellent protection and aesthetic functions. Moreover, they are easy and simple for application. Such designs could be good options to reduce VOC and make coatings process efficient. n

Contact Author: Mr Futao Cheng, Coating Formulation Team Manager, Dispersions and Resins (ED) R&D Center, Asia Pacific, BASFTel: +86 21 2039 3737; Email: futao.cheng@basf.com

Acknowledgements

The authors would like to thank Dr Vijay Immanuel Raman and Dr Qiuling Feng’s consistent support on the work; Ami Ganatra and Kin Sum Kong’s professional inputs.

Fast dry (FD) Benchmark

1 Solid content 44% 46%

2 Viscosity (DIN-4, 23° 17 16.8

3 Potlife (double viscosity) 95min 75min

4 Drying condition 30min air dry 30min air dry

5 Dry film thickness (μm) 45-50 45-50

6 Touch free time 10min 10min

7 Dry through time 30min 30min

8 Hardness (pendulum/pencil)

8.1 One day 25/2B 30/2B

8.2 Two days 29/B 35/2B-B

8.3 Seven days 36/HB 40/HB

9 Gloss (20/60/85) 89.4/93.4/98 89.5/93.2/94.3

10 Appearance (DOI/LW/SW) 93.4/3.6/17.3 93.3/4.6/18.4

11 Normal adhesion OK OK

12 Cupping (mm) 8.4 8

13 Chemical resistance (20° gloss retention after gasoline/alcohol/2%HCI rubbing for 50 times)

100%/100%/100% 100%/100%/100%

14 10 days of humidity resistance (100%RH, 40°C)

14.1 Appearance No visual change No Visual change

14.2 Adhesion 5B 5B

14.3 Gloss retention 99.0% 91.0%

14.4 DOI retention 98.0% 98.0%

15 QUVA (delta E,500hr) 1 0.9

16 Pinhole resistance Strong Weak

Table 5. Test results of fast dry and Benchmark products

Figure 6. Performance comparison between fast dry formula and Benchmark

Figure 5. Appearance of fast dry product and Benchmark with different pinhole resistance performance (including mechanism and related parameters)

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