New type of cashew nut shell liquid water-based curing agent-specially designed for high-performance, low-VOC epoxy protective coatings

In order to meet more stringent regulations and high performance requirements, a series of zero volatile organic compound (VOC) waterborne epoxy curing agents based on non-food chain renewable biomaterial cardanol have been developed

The test results show that the new water-based curing agent enables low VOC (<75g/L) formulations that can be directly used in metal surface primer systems with excellent performance, such as balancing fast curing and pot life, and excellent attachment Focus on long-term corrosion protection coatings for a variety of metal substrates online coatingol.

｜Introduction｜

CNSL-Cashew Nutshell Liquid (CNSL-Cashew Nutshell Liquid) is a sustainable non-food chain biological material that can be obtained from a by-product of the cashew industry

Figure 1 Cashew pear and husk

Cardanol is the main component extracted from CNSL by decarboxylation and extraction

Figure 2 Average cardanol structure

Traditionally, phenolalkaneamines and phenolalkanamides have been used in solvent-based and high-solid epoxy coating systems for marine and protective coatings

This article introduces the performance evaluation of two new water-based curing agents, one is CNSL curing agent and the other is non-CNSL curing agent, which are used in several low-VOC water-based primer systems

｜Materials and Experiments｜

Table 1 lists two new water-based curing agents, referred to as WB-A (a modified polyamine-based water-based curing agent) and WB-B (a CNSL-modified polyamine-based water-based curing agent), and the first The typical performance of the first generation CNSL-based water-based curing agent (abbreviated as WB-C) and a commercial water-based curing agent abbreviated as COM (a conceived polyamine adduct type water-based curing agent) for comparison

Table 1 Typical properties of water-based curing agent

This study used five solid epoxy resin dispersion resins, namely Resin 1, Resin 2, Resin 3, Resin 4, and Resin 5
.
Their typical properties are listed in Table 2
.

a: Based on epoxy dispersion solids

Table 2 Typical properties of solid epoxy resin dispersion resin

The linear drying time test is carried out according to ASTM D5895-03
.
Use an 8 mil (200μm) coating rod to apply a clear (unpigmented) paint on a 12x1x0.
125 inch glass strip
.
The glass strip is immediately placed on the dry film recorder that has been placed in a 25°C thermostat, the test needle of the instrument drops on the wet film, and the linear drying time test is started
.

Spray the water-based primer onto different types of substrates
.
After curing at room temperature for 7 days or baking in a 60°C oven for one to two hours, the samples are subjected to adhesive tape test (ASTM D3359) and salt spray resistance (ASTM B117) test
.

｜Results and discussion｜

Part 1: Improved application performance of new water-based curing agent

WB-C is part of the first generation CNSL-based water-based curing agent, and its calculated biological content is 44.
1%
.
In previous studies, the WB-C system showed very interesting properties, such as fast curing, excellent early water resistance and good compatibility with various epoxy resins
.
However, the WB-C system needs to further improve some application properties, such as the ability to be easily diluted by water and further balance the curing speed and construction time limit
.
WB-A and WB-B are newly developed high-performance water-based curing agents
.
WB-A is not based on CNSL, while WB-B is based on CNSL, with a calculated bio-based content of 44.
0%
.
It can be seen from Table 1 that although WB-A (5600cPs) and WB-B (10798 cPs) curing agents are supplied at 80% solids, their viscosity is still much lower than 50% solids WB-C (15000- 55000 cPs)
.
The viscosity of WB-A and WB-B is also lower than the commercial product COM
.

If you ignore the initial solid difference of the four water-based curing agents and pay attention to the viscosity changes of each water-based curing agent after adding a certain proportion of water, it can be seen in Figure 3 that the dilution performance of WB-A and WB-B has been improved ：Only adding 10% water can reduce the viscosity of WB-A and WB-B from 6300 cPs and 14700 cPs to 4200 cPs and 10000 cPs respectively; while the viscosity of WB-C system remains at 20000 cPs after adding 25% water Around
.
The results show that these two new water-based curing agents have more user-friendly application properties
.

Figure 3 The viscosity of the water-based curing agent as a function of the percentage of water added

It is common for water-based curing agents to have different initial viscosities when mixed with various solid epoxy dispersion resins
.
For example, the initial viscosity of WB-A (in stoichiometric ratio) mixed with Resin 1, Resin 3 and Resin 4 is 9650 cPs, 14920 cPs and 21250 cPs, respectively, as shown in Figure 4.
No matter which resin you choose, dilute The results show that mixing with WB-A can be easily diluted with a small amount of water to a low viscosity
.
Less than 15% of additional water is sufficient to reduce the mixed viscosity of WB-A and resins 1 and 3 to below 2000 cPs
.
Even when the viscosity of WB-A and resin 4 is high, 20% additional water can effectively reduce the viscosity to about 3000 cPs
.
WB-A has good dilution performance, indicating that WB-A is suitable for high solids (>60% solids) aqueous formulations
.

Figure 4 Dilution performance of WB-A and various solid epoxy dispersions

It is desirable for a coating system to exhibit fast curing performance while having a longer working window (application time limit) for easier application, although this is usually a difficult task
.
For example, CNSL-based WB-C curing agent mixed with various solid epoxy dispersion resins has excellent fast curing performance (hard-drying time is less than 2h at 25°C)
.
However, as shown in Figure 5, its average construction time limit is about 1.
5 hours (shown as a red dot), which can be considered shorter in some applications
.
Through a better design of the polymer structure, the new water-based curing agents WB-A and WB-B show better performance, fast curing, and long construction time
.
As shown in Figure 5, WB-A and WB-B mixed with Resin 1 and Resin 3 can achieve a hard drying time of 3 hours or less, while extending the construction time limit to 3.
5 hours
.
Competitive product system (COM) also showed a longer construction time limit, but a slower hard-drying time
.

Figure 5 Linear drying time data of various water-based systems

The Persoz hardness data in Figure 6 confirms that the WB-A and WB-B systems provide faster hardness build-up than the COM-based system
.
In addition, the fast curing characteristics of WB-A and WB-B are also conducive to the construction of wet-on-wet topcoats, which will be described in detail later in this article
.

Figure 6 Persoz hardness development of various water-based systems

Part 2: Water-based primer/medium coat system based on the new WB-A and WB-B curing agents

Excellent long-term corrosion protection is a key feature necessary for waterborne epoxy primer systems and midcoat systems; however, achieving this is also very challenging
.
In this study, the newly developed WB-A and WB-B curing agents were evaluated to evaluate their corrosion resistance after prolonged salt spray exposure
.
Different performance aspects, such as rust or bubbles on the board surface, and crack changes along the scribe line, were checked after a certain exposure interval, and the wet adhesion of the test panel was tested after 800h of salt spray exposure
.

Table 3 Water-based primer formulations based on WB-A, WB-B and COM

In Table 3, the formulations of MC#1, MC#2 and MC#3 three low-VOC water-based primer systems based on WB-A, WB-B and COM curing agents are listed
.
For comparison purposes, all three primer systems were formulated with the same solid epoxy dispersion (resin 3), and had similar solids content (approximately 57%), and equivalent pigment volume concentrations (PVC from 27% to 30%) and use the same stoichiometric ratio of 1.
25
.
It can be seen that the VOC value of these water-based primer systems is less than 75g/L
.

The anti-corrosion performance of MC#1, MC#2 and MC#3 formulations was evaluated in DTM-direct-to-metal primer
.
These primers are sprayed directly on various metal substrates without pretreatment by air spraying methods, such as SA2.
5 sandblasted steel plates, cold rolled steel (CRS) plates, galvanized steel plates, aluminum alloy AA2024T3 plates and stainless steel plates
.
The samples coated with these water-based primers used two curing conditions: curing at room temperature for seven days or baking in a 60°C oven for one to two hours
.
The final dry film thickness (D FT) of the water-based primer film after curing is about 55~80μm
.
The back and edges of the test panel are sealed with tape or coating, and then placed in the Q-FOG room of Q-Lab for ASTM B-117 testing
.

Figure 7 Photographs of test panels of WB-A/MC#1, WB-B/MC#2 and COM/MC#3 systems after 1162 hours, 949 hours and 1162 hours of salt spray exposure (SA2.
5 steel plate, baking Cured, dry film thickness ≈60-75μm)
.

Figure 7 shows the sample photos of MC#1, MC#2 and MC#3 systems after approximately 1000 hours of salt spray exposure and 800 grit sandpaper to remove surface rust stains
.
These coating films are applied to SA2.
5 sandblasted steel plates, and the dry film thickness is about 60~75μm
.
It can be seen that only some small bubbles are formed near the scribe line of the WB-A system coating film, and the coating film still has good adhesion to the steel substrate along the X-shaped scribe line
.
Compared with the WB-A system, the WB-B system has smaller bubbles, but this may be because its salt spray exposure is 200 hours less than that of the WB-A system
.
The COM system also showed good anti-corrosion performance.
There were few bubbles formed on the surface of the coating film, but some cracks were observed along the scribe line
.

As part of this research, one can also pay attention to the significant impact of coating film thickness on long-term anti-corrosion performance, especially on sandblasting boards
.
For example, MC#1 formulation paint is coated on steel plate with various dry film thicknesses of 37μm, 50μm, 65μm and 100μm, and then exposed in a salt spray box
.
After 500 hours of salt spray exposure, the system with a dry film thickness of 37μm has been severely rusted and blistered, while the other three systems still have a complete coating; by 1100 hours, the system with a dry film thickness of 50μm has been There were denser bubbles on the scribe line than the 65μm dry film thickness system, but no bubbles appeared in the 100μm dry film thickness system
.
As expected, the higher coating film thickness provides better and longer anti-corrosion protection for the metal substrate
.
In addition, the test results of this study indicate that the influence of the film thickness of the water-based primer system on the anti-corrosion performance may become more important on the sandblasted steel plate
.
This may be because the water-based primer tends to penetrate and settle in the bottom crevices of the rough surface of the sandblasted steel plate, which results in some weak areas with lower coating thickness where corrosion can begin
.

Figure 8 shows the sample photos of MC#1, MC#2 and MC#3 systems after approximately 1000 hours of salt spray exposure, using 800 grit sandpaper to remove surface rust stains
.
These water-based primers are applied to cold-rolled steel baseplates with a coating thickness of approximately 75μm
.
In all three systems, only bubbles were observed along the scribe line, but the adhesion of the coating film to the cold-rolled steel substrate along the scribe line was poor; MC#1, MC#2 and MC#3 systems were 3mm, 1.
5mm and 3.
5mm bottom cracks
.
These test results show that the new water-based primer system can provide good corrosion protection on cold-rolled steel, but it is still a major challenge to obtain excellent adhesion on smooth steel plates after long-term salt spray exposure
.

Figure 8 Photographs of samples of WB-A/MC #1, WB-B/MC #2 and COM/MC #3 systems after exposure to salt spray for 1162 h, 949 h and 1162 h (S-36 cold-rolled steel substrate , Bake curing, dry film thickness 75 μm)
.

Next, this study evaluated the anti-corrosion and adhesion properties of the new water-based curing agent on various metal substrates.
These are metal substrates commonly used in industrial coating applications, including aluminum alloys, stainless steel, and galvanized steel
.
Generally speaking, for these substrates, good adhesion and long-term corrosion protection performance may be difficult to achieve, especially for coatings with low VOC formulations
.

Figure 9 shows the sample photos of the MC#1, MC#2 and MC#3 systems on the aluminum alloy AA2024T3 substrate, which have been exposed to salt spray at 2018h, 1852h and 2018h, respectively
.
(The surface of the bottom plate was polished with 220 sandpaper, and then cleaned with acetone and paper towels
.
) It can be seen that the MC#1 and MC#3 systems have shown good protection performance.
After the salt spray exposure in 2018h, only a few lines were formed on the score line.
A very tiny bubble
.
The MC#2 system also showed good anti-corrosion performance on the sample plate.
After 1800h of salt spray exposure, there were no bubbles and delamination on the sample surface, only some small bubbles were formed along the scribe line
.
Wet adhesion was measured on the test panels exposed to continuous ion and water erosion for more than 800 hours in the salt spray chamber (upper right corner of each panel); therefore, good results were observed from the three primer systems on the AA2024T3 sample panel The wet scratch adhesion indicates that it has good long-term corrosion protection
.

Figure 9 Sample photos of WB-A/MC#1, WB-B/MC #2 and COM/MC #3 systems after exposure to salt spray for 2018 h, 1852 h and 2018 h (AA 2024 T3 substrate, baked Cured, the dry film thickness is 65 μm)
.

After about 1000 hours of salt spray exposure, the sample photos of the MC#1, MC#2 and MC#3 systems applied on the stainless steel substrate are shown in Figure 10
.
No bubbles or stratification were observed in these three systems
.

Figure 10 Photographs of test panels of WB-A/MC #1, WB-B/MC #2 and COM/MC #3 systems after exposure to salt spray for 996 h, 949 h and 996 h (stainless steel substrate, cured at room temperature, dry Film thickness ≈65 μm)
.

In addition, MC #1, MC #2, and MC# 3 systems are also applied to galvanized steel substrates after simply wiping with acetone
.
Figure 11 shows the sample photos of these systems after exposure to salt spray for about 1000 hours: MC#1 system has 2-3 large bubbles and medium density bubbles with a size of 6-8 along the scribe line; MC #2 system appears along the scribe line Two large bubbles and some 8-size bubbles; while the MC#3 system scribes lines from the center to the sides to form larger and denser bubbles
.
The results show that, compared with other substrates evaluated in this study, the bubbles formed on the galvanized steel sheet are more serious under similar salt spray exposure time
.
Although galvanized steel is the most challenging substrate, MC#1 and MC#3 systems still exhibit good wet adhesion properties and can be used as a good choice for formulations
.
The third part introduces the formulation research to improve the corrosion resistance of galvanized steel substrate
.

Figure 11 Photographs of test panels of WB-A/MC#1, WB-B/MC #2, COM/MC #3 system after exposure to salt spray for 1115 h, 949 h, and 1115 h (galvanized steel substrate, cured at room temperature , Dry film thickness ≈65 μm)

Table 4 summarizes the dry and wet cross-cut adhesion of MC #1 and MC#2 primer systems on four types of metal substrates.
The four types of metal substrates are: Smooth and polished bare without pretreatment Cold rolled steel, galvanized steel wiped with acetone, polished with 220 grit sandpaper, then AA 2024 T3 plate cleaned and wiped with acetone, stainless steel without surface treatment
.
The dry adhesion value is measured on the sample plate without salt spray test after curing, and the wet adhesion value is measured on the sample plate after salt spray exposure for more than 800 h
.
It can be seen that no matter what type of metal substrate, MC # 1 and MC # 2 primer systems have good dry adhesion; after 800 h of salt spray treatment, MC # 1 and MC # 2 systems The wet adhesion on the aluminum alloy substrate is still very good
.
On the stainless steel substrate, the MC# 1 system still maintains good wet adhesion, while the MC# 2 system's wet adhesion drops to 3B
.
The MC # 1 and MC # 2 systems on bare cold-rolled steel can still achieve 3B wet adhesion
.
The main difference between MC # 1 and MC # 2 is that MC# 1 system still has good 4B wet adhesion when applied to galvanized steel substrates, but MC# 2 system loses adhesion after prolonged salt spray exposure.

.
The test results in Table 4 show that, except for the poor wet adhesion of the MC #2 system on galvanized steel substrates, the two new water-based curing agents have good wet and dry adhesion to various metal substrates
.
The excellent adhesion of the new water-based curing agent samples is further conducive to the long-term corrosion protection of different metal substrates
.

Table 4 Dry and wet adhesion of WB-A, WB-B and COM systems on various metal substrates

The third part: wet-on-wet performance

Fast self-recoating or recoating with polyurethane coatings is a very ideal industrial application, such as transportation vehicle coatings, agricultural, construction and earth-moving equipment coatings, railway vehicle coatings and other applications
.
The recoating interval between the primer and the second coating must be very short, for example 30 minutes or less at room temperature or elevated curing conditions
.
Since the primer may not be completely dry when the second coat is applied, this curing process is usually called wet-on-wet construction
.
If the primer cures slowly or has poor compatibility with the PU topcoat, it is likely to cause surface layer problems, which means that the cured PU topcoat no longer has the original high gloss, and may exhibit poor adhesion to the primer.
Focus on
.

Table 5 Water-based primer formulation based on WB-A

Figure 12 The photo of the test board after the wet-on-wet adhesion test

The MC#4 system based on WB-A curing agent and No.
3 resin evaluates the performance of wet-on-wet topcoat; the VOC of MC#4 primer system is less than 75g/L
.
The primer system is air sprayed on the cold-rolled steel substrate with a wet film thickness of 50-65 μm
.
The test panels are divided into 6 groups: groups 1 to 3 are cured at room temperature for 15, 30, and 45 minutes, respectively
.
At the same time, the test panels of the 4th to 6th groups were baked in an oven at 60°C for 15, 30, and 45 minutes, respectively
.
Subsequently, a commercial two-component solvent-based polyurethane coating system was air sprayed on all six test panels
.
After curing at room temperature for 24 hours, the gloss of the PU topcoat and the adhesion between the primer and the PU topcoat were measured.
The results are shown in Table 6
.
Figure 12 shows a photo of the test panel after the cross-hatch adhesion test
.
The test results show that no matter what curing conditions and recoating intervals are used, the MC# 4 system can have higher gloss retention (98%) and good adhesion (5B)
.
In addition, after the test panels from group 1 to group 3 were exposed to salt spray for 881 hours, no bubbles appeared on the surface area, only cracks less than 2mm appeared along the scribe line (as shown in Figure 13)
.
These wet-on-wet applications prove that the sample has good adhesion to the polyurethane topcoat after long-term salt spray testing
.
These comprehensive test results show that the WB-A system is suitable for industrial coating applications due to its rapid wet-on-wet performance and excellent long-term anti-corrosion performance
.

a: The original gloss of PU refers to the gloss of the coating film with only one layer of PU coating on the bare cold-rolled sheet

Table 6 Gloss retention of test panels after wet-on-wet application

Figure 13 The photo of the wet-on-wet application test board after salt spray exposure for 881 h

Part 4: Formulation Research

In this study, the influence of some factors in the formulation on the anti-corrosion performance was evaluated, such as different solid epoxy dispersions, different stoichiometric ratios, and the addition of different co-solvents
.

In the formulation of water-based coatings, the good compatibility of the solid epoxy dispersion with the water-based curing agent is crucial to the overall performance of the coating
.
In this study, MC # 4, MC # 5 and MC # 6 three primer system formulations are designed with pigment composition, PVC is very similar, solid content percentage and co-solvent content are similar, using the same water-based curing agent WB-A, stoichiometric The ratio is 1.
24
.
When resin 3, resin 2 and resin 5 are used in MC #4, MC #5 and MC #6 systems respectively, only the solid epoxy dispersion is changed, and the solid epoxy dispersion resin 3, resin 2 and resin 5 are used respectively For MC #4, MC #5 and MC #6 systems
.
The test results show that regardless of the type of metal substrate, the primer systems of MC #4 (based on resin 3) and MC #5 (based on resin 2) have better corrosion resistance than the MC #6 (based on resin 5) system
.
For example, the three test panels shown in Figure 14 show the MC #4, MC #5 and MC #6 systems from left to right, which are applied to galvanized steel sheets
.
After exposure to salt spray for 750 h, some bubbles appeared at the scribe line of MC #4 and MC #5 systems, but there were no cracks or bubbles in the entire area
.
However, the coating film of the MC #6 system became fragile, and some areas began to delaminate from the substrate
.
The test results show that choosing the right solid epoxy dispersion has an important influence on the final performance of the water-based primer system
.

Figure 14 Photo of test panels of MC# 4 system (left panel), MC# 5 system (middle panel), MC# 6 system (right panel) after salt spray exposure for 750 h (galvanized steel substrate, cured at room temperature, dry film thickness = 50 -60 μm)
.

In addition to the choice of epoxy resin, the stoichiometric ratio of epoxy to amine also has a significant impact on the overall performance of the water-based primer system
.
In our previous research, we found that when the water-based primer is applied to sandblasted or cold-rolled steel substrates, a stoichiometric ratio of 1.
25 has the best effect
.
The test results in the second part of this article confirm that a stoichiometric ratio of 1.
25 can obtain a good long-term anti-corrosion effect on the steel plate, but it is poor on the galvanized steel plate
.
Therefore, two different epoxy-to-amine stoichiometric ratios of MC #4 and MC #7 primer systems were tested on galvanized steel sheets of 1.
25 and 1.
10, respectively
.
After 750 hours of exposure in the salt spray chamber, it can be seen from Figure 15 that the MC #7 system with a stoichiometric ratio of 1.
10 has better corrosion resistance than MC #4 because there are fewer bubbles formed along the scribe line
.
The results show that increasing the amount of WB-A in the formulation can effectively eliminate air bubbles on the surface of the galvanized sheet
.

Figure 15 Photographs of test panels of MC# 4 system (left picture) and MC# 7 system (right picture) after 750h of salt spray exposure (galvanized steel, cured at room temperature, dry film thickness ≈50-60 μm)
.

In order to achieve the low VOC target of less than 75g/L for the water-based primer system, the amount of co-solvent used in the formulation was significantly reduced; next, "Which type of co-solvent can effectively help the formation of the film with a small amount of use "Become an important topic
.
In this study, five glycol ether solvents were used as the research object, and they were evaluated in terms of storage stability, film appearance and hiding power
.
These solvents are propylene glycol monomethyl ether (PM), dipropylene glycol n-butyl ether (DPnB), hexyl carbitol (HC), propylene glycol propyl ether (PNP) and propylene glycol phenyl ether (PPH)
.
The storage stability test is to mix the WB-A curing agent, co-solvent and deionized water in a ratio of 3:1:1, place in an oven at 50°C for two months, and then check for any phase separation
.
The test panel for measuring gloss is coated with a white water-based primer containing various co-solvents on a cold-rolled steel base plate
.
In order to measure the contrast ratio, these water-based primers were placed at room temperature for 20 hours and then coated on Leneta-closed black cardboard
.
The contrast ratio and gloss of the cured film were tested using Datacolor Check II Plus and BYK Gardner mini three-angle gloss meter
.

Table 7 shares the results of storage stability, gloss and contrast ratio
.
It can be seen that there is no phase separation between PM and PNP systems after two months of storage at 50°C, indicating that the compatibility of PM and PNP with WB-A is better than that of the other three solvent systems
.
The gloss and contrast ratio data show similar trends: PM and PNP systems have higher gloss than the other three solvent systems because they have better film-forming effects
.
The contrast ratio of PM and PNP systems is higher, which shows that these two systems have better covering ability, that is, the ability to cover black background, as shown in Figure 16
.
Since these five primer systems were placed on the black cardboard sealed by Leneta after being placed at room temperature for 20 hours, the better hiding power showed that the water-based primer system based on WB-A and resin 3, PM and PNP can extend its construction time limit
.

Table 7 Performance test using different solvents

Figure 16 Photographs of test panels of water-based primers containing five different co-solvents after being placed at room temperature for 20 hours

｜Conclusion｜

This article evaluated the performance of a series of low-volatile organic compounds (<75g/L), high-performance water-based primers directly applied to metals.
These water-based primer formulations are based on two new solvent-free water-based curing agents
.
The results show that both CNSL type and non-CNSL type water-based curing agents can provide curing performance with a balance between fast drying time and construction time limit, and good dilution performance
.
In addition, the new water-based primer system exhibits excellent dry and wet adhesion on various metal substrates after 800 hours of salt spray exposure, resulting in excellent corrosion resistance
.
In addition, the test results of wet-on-wet polyurethane topcoats on epoxy primers show that the new water-based curing agent can make the recoating interval of multi-layer coating systems as short as 15 to 30 minutes, and has excellent adhesion and anti-corrosion properties.
, And keep the appearance beautiful
.

In addition, the formulation studies of different solid epoxy dispersions, stoichiometric ratios and co-solvents show that:

(1) The good compatibility of the curing agent and the solid epoxy dispersion has a significant impact on the overall performance of the water-based coating system;

(2) Increasing the amount of WB-A curing agent relative to the solid epoxy dispersion from 0.
8 to 0.
9 can improve the anti-corrosion performance of the water-based primer system on some substrates, such as galvanized steel;

(3) Choosing a suitable co-solvent in the water-based primer system can not only improve the storage stability, but also may help to extend the construction time limit
.