Solution to the pain and difficulty of water-based two-component industrial paint

In recent years, with the continuous promulgation and implementation of China's environmental protection policies, the "oil-to-water" work in the metal coating market has been in full swing, and the market share of water-based coatings has continued to increase

United Cure Science has focused on independent research and development of water-based two-component resins and curing agents for more than ten years.

Problem 1: Water-based epoxy emulsion is expensive and has high VOC and is difficult to thicken

In 2021, the price of chemical raw materials is deep in a frenzy, and the shortage of epichlorohydrin has caused the epoxy resin to soar

Table 1: Two sets of low-VOC, cost-effective epoxy dispersion solutions: 2093, 2095

The viscosity of Banco 2093 is only 700cp.

The viscosity of Banco 2095 is 3000cp.

Question 2: Water-based epoxy primer polishes sticky sandpaper

After the epoxy primer is sprayed for medium and high-end engineering machinery, high-speed rail, subway, and bus paint, in order to ensure the surface effect of the middle or topcoat, the epoxy primer is generally polished

Question 3: The gloss of the water-based epoxy bottom surface all-in-one paint is unstable

For scenes that do not require high weather resistance, such as automobile parts, indoor equipment, and large underground pipelines, medium-gloss or high-gloss epoxy DTM will be selected
.
The gloss of many medium-gloss or high-gloss waterborne epoxy DTMs will change continuously with the pot life and baking process, as shown in Figure 3
.
The gloss is inconsistent, and the color is not consistent
.
In the end, the colors of different spraying batches of workpieces or different areas of large workpieces are inconsistent
.
To this end, Liancure Science has launched Banco970, a long-lasting and stable gloss epoxy curing agent, which can achieve stable gloss of medium and high gloss DTM under different drying conditions and application periods, as shown in Table 2
.

Figure 3: The gloss of water-based epoxy DTM continues to decrease with the pot life

Table 2: The pot life of Banco970 and the gloss change under different drying conditions

Question 4: "Wet-on-wet" water-based epoxy base polyurethane topcoat

Based on the consideration of speeding up production cycle, energy saving and emission reduction, and reducing manpower, more and more construction machinery coating lines have changed the traditional two spray and two bake (2C2B) to two spray and one bake (2C1B): When the paint is not completely dry, spray the polyurethane topcoat and then bake it, which is the so-called "wet-on-wet"
.
The "wet-on-wet" process poses a great challenge to the application of water-based paint
.
Waterborne epoxy primers generally dry slowly.
Spray polyurethane topcoats on "wet" epoxy.
Fatty amines will react preferentially with isocyanates.
The topcoats are prone to rashes and cause surface defects such as polyurea particles
.
To this end, United Curing Science has launched two technical routes of "wet-on-wet" solutions
.

Technical route 1: Use a water-based epoxy primer with fast surface drying to anchor the fatty amine in the primer and reduce side reactions with the topcoat
.
Epoxy primer pushes Banco2091A+Banco920, and uses Banco2091A's high glass transition temperature to achieve rapid surface drying.
The "wet-on-wet" effect is shown in Figure 4
.

Or Banco2095+ethanol+Banco920, using the non-breaking characteristics of 2095 and ethanol, ethanol will take out the water under high humidity conditions to achieve surface drying
.
Technical route one requires that the primer surface is dried before painting.
The surface drying of the primer is usually 15-30min.
The specific surface drying time depends on the air humidity and temperature
.
It is recommended to install ventilation equipment to maintain low humidity, or to pre-heat the workpiece and spray primer to accelerate the surface drying of epoxy primer
.

The second technical route is to use aliphatic amine curing agent that reacts slowly with the isocyanate curing agent to reduce the side reaction of "wet-on-wet"
.
Epoxy curing agent recommended Banco933, matched with Banco2095+ ethanol, the topcoat can be sprayed without waiting for the surface of the primer to dry, which can achieve a shorter time-5min "wet on wet"
.
As shown in Figure 5, "wet on wet", under 23℃100RH% breeze, spray 50um 2095+933, leveling for 5min, then spray 50um2720+3580A medium chrome yellow, leveling for 20min, baking at 60℃ for 30min, 20° gloss 86.
1, 60° gloss 92.
7, good board surface
.

Question 5: Waterborne polyurethane thick coating will cause rashes when it dries quickly

Because NCO easily reacts with water to generate carbon dioxide, compared to oil-based paints, water-based polyurethane topcoats have always been at risk of rashes since they were first introduced
.
For some poor water-based polyurethane topcoat formulations, the dry film thickness should be less than 50um during construction, and the leveling should be at least 15min or more, and then bake after the foam is removed to ensure that the board surface is free of heat rash
.
These construction problems of waterborne polyurethane topcoats have been plagued by customers who are pursuing efficiency, poorly controlled spraying techniques, and short coating lines
.
For this reason, United Cure Science introduced Uahard 3580A isocyanic acid curing agent, which is hydrophobic and lipophilic.
After two-component mixing, it has less side reactions with water.
It can be used with high-resistant rash hydroxypropyl dispersion Uacryl 2720 or Uacryl 2150.
It is easy to spray a 90um dry film without leveling, and no heat rash can be caused by rapid drying at 60°C.
The board surface has a good effect.
The 20° gloss is greater than 80, and the 60° gloss is greater than 90, as shown in Figure 6
.

Picture 6: 2720+3580A is not leveling, it won't cause heat rash directly at 60℃

Question 6: Water-based polyurethane topcoat has short application period and high formulation cost

Regardless of national standard requirements or on-site construction requirements, water-based or oily, mid-to-high-end polyurethane topcoats require a room temperature pot life of 4 hours and a summer high temperature (35°C) pot life of 2 hours
.
The usual practice is to incorporate IPDI into the curing agent to extend the pot life
.
However, the cost of IPDI is high, and there will even be out of stock in 2021
.
On the other hand, the cost of water-based polyurethane topcoats has always been high.
The reason is that the price of water-based isocyanic acid curing agents is too high
.
For this reason, United Cure Science has launched Uacryl 2766, a hydroxypropyl dispersion of long pot life and oilable isocyanate curing agent
.
The product innovation uses the organic coating technology to circumscribe the branch coating layer on the core-shell structure of the hydroxypropyl dispersion, and uses the steric hindrance effect of the coating layer to slow down the reaction of the active hydroxyl group and the isocyanate curing agent to obtain a long pot life
.
The molecular structure design principle and pot life viscosity change data are shown in Figure 7.
Without IPDI, the pot life is 4h at 25°C and the viscosity remains unchanged.
At 35°C, the pot life is 3h and the viscosity does not change.
Change
.
In addition, the product has excellent emulsification properties and can be matched with oily HDI curing agents, such as HT100, 3300, etc.
, whether it is made of inorganic or organic colors, 60° gloss can reach more than 90, as shown in Figure 8
.

Figure 7 Schematic diagram of 2766 ultra-long pot life and performance of pot life at 35°C

Figure 8 High-gloss photo of inorganic/organic color 2766+oily HDI curing agent

Question 7: Waterborne polyurethane topcoat has high VOC, poor fullness, high temperature yellowing

High-end polyurethane topcoat requires good leveling, high gloss at 20° angle, and good fullness.
In some scenes such as CRRC Q/CRRC J 44-2019, the waterborne polyurethane topcoat for electric trains is also required to be resistant to high temperature yellowing (150°C).
1h), VOC is low
.
However, due to the side reaction of water and isocyanate, the water-based polyurethane topcoat has poorer fullness than oil and poor high temperature yellowing resistance, as shown in Figure 9
.

Figure 9: High temperature yellowing resistance of waterborne polyurethane (150℃1h)

In addition, in order to ensure a foam-free film thickness, the amount of slow-drying solvent is usually added to the formulation to extend the open time of the paint film, so that the final VOC is relatively high
.
To this end, Liancun Science launched Uacryl 2739, a low-VOC, high-gloss, high-temperature yellowing hydroxypropyl dispersion
.
Its fullness is high, 20° gloss>85, and VOC is only 10g/L
.
With the water-based isocyanate curing agent Uahard3598A, the thickness of the non-foaming film can be easily controlled at 120um in the dry film, and it is resistant to yellowing at 150℃ for 2h, as shown in Figure 10
.
Uacryl 2739 can well meet the user requirements of high-end water-based polyurethane topcoats, such as water-based bus paint, high-speed rail paint, and subway paint
.

Picture 10: 2739+3598A high gloss and fullness, 150℃ 2h without yellowing

Concluding remarks

2021 is a difficult year for the chemical industry.
Domestic and international epidemics continue to spread, upstream raw material factories have stopped working, import and export trade has been blocked, raw material prices continue to be high, and even out of stock, paint companies have issued price increase letters one after another
.
"Oil-to-water conversion" is struggling to move forward under the double attack of technical difficulties and rising prices.
Fortunately, the country's determination and policy to promote the "blue sky and white cloud defense" remain unchanged, and the market's continuous demand for water conversion is increasing
.
Liancure Science adheres to the concept of "your water paint, Liangu escort".
For more than ten years, it has focused on the development and application of resins and curing agents for water-based two-component industrial paints, and has been committed to providing customers with the most practical, reasonable, and The most appropriate innovative solution
.
This article analyzes and summarizes some of the pain points and difficulties encountered in the "oil-to-water" process of two-component industrial paints in recent years, and provides corresponding water-based resin and curing agent solutions to help the sustainable development of China's water-based industrial paints
.