A new route-hydroxyl emulsion for waterborne high performance two-component polyurethane coatings

By Steven Mao, Technical Manager; Marcelo Herszenhaut, Americas Market Development Manager; David Vanaken, Global Technical and Marketing Director; Hexion Ltd.

The ongoing water-based globalization of industrial paints caters to increasingly stringent VOC regulations and public awareness of environmental protection1,2

Coatings Industry Demands and Trends

We've highlighted some recurring needs in the coatings industry, each of which generates its own specific trends

Sustainability and low VOC requirements, they will bring us:

· Improving the solids content of existing solvent-based (SB) systems is the first step in achieving environmental compliance

· Improve the performance of water-based coatings

· Production efficiency as a measure of sustainability

Excellent performance

While users understand environmental needs and demand action from their suppliers, they also need performance close to traditional oil-based coatings

Traditional base performance requirements include:

· Appearance, (gloss and flow, and ability to control paint gloss);

chemical resistance (for water, acids, bases or solvents);

· Hardness;

· Aging resistance

globalization of technology

Global resin suppliers and global coatings manufacturers are looking for solutions that can be easily used in the wider region
.
This involves resins that are easier to produce in more readily available equipment and less sensitive to coating formulation techniques and practices
.

Control costs

Similar to the performance requirements mentioned above, the end user may not accept the premium price for high performance waterborne coatings
.
As a result, resin scientists and coating formulators alike must look for alternatives that offer lower environmental impact and better performance at a reasonable cost
.

Give full play to the various excellent properties of large groups

In the 1950s, Dr.
Herbert Koch of the Max Planck Institute in Mülheim, Germany discovered that alkenes react with carbon monoxide and water under the influence of strong acids to form highly branched tertiary carbon carboxylic acids (Fig.
1) 3,4
.

These tertiary carbonic acids can be converted to the corresponding glycidyl esters by reaction with epichlorohydrin
.
Versatic™ Acid 10 (VA10) is a tertiary carbonic acid containing 10 carbon atoms, which is converted into Cardura™ E10P Glycidyl Ester (CE10P) by this pathway
.
In turn, CE10P reacts with acrylic acid to form acrylated CE10P (ACE) (Figure 2)
.

ACE is a highly versatile molecule containing acrylic unsaturated double bonds, hydroxyl (-OH) functional groups (mainly primary), and a very hydrophobic and highly branched tertiary carbon structure
.
Acrylic functional groups are used to incorporate ACE into polymers by reaction with other unsaturated monomers, -OH groups can be used to crosslink with isocyanates, and the branched alkyl chain of VA10 imparts excellent properties
.

Establishment of water-based two-component 2KPU system

WB 2K PU systems are based on (A) -OH functional resins (mostly with acrylic backbone) or such hybrid resins and (B) isocyanates or mixtures of isocyanates
.
We will discuss the (A) component here
.
Figure 3 shows the logical evolution of isocyanate-crosslinked water-based acrylic polyol (WBAPO) resin technology
.

Water-soluble acrylic polyol (WSAPO) resin solutions are based on polymers with relatively short molecular backbones and high acid numbers
.
To provide a sufficiently low molecular weight, special equipment or a certain amount of chain transfer agent may be required
.
After the polymerization step, the carboxyl groups are neutralized with amine and the resin is dispersed in water
.
However, this technique still requires the use of large amounts of co-solvents to ensure complete dissolution of the resin in water
.
Additionally, high concentrations of carboxyl groups may negatively affect the properties of the cured coating
.
Figure 4 shows the relative acid number range for the technology shown in Figure 3
.

Waterborne acrylic polyol resins typically use solvent-borne polymerization, much like their solvent-borne analogs (Figure 5)
.
Waterborne acrylic polyols differ from solventborne acrylic polyols in that they contain a certain amount of carboxyl monomers, such as acrylic or methacrylic acid, to impart anionic properties to the polymer
.
After polymer synthesis, these carboxyl groups are neutralized with amines to give water dispersibility
.
One disadvantage of polyols prepared by conventional methods is their solvent content, the polymerization process requires a large amount of solvent as a medium for the reaction, and this solvent will remain in the polymer without additional processing steps
.
In order to reduce the solvent content of the polymer to acceptable levels, an energy and time intensive distillation step is required
.

When used as one of the reaction raw materials for APO synthesis, CE10P replaces the solvent originally used for polymerization
.
CE10P is gradually grafted into the polymer backbone during the monomer feed step by reacting its epoxy functional groups with carboxyl groups contained in acrylic or methacrylic acid present in the monomer
.
During this process, two reactions occur simultaneously: the free-radical polymerization of monomers, and the epoxy-carboxylic acid reaction for grafting CE10P into acrylic polyols
.

The use of CE10P in the synthesis of oily and waterborne acrylic polyol resins has been extensively discussed in literatures 5, 6, 7
.
This article will focus on the use of acrylate adducts
.

Primary Dispersion vs Secondary Dispersion

Figure 3 depicts a hydroxyl emulsion, a better form of waterborne acrylic polyol technology
.
To answer the "why?" question, we will compare the two types of resins, evaluating the production process and chemical properties
.

craft

As mentioned above, secondary dispersions include:

(i) polymerization (with or without co-solvents);

(ii) neutralization of carboxyl groups;

(iii) the process of dispersion in water;

There is also a step that may require solvent removal upon request
.
It's just that the removal of the solvent can take place in different stages - after the dispersion stage or after the polymerization stage in the first step
.

While steps (i) and (ii) can be carried out in the same vessel, step (iii) will require high shear mixing to ensure the formation of a stable dispersion
.
This means there are at least two vessels in addition to the solvent removal series
.

Emulsion polymerization is a simpler process, carried out in a single vessel, itself has a fairly simple design, does not require the use of solvents, but directly uses water as the polymerization medium, and can be performed at lower process temperatures than solvent-based polymerization proceed
.

chemical principles

Acrylic polyol secondary dispersions are stabilized by neutralizing acid groups distributed along the molecular chain
.
This relatively high concentration of carboxylic acid groups can cause problems during curing (interference with isocyanate crosslinkers) and humidity sensitivity of the cured film
.
Furthermore, such chains cannot be too long in order to produce stable dispersions, and their Mw values ​​are usually around 10,000 Da
.
Under similar curing conditions (catalyst and isocyanate concentration), these shorter chains exhibit a slower cure response because of their insufficient performance in terms of physical drying
.

Emulsion polymerization can be equated to a large number of bulk polymerizations performed in parallel in nanoscale reactors (polymeric micelles)
.
Under the action of surfactants, the emulsions are very stable and can achieve higher Mw values ​​(up to 200,000 Da)
.
Therefore, once an emulsion-based coating has been applied and its film has coalesced, it will take on its physical properties before curing begins, simply because of the higher molecular weight
.
This will affect the apparent drying time and other properties such as processing time
.

Figure 6 summarizes and compares the two approaches
.

Novel Hydroxyl (-OH) Emulsion for High Performance Waterborne 2KPU Coatings

Having established the appeal of hydroxyl (-OH) emulsions for high-performance waterborne 2KPU coatings, we will now address some of the shortcomings of the current technologies used to produce these emulsions, as well as some unmet process and performance needs
.

In most cases, the source of the hydroxyl functionality of the polymers in these emulsions is the hydroxyethyl methacrylate (HEMA) monomer
.
HEMA is a low molecular weight polar molecule that is easily miscible with water
.
The solubility of HEMA in water can cause some process problems:

Monomer pre-emulsions with high levels of HEMA tend to be less stable than pre-emulsions based on less water-soluble monomers8,9;

HEMA tends to be more likely to homopolymerize in the aqueous phase (rather than in the micelles) once fed into the polymerization reactor - this results in more slagging of the emulsion and reduces the concentration of HEMA in the polymer micelles ( HEMA does aggregate, but in the "wrong place")
.

The use of ACE (CE10P acrylate) as another source of OH functionality addresses both of the above problems
.
Since ACE has negligible solubility in water, it easily migrates into organic micelles while carrying HEMA into the micelles
.
During this process, ACE helps stabilize the polymer pre-emulsion and reduce slagging
.
Figures 7 and 8 illustrate these advantages of 4.
2% OH (in solids) emulsions and pre-emulsions
.

In addition to solving process problems, the use of ACE as a comonomer also improves the properties of the emulsion polymer
.
As mentioned above, HEMA is extremely hydrophilic
.
Therefore, HEMA tends to stay on the surface of polymer particles, either on the polymer backbone or in the form of oligomers
.
Therefore, emulsions that rely solely on HEMA as the source of OH functional groups will have particles that are predominantly cross-linked at their surfaces
.
On the other hand, when ACE is used as a comonomer, it helps to distribute the OH groups within the micelles, resulting in a more uniform distribution of reactive OH groups in the polymer
.
Thus, cross-linking occurs inside and outside the polymer particles
.
This effect is shown in Figure 9
.

Finally, another advantage of using ACE as a comonomer is the compatibility with isocyanate crosslinkers
.
In order to cure properly, the emulsified resin must be compatible with the crosslinking agent
.
Aqueous isocyanates are generally too viscous, so they are usually diluted with a hydrophobic solvent such as PMA
.
The non-polar large alkyl groups in ACE increase the compatibility of the emulsified polymer with non-polar (hydrophobic) solvents
.
This facilitates better fusion of the crosslinker and polymer, which in turn improves the properties of the cured paint film
.

Figure 10 compares the properties of varnishes, where one emulsion was made with conventional HEMA only and the other was made with a 50/50 mole HEMA/ACE monomer blend
.
In both cases, the hydroxyl content was 4% of solids
.
In addition to the process advantages, the use of ACE as a comonomer can significantly improve coating performance
.
Hexion has developed a toolbox to customize polymers based on desired properties including drying speed
.

in conclusion

In the synthesis of hydroxyl emulsion, the use of acrylic acid-Cardura E10P adduct ACE as the comonomer of HEMA solves some of the problems when using HEMA alone
.
Monomer pre-emulsions containing ACE will be more stable and the process will produce less slag, resulting in less polymer loss
.
In addition, the hydrophobicity of the ACE monomer ensures that the hydroxyl groups in the final polymer are more uniformly distributed within the polymer particles, rather than just on their surface, allowing for better crosslinking
.
In addition, the hydrophobicity of ACE monomer improves the miscibility of isocyanates used in waterborne 2KPU coatings, resulting in better coating performance
.
Finally, the performance of emulsions based on ACE monomers was improved over that of emulsions without this monomer
.

references

1 V.
Kumar and A.
Bhattacharya, Demand for low VOC coatings continues to rise, Paints and Coatings Industry, 5 May 2020

2 C.
Challener, Waterborne Industrial Coatings: Regulatory Change Slowly Drives Shift to Waterborne, Coatings Tech, October 2017, Issue 14, Issue 10

3 H.
Koch, Production of Carboxylic Acids from Olefins, U.
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Patent 2,831,877, filed March 17, 1952

4 H.
Koch, Recent Results on the Synthesis of Branched Carboxylic Acids, Fette, Seifen, Anstrichmittel, Vol.
59, No.
7, 493-498, 1957

5 C.
Steinbrecher, C.
Le Fevere, D.
Heymans, Hybrid Acrylic and Polyester Chemicals: High-Performance Polyols for Solvent-borne and Waterborne Polyurethane Topcoats, Conference "Automotive Coatings", 2011, Berlin, Germany

6 C.
Cavallin, Z.
Yan, D.
Vanaken, D.
Heymans, High Solids Acrylic and Polyester Polyols Made Easy: Glycidyl Esters of Neodecanoic Acid Provide Performance and Competitiveness, ABRAFATI Conference, 2015, São Paulo, Brazil

7 C.
Cavallin, M.
Herszenhaut, Using Glycidyl Neodecanoate Technology to Ease Production of High-Performance Waterborne Acrylic Polyols, Coating Trends and Technologies, 2016, Chicago, USA

8 Zhang Chunyan, Zhu Ziwei, Gong SL.
Synthesis of stable high hydroxyl self-emulsifying aqueous polyacrylate emulsion, Journal of Applied Polymer Science, 2017, DOI: 10.
1002/APP44844

9 Zhang Fulin, Wang Yaoyuan, Chai Liyuan, Synthesis of High Hydroxyl Acrylic Emulsion, Journal of Polymer Science, Series A, Vol.
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15-27