The Secret to Formulate Waterborne Metal Epoxy Coatings That Work

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Worldwide, all coatings sectors experience a gradual shift towards water-based technologies as limits on emissions, VOC content and safety rulings become more prevalent. Yet this change of media is not without consequence on your formulation work and practices.

Especially, epoxy resins by their very nature are fundamentally incompatible with water. A careful design is crucial to deliver the combined properties one usually expects from such systems. These properties include chemical resistance, adhesion, corrosion resistance, mechanical strength, and high flexibility in some cases.

Epoxies are one of the best materials available when comparing performance against relative cost, and this has, in turn, led to their use in many high-performance applications where UV stability is not a must-have requirement. Metal corrosion protection is one of them.

After quick starting reminders about epoxy systems, we will review how to well select your waterborne resin and curing agents, so your formulation can match the performance of a traditional solvent-based coating. At the end, you will be able to see it by yourself with a case study comparing the performance achieved with water-based and solvent-based anticorrosion coatings.

Fundamentals of Epoxy Systems

Epoxy Chemical Structure and Linked Properties

Epoxies favorable properties are directly related to the resin chemical structure, which is available most commonly in the form of Bisphenol A Diglycididyl Ether (BADGE). BADGE is formed from the reaction of Bisphenol A and Epichlorohydrin, which when further reacted with an appropriate curing agent will eventually form a thermoset polymer, with aromatic groups from the Bisphenol A component evenly distributed throughout the entire structure.

Crosslinking Mechanism of Epoxies

Epoxy resins can be reacted with a range of different curing agents. The combination of a particular resin and curing agent is often referred to as a system. The figure below shows the two different points of reactivity in a typical resin molecule, namely:

• The end epoxy ring
• The side hydroxyl group

Both functional groups can react at either ambient or elevated temperatures, thus allowing the formulation of both 1-pack and 2-pack curing systems from the majority of epoxy resins available commercially.

• For a 1-pack system, the resin and curing agent can be combined well in advance of its actual application. The system will either not react at all or react very slowly at ambient temperature. After application of these 1-pack systems, when heat is applied, the curing reactions occur much faster and the final polymer is formed.
• For a two-pack epoxy system, the resin and curing agent must be kept apart until just prior to their application. As soon as they are combined, chemical reactions occur and form the final polymer. Therefore the applicator has a finite time period after mixing of the resin and curing agent, to use and apply the material before the growing molecular weight of the system becomes too high that it cannot be used anymore. This time period is commonly referred to as the pot life.

Epoxy Coatings – Film Formation in a Solvent-based System

By their very nature, most polymeric coating applications require the resin to be applied as a thin film onto another material substrate. For coatings applied in liquid form, the viscosity has to be relatively low to allow a consistent thin film to be achieved on the substrate surface.

In the case of ambient-cured 2-pack epoxy systems, the low viscosity is generally achieved by dissolving the resin and curing agent monomers into a type of solvent. This solvent will evaporate over time after application, allowing a liquid film to form and subsequently convert into a solid thin film.

Here are the main stages that occur during the film-forming mechanism of an applied liquid coating:

STEP 1: For liquid epoxy systems, the mixed resin and curing agent either dissolved or dispersed in a liquid is applied onto the substrate. STEP 2: The solvent gradually evaporates forcing the monomers to move closer together. STEP 3: When most of the solvent has left the system, the monomers start to coalesce together as the growing polymer chains start to intertwine with each another. STEP 4: The solvent has completely left the coating, and curing reactions continue in the solid film-forming, high molecular weight thermoset polymer.

The solvent used during this process is effectively emitted into the atmosphere and is classed as a Volatile Organic Compound (VOC) thus creating a degree of air pollution during the coating process. Yet, increasing environmental concerns over the past decades have resulted in limits being imposed worldwide, on the amount of emissions allowed during various industrial and commercial coating applications. It calls for a change to water-based technologies.

Careful Design of a Water-based Emulsion Epoxy Coating

In its simplest definition, water-based coating technologies involve using water as the main carrier medium of a liquid applied coating instead of an organic solvent, thus effectively reducing the amount of VOC’s that are emitted during application, as water is not classed as a VOC.

Epoxy resins, by their very nature, are fundamentally incompatible with water and this is one aspect of what makes them very useful as protective coatings, in particular for corrosion protection. If an organic solvent is replaced with water, careful design of the monomer structure is crucial, be it a curing agent or an epoxy resin, in order to deliver a high-performance coating. The liquid coating has to be delivered in aqueous media, but when fully dried and cured it must give the same high performance as conventional epoxy-based materials.

There are 2 main strategies for the epoxy resin:

• Either the epoxy part comes as 100% solid (no solvent), in such case we speak of liquid epoxy coatings.;
• Or the epoxy part comes as an emulsion (micelles in water).

In both cases, it involves a water-based curing agent.

From now on, let us focus on how to choose the right epoxy resin, surfactants and curing agents for your WB emulsion epoxy coatings and whether it is applicable to liquid coatings or not.

Choosing Your Epoxy Resin

First, and this is good news, since the base epoxy resin is not in direct contact with water, there is no need to drastically modify its chemical structure. It exists in essentially the same form as a conventional epoxy resin with its chemical structure unaltered.

The two-phase nature of emulsions means that their final viscosity at a fixed solids level remains independent of the molecular weight of the epoxy resin within each micelle. Therefore, it is possible to supply a wide range of epoxy resins in emulsion form at relatively high solids level (> 50 %) but still at a usable viscosity.

This is not the case with solvent-based technologies, nor with liquid epoxy coatings where the solution viscosity increases significantly with the molecular weight of the epoxy resin being solubilized.

Having epoxy resins with higher molecular weight and consequently higher epoxy equivalent weight (EEW) is generally preferred as it enables to achieve longer pot lives coupled with faster drying times. This is a clear advantage of emulsion epoxy resin over liquid epoxy coatings.

This effect on pot life and drying time is shown in the graphs below.

• The first epoxy system is made with liquid epoxy resin – Epotec® YD 128 (EEW = 190 grams per equivalent)
• The second system is made with water-based type 1 epoxy resin (EEW of 475 grams per equivalent)

The curing agent is the same in both systems.

Effect on Pot Life

The pot life is determined by measuring the gloss retention of the dried coating when applied over time, the gloss level is maintained for nearly 3 hours before it starts to drop significantly. Therefore, the usable pot life of this system is 3 hours.

• The longer pot-life observed with the water-based epoxy is very apparent by its very slow viscosity rise which is almost constant for the first 3 hours.
• The liquid epoxy resin, on the other hand, shows a very rapid increase in viscosity within the first 30 – 40 minutes.
• The water-based emulsion epoxy system consequently does not have an end of pot life indication.

Effect on Drying Time

The graph below shows the other main advantage of using higher molecular weight epoxy resin in coating applications. When monomers already have higher molecular weight, then an ultra-high molecular weight polymer is formed much more quickly, and the coating has an overall much faster drying time.

The water-based emulsion system dries at less than half of the time of the liquid epoxy resin-based coating. In practice, it means fast ‘tack-free’ and shorter overcoat times.

Plot Showing the Drying Times of a Water-based Type 1 Epoxy Emulsion (WB Epoxy) and a Liquid Epoxy Resin (YD 128) Cured with the Same Curing Agent

What Surfactants Can You Use for Your Water-based Epoxy Emulsion?

As with most emulsion technologies, the dispersed phase is created using surfactants which form micelles in water above a specific concentration.

The use of surfactants, however, does form a ‘weak link’ in the final dried coating in terms of performance. Since surfactants are required to have part of their chemical structure as compatible with water, the presence of a surfactant in the final solid coating will lead to some water sensitivity. Moreover, if the molecule is small, it can migrate to the coating surface, leading to increased water sensitivity of the coating, in addition to affecting inter-coat adhesion.

It is best to use surfactants at a minimal concentration, where associated with high-shear mixing, you achieve appropriate micelles size as well as a stable emulsion over time.

On top of this, to counteract this unwanted water sensitivity, it is recommended to use a surfactant which contains reactive epoxy groups. These epoxy groups will cure alongside the base resin and effectively ‘lock’ the surfactant in place, preventing any migration to the coating surface. This is illustrated in the figure below.

High-performance & Compatible Water-based Curing Agents

The curing agent molecules diffuse into the epoxy-containing micelles to allow reaction and creation of the final polymer. In this case, it is important that the epoxy micelles are as small as possible so that a full cure is achieved in the final coating.

First, let us remember that:

• Coating performance is heavily influenced by the compatibility between the resin and curing agent.
• For all two-pack systems, the components should be used in the correct mixing ratio and thorough mixing for both parts should be achieved prior to application.

The curing agents used in ambient-cured 2-pack waterborne epoxy systems are predominantly based on either aliphatic or cycloaliphatic amines. Since the A+B mixed emulsion ultimately determines the final performance, the amines used in curing agents are often chemically modified to make them more compatible with epoxy resin. Modification can take several forms, including:

• Reacting with fatty acid to produce an amidoamine or polyamidoamine, or
• Pre-reaction with an epoxy resin to form an epoxy-amine adduct.

The epoxy section of the adduct thus aids resin compatibility.

Related Read: Select Curing Agents for Coating Formulations

With water-based curing agent technology, the design of the epoxy-amine adduct structure gets more complicated since good compatibility is required between the molecule and water and between the molecule and epoxy resin, which are two opposing forces. Therefore, a high-performing waterborne curing agent would have a molecular structure that behaves similar to a surfactant with a block copolymer consisting of hydrophilic, hydrophobic, and some partially hydrophilic/hydrophobic segments to allow the molecule to be compatible with all species.

The hydrophilic blocks mainly come from the amines used (AM), whilst the hydrophobic portions come from pure epoxy used in the adduction (EP). There are many additional di-epoxide species available commercially that can be used in the molecular synthesis to influence a partial hydrophilic interaction (HP) in the end product. A generic description of the overall curing agent molecule is shown below.


When the liquid epoxy resin is used, the water-based curing agent serves two main functions.

• It chemically reacts with the epoxy resin as a curing agent to form the final polymer.
• It also acts as an emulsifier so that when the parts A and B of the system are mixed together, a homogenous emulsion is formed with the resin and curing agent reacting together within micelles which will eventually coalesce and form a solid polymer as the water evaporates.

WB vs. SB Anti-corrosion Coatings – A Practical Proof that ‘It Works’

Characteristics of Water-based Corrosion Protection Coating

Here are the resin and curing agents that were used:

1. CeTePox® 484R is a water-based emulsion of Type 1 epoxy resin supplied at 53 % solids level in mainly water, plus a small content of Methoxy Propanol. Being an emulsion, the viscosity of the product is very low (< 1500 Centipoise at 25°C), making it easy to incorporate pigment and fillers or to develop sprayable water-based coatings without having too low a solids level. The mean particle size is also less than 1.1 microns to allow good penetration and through-cure of the curing agent within the epoxy micelles.
   
   

   CeTePox® 484 R
   Properties	Value	Units
   Appearance	Milky white	-
   EEW (Solid)	490-550	g/eq
   Viscosity @ 25°C	300-1300	cPs
   Solids content	50-55	%
   Density @ 23°C	1.06-1.1	g/cm3

   
   Waterborne Epoxy Resin


2. Epotec® THW 4510 is a water-based curing agent based on a relatively high molecular weight epoxy-amine adduct. It is supplied in solution form at 60% solids level and contains no added solvent.
   
   

   Epotec® THW 4510
   Properties	Value	Units
   Appearance	Yellowish -liquid	-
   AHEW (F.O.D)	227.2	mgKOH/g
   Viscosity @ 25°C	23575	cPs
   Solids content	60	%
   Density @ 23°C	1.0912	g/cm3

   
   Waterborne Curing Agents

These water-based epoxy and curing agent are fully compatible when used in combination and employed in a variety of coating applications. The minimal solvent content in both the resin and the curing agent allows the formulation of coatings with a very low VOC level.

Determination of Pot Life

When the epoxy and curing agent is mixed, a stable homogenous emulsion is formed where the curing agent molecules can penetrate the micelles of the epoxy resin-emulsion, and polymerization reactions commence.

The chain extension and crosslinking reactions ultimately form the final thermoset polymer within the micelles, but there is no breakdown of the micelle structure, and hence this system does not have a visible end of pot life. The figure below, shows the viscosity profile over time of the mixed system, and is virtually constant over several hours and since the pot life does not visibly end.


The actual usable pot life of the system has to be determined by measuring other coating properties. The growing thermoset polymer will eventually attain a molecular weight high enough so that coalescence is not possible after the liquid emulsion has been applied, resulting in a poor final coating appearance and very poor protective properties.

The figure below shows the coalescing behavior of the system when applied as a clear film overtime after the initial mixing. It is evident that the clarity of the film starts to decrease around the 4-hour mark, and cloudiness becomes more evident as the clear film dries. This signals a restriction in the coalescence of the coating, and thus the end of the usable pot life for this system.

Get Desired Performance with Fully Formulated System

For use as a viable corrosion protection coating, the water-based system based on CeTePox® 484R and Epotec® THW 4510 must be further formulated to give the desired overall performance. This generally involves incorporating pigments and fillers into either the resin or the curing agent component. It is more advantageous to formulate into the resin component since its low viscosity allows the incorporation of many pigments and fillers without the viscosity of this part becoming excessively high.

To avoid affecting the emulsion stability of the water-based epoxy by applying excessive shear, the formulated part A is prepared by first making an aqueous pigment dispersion and adding the epoxy as one of the last components. The water-based system is best formulated with a Pigment Volume Content (PVC) in the range 30 – 35%, and it is typically formulated using zinc phosphate-based anti-corrosive pigments. Appendix A describes the formulation used for generating the performance data.

Part A
DI Water	9.49
Bentone® EW	0.29
DisperBYK 194n	1.43
BYK® -018	0.14
Additive F2M	0.38
HALOX® 550	1.59
TiO2	11.54
BAYFERROX® 318M	1.71
FinnTalc M-15	8.41
BaSO4	10.58
HEUCOPHOS® ZAM-PLUS	7.70
Isopropyl Alcohol	1.92
DOWANOL™ PM	0.45
CeTaPox® 484R	44.25
BYK® -024	0.10
Part B
Epotec® THW 4510	6.88
DMP-30	0.07
DI Water	0.88

Appendix A: Formulation for a 2-pack Water-based Corrosion Protection Coating

The formulation itself has a pigment volume content of 30 % by weight, and offers a VOC level of less than 100 grams per liter, and provides a range of other favorable coating properties which are described in Appendix B.

Test Property	Units	Primer Type II Specification*	Test Result
In-can Stability	-	Homogenous no hard settlement after stirring	Pass
Film Appearance	-	Smooth, no abnormal	Smooth
Storage Stability	-	No abnormal	Pass
Film Color	-	-	Gray
Fineness of Grind	µm	≤50	5-10
Recoat Interval Time	mins	≤30	30
Drying Time	mins	≤1hr	30
Pot Life	hrs	3	3.5
Solids Content (by weight)	%	≥40	47.6
Solids Content (by volume)	%	≥55	64.09
Adhesion	-	1	0
Flexibility	mm	3	3
Impact Resistance	kg/m2	0.5	2.00

Appendix B: A Summary of the Main Coating 2-Pack Waterbased Corrosion Properties of the Formulation in A

Again, for an absolute determination of the usable pot life of this coating formulation, it is necessary to check the overall performance of the coating overtime after the initial mixing of Parts A and B. Figure below shows the results of one such test where the adhesion of the fully formulated coating applied onto mild steel panels, was measured with time.

Strong adhesion to the substrate is just one of the important properties a coating must have if it is to exhibit good protective properties. The figure below illustrates the results of a cross-hatch adhesion test following the standard method ISO2409:1992, which shows how there is no adhesion loss of coating even after 4 hours.

Performance Testing of Water-based Corrosion Coatings

#1 Salt Spray Chamber

The main purpose of this type of coating is for corrosion protection of metals, and the performance is typically evaluated with the aid of a salt spray chamber that provides accelerated corrosive conditions for evaluating coated test panels.

Figure below shows the appearance of mild steel panels coated with the water-based coating described in Appendix A at a dry film thickness of 50 microns. These panels underwent salt spray exposure for 2000 hours in line with the standards ASTM B117-16 and ISO 9227:2006, and the two pictures show both the fully coated panel after removal from the salt spray chamber (slight clean-up of the surface) and the panel with the coating around the scribe mark deliberately removed.

The panels are evaluated after exposure in terms of blisters on the surface of the coating, the creepage of the rusting from the center of the scribe mark and the degree of rusting on the metal surface underneath the coating after it has been removed.

The figure also shows how the creepage from the center of the scribe line is not greater than 0.5 millimeters in length, there is minimal blistering on the surface of the coating and on the exposed metal underneath the coating.

Adhesion testing after salt spray exposure also shows that no loss of adhesion occurs over time, thus confirming the formulation as a high-performance corrosion protection coating even when applied at relatively low film thickness.

#2 Salt Water Resistance Test

Since water-based corrosion protection coatings are increasingly being used in marine environments such as for the protection of shipping containers, another common performance test is the saltwater resistance test. This involves immersing a coated panel in a 5% Sodium Chloride aqueous solution and once removed the panel is evaluated in a similar fashion to salt spray exposure.

The figure shows the data and appearance of the coated panels after 168 hours of saltwater immersion. As with the salt spray exposure result, the coating formulation exhibits no blistering, minimal rust creepage from the center of the scribe and coating adhesion is not affected by the immersion.

Test Data and Coating Appearance After 168 Hours Salt Water Immersion
Test Description		Test Standard	Primer Type II Specification	Test Result
Panel Evaluation	Coating Appearance	ASTM D714-02 (2009)	No Film Change	Pass
	Blistering		10	10
	Rust Creepage		≤0.5	≤0.5
Adhesion		GB/T9286/ ISO2409:1992	≤1/1	0/0

Test and Appearance Data After Salt Water Immersion

For many years, the main technology for corrosion protection coatings has been two-component solvent-based epoxy coatings. The epoxy portion would typically be based on Type 1 solid epoxy resin that had been dissolved in a solvent, and the hardener portion would typically be polyamidoamine based. 

The actual formulation of the solvent-based coating is shown in Appendix C.

S.No.	Part A	wt/g	Supplier
1	YD011X75	280.00	Aditya Birla Chemicals Ltd.
2	HEUCOPHOS® ZPA	150.00	Heubach
3	Portaryte® B 15	100.00	Sibelco Specialty Minerals
4	FinnTalc M-15	100.00	Elementis
5	BAYFERROX® 130M	150.00	Lanxess
6	MIBK	80.00	Local
7	Xylene	110.00	Local
8	n-Butanol	30.00	Local
	TOTAL	1000.00
S.No.	Part B	wt/g	Supplier
1	TH7515X70	106.00	Aditya Birla Chemicals Ltd.
2	Xylene	59.00	Local
3	DOWANOL™ PM	35.00	Dow Chemicals
	TOTAL	200

Appendix C: Formulation for a 2-Pack Solvent-based Corrosion Protection Coating

The figure below compares the coated panel appearance of both technologies when tested for corrosion resistance under the same accelerated conditions. For both panels, blistering on the surface of the coating is completely absent, and the adhesion of both coatings has not been diminished. The length of the rust creepage from the center of the scribed areas on the metal panel is also generally the same with some localized areas on the solvent-based panel being more than over one millimetre.

Panel Ratings: 2000 Hours Salt Spray Exposure
Test Description	Test Standard	Water-based 2-K Coating	Solvent-based 2-K Coating
Rusting	GB/T1771-2009	10	9
Blistering		10	9
Maximum Rust Creepage	ASTM D714-02(2009)	0.5 mm	1.25 mm
Adhesion	GB/T9286/ISO 2409:1992	0/0	0/0

 
The Appearance of a Waterbased Coated Panel and a Solvent-Based Coated Panel After 2000 Hours Salt Spray Exposure (ASTM B117)

Thus, when considering the overall data, it can be concluded that this water-based technology can deliver performance equal to that of standard solvent-based in corrosion protection applications.

Summary

Water-based epoxy resin and curing agents have been developed that can used together as a system to provide corrosion protection coatings with low VOC content, less than 100 grams per liter. The epoxy resin is an emulsion with higher molecular weight which produces coatings with longer pot lives and fast drying times, especially when compared to coatings based on liquid epoxy resins. The resin and curing agent combination does not provide an end of pot life indication but can still be applied up to three hours after the initial mixing as a clear coating and up to four hours as a fully formulated coating without the loss in performance.

The system can be formulated with intermediate Pigment Volume Content in the range 30 – 35 % to give high corrosion resistance even at low coating thickness. Salt Spray resistance in excess of 2000 hours is achievable without any detrimental effect to the coating or significant loss of performance. The waterborne metal coating was also compared against a typical solvent-based epoxy coating and no significant difference was observed.

Find Suitable Epoxy Resin Grade for your WB Coatings