Table of contents

Table of contents

Air-drying paints have long played a pivotal role in the architectural coatings sector. Traditionally, Cobalt has been a staple drying agent for these paints, but its associated complications are nudging the industry towards other options. Europe’s impending cobalt regulations are intensifying the search for reliable alternatives to Cobalt.

Paint drying in two stages

1. Physical drying

After application, the solvent in the paint evaporates, allowing the paint particles to come together and form a solid film.

2. Chemical drying

The binder compound reacts with air’s oxygen, transforming the paint molecularly to harden and increase durablity.

Creating a durable paint film

Oxidation is key for the paint’s hardening and provides it with lasting durability. The synergy of these stages ensures alkyd paints perform as intended, with the balance between solvent evaporation and oxidation is crucial for a durable paint film.

The chemistry of drying

Below is a brief overview of the chemistry of the paint drying process. It shows the importance of the autoxidation sequence, especially in alkyds:

The paint drying process

Formation of hydroperoxide (oxygen uptake)

Figure 1: Metal driers are instrumental in facilitating the ingress of molecular oxygen throughout the paint's coating. As the drier and oxygen interact, a hydroperoxide compound is formed.

Decomposition of hydroperoxide

Figure 2: The formed hydroperoxide undergoes rapid decomposition, leading to the generation of highly reactive radicals.

Combination with another unsaturated side chain

Figure 3: These radicals initiate the radical chain reaction when they combine with unsaturated side chains like the ester of linoleic acid. This marks the beginning of the auto-polymerization process.

Generation of a carbon- based radical

Figure 4: These radicals can combine with fatty acids in the paint, setting off an auto-polymerization process which essentially determines the drying speed of the alkyd paint.

The drying process of alkyd paints is a complex interplay of reactions. From oxygen uptake to hydroperoxide formation and the intricate auto-polymerization process. Metal driers drive these mechanisms, influencing drying speed and the end performance of the paint.

Comprehending these fundamental principles enables paint formulators and manufacturers to fine-tune their products, ensuring optimal drying times, enhanced performance and superior finishes across applications.

Oxygen transition

Oxygen transfer involves a series of chemical reactions catalyzed by transition metals known as drying agents. These metals can be broadly categorized into two groups: primary and auxiliary. Primary metals are the main catalysts that facilitate the oxidation reactions needed as paint driers. Auxiliary metals are used in conjunction with primary metals to optimize the drying process, mitigating any undesirable effects caused by the primary metals.

Primary drying metals

Among the variety of transition metals, Cobalt stands out historically as the most important and widely used primary drying metal.

allnex ECOWISE™ CHOICE cobalt chemical symbol
  • Strong surface drying
  • Low to medium discoloration impact
  • Tendency to cause surface wrinkling
  • Acceptable through drying
  • High hardness
  • Reclassified “reprotox Cat. 2 (CLP)”

Cobalt, although effective in facilitating strong surface drying and imparting a desirable bluish undertone, has drawbacks such as a tendency to cause surface wrinkling, less effective slow drying and reclassification as reprotoxic (Cat. 2 under CLP regulation).

  • Longer induction time than Cobalt, but faster polymerization rate
  • Very good through drying performance
  • low risk of surface wrinkling
  • Higher risk of discoloration in white paints

Manganese and Cobalt both pose a risk of discoloration in the drying process, but the types of discoloration differ. Compared to Cobalt’s blueish undertone, Manganese results in a yellow to burnish undertone, which is typically less desirable in white paints.

  • General low efficiency at room temperature
  • More reactive in aqueous solutions, but less redox potential in polar solvents
  • Negative impact on anti corrosion performance in industrial applications
  • Low risk of discoloration

Selecting the correct transition metals as primary paint driers is crucial for optimizing paint performance and quality. However, the role of auxiliary drying metals is essential in fine tuning specific requirments of the paint drying process.

Auxillary drying metals

Auxiliary driers, a category of supportive metals, are integral to the performance of certain paints. This is particularly relevant when it comes to meeting requirements such as application with a brush or roller at higher film thicknesses.

  • Lowest discoloration risk
  • Good through drying support
  • Combined with lithium or barium for better room temp drying
  • Longer open time allows for better application, flow and leveling
  • Good support of through drying in combination with primary paint driers
  • Very good through drying support in both High Solid and Waterborne paints
  • Reduces wrinkling in paints with high film thickness
  • Very efficient at room/low temperature

While all these supportive dry metals offer beneficial properties, lithium stands out as an exceptional element due to its efficacy in high solids and waterborne architectural paint systems. Its ability to mitigate wrinkling, facilitate high film buildup, and enable low-temperature curing make it highly valuable in various applications.

Cobalt toxicology and regulations

The following Cobalt substances have been reclassified as reprotoxic Cat.2 (H361) in the European CLP regulation:

Substance CAS number
Cobalt, borate neodecanoate complexes 68457-13-6
Neodecanoic acid, cobalt salt 27253-31-2
Cobalt(2+) neodecanoate 52270-44-7
Naphthenic acids, cobalt salts 61789-51-3
Fatty acids, tall-oil, cobalt salts 61789-52-4

Cobalt, both as an element and in the form of various soaps such as borate neodecanoate complexes or neodecanoic acid complexes, has raised significant health concerns, particularly in relation to DIY paints available in home markets. These materials mentioned fall under Category 2, as per the H361 sanctions outlined in the European CLP regulation.

Moreover, Cobalt is anticipated to be reclassified as a carcinogenic material, falling under Category 1B classification (CMR) once the relevant progress is made in REACH and ECHA. This development presents another challenge as it would limit the permissible dosage of Cobalt in ready-to-use formulations to below 0.1 percent.

Extensive testing conducted in allnex in-house labs revealed that formulating paints with a Cobalt content lower than 0.1 percent is unfeasible. This reclassification would discourage paint manufacturers from using Cobalt in their formulations, leading to its phase-out in the EU. Other countries including the USA and China also have ambitions to eliminate Cobalt from future oxidative paint driers.

Cobalt alternatives

Fortunately, there are already Cobalt alternatives available in the market. Let’s explore some of them.

Polymeric Cobalt driers

  • Meets current regulations (CLP cat. 2 and CMR Cat. 1B)
  • Performance compararable to cobalt paint driers
  • Higher formulation costs
  • Formulations still contain Cobalt

Cobalt polymers comply with current regulations such as CLP and CMR. However, from a marketing and sustainability standpoint, having cobalt as an atom in the polymer is a disadvantage. Another drawback is that higher dosages of Cobalt polymer are required to achieve the same efficiency as cobalt metal, leading to increased formulation costs.

Iron complex driers

  • Acceptable drying performance in WB alkyd paints
  • Compatibility issues in SB and HS paints
  • Negative impact in WB anticorrosion formulations
  • Performance loss during aging (absorption on particle surfaces)
  • Anti-skin agents (MEKO) ineffective

Iron complex driers are well-suited for waterborne alkyd paints, but compatibility becomes challenging in solventborne high solid alkyd paints. Furthermore, Iron has a negative impact on the anti-corrosion performance of waterborne systems as it promotes corrosion. It’s important to note that both the first-generation Iron and Manganese replacement driers experience performance loss as the paint ages.

Manganese driers

  • Partially acceptable discoloration levels.
  • Good gloss retention
  • Lower set drying/hardening in WB paint
  • Longer overall drying in SB paints and skinning tendency
  • Performance loss during paint aging (absorption on particle surfaces)
  • Anti-skin agents (MEKO) ineffective

Manganese exhibits good color retention and acceptable discoloration rates, making it suitable for undercoats and primers. However, it may not perform as effectively in top coats.

In summary, while alternatives to cobalt exist, each option comes with its own set of advantages and limitations. Careful consideration is necessary to choose the most appropriate replacement based on the specific requirements of the paint application.

ADDITOL® dry CF range

Allnex has developed innovative solutions that eliminate the need for cobalt by utilizing manganese as a complex metal in combination with a specific composition of various complexing partners, accelerators and ligands.

Additol® Dry CF100 and CF103 are both cobalt-free. These products incorporate a patented multi-acceleration system that addresses the initial drying stage (known as set-dry) by compensating for the inherent weaknesses of Manganese. These paint driers exhibit an exceptionally rapid polymerization rate, reducing the risk of skin formation and minimizing the need for anti-skin additives.

ADDITOL® dry CF100

02 Cured
safer materials allnex ecowise logo

Key Features

  • Multi-accelerated Manganese ligand drier

  • No loss of dry/long-term performance
  • Faster tack-free than Cobalt

  • Less surface skinning

Applications

  • Decorative

  • Industrial metal

  • Industrial wood

  • Marine & Protective

Eco Advantage

  • Cobalt free

  • APEO free

A Cobalt replacement drier with patented twin accelerated Manganese. Designed for fast incorporation and universally compatibility in all SB and WB alkyd paints.

ADDITOL® dry CF103

Strictly natural
safer materials allnex ecowise logo

Key Features

  • Accelerated Manganese ligand drier

  • Designed for natural oils and sealants

  • Zero solvent/near zero VOCs for safe handling.

  • Best new ink drier

Applications

  • Inks

  • Stains

  • Architectural

  • Anti corrosive

Eco Advantage

  • Cobalt free

  • Low/zero VOC
  • Organo Tin free

Cobalt free, Manganese-based accelerated primary drier. High performance set and through drying in alkyd paints, stains, natural oils and printing inks. Universally compatible in SB and WB oxidative drying systems. Solvent-free, ultra-low VOC, high flash point and long-lasting drying performance.

For applications requiring a high film build, allnex offers co-designed auxiliary driers ADDITOL® dry CF200 and CF300. These can be combined with Lithium, Barium and Zirconium to enhance the performance of the primary paint driers. This combination is beneficial for WB and HS systems, including UHS formulations. It has been successfully used in heavy-duty coatings and mid-duty corrosion protection, specifically in direct-to-metal applications.

ADDITOL® dry CF200/CF300

Boosted
safer materials allnex ecowise logo

Key Features

  • Combination of auxiliary driers (Li/Ba/Zr)

  • For WB and SB HS/UHS alkyd systems

  • Boosting set and through dry, low temp and high DFT

Applications

  • Decorative

  • Industrial metal

  • Industrial wood

  • Marine & protective

Eco Advantage

  • Cobalt free

  • APEO free

  • Low/zero VOC

Balanced auxiliary drier metal combinations, optimized for oxidative drying in WB alkyd formulations. They are recommended to be used with the Cobalt-free primary drier ADDITOL® dry CF100 for all oxidative drying WB alkyd resin paints, serving as universal Cobalt free driers.

By utilizing these innovative solutions from Allnex, manufacturers can effectively eliminate the use of cobalt while achieving superior drying performance in various coating applications. The choice of materials in the transition away from cobalt is ultimately in the hands of our customers and formulation partners. Our experts offer guidance throughout different stages of the cobalt exit process.

Choosing the right paint driers

At present, allnex provide waterborne formulations that contain emulsified Cobalt materials. These serve as an intermediate stage in the cobalt exit strategy. Moving forward, we have phase two Cobalt polymeric driers that offer an alternative to traditional Cobalt-based solutions. These polymeric driers help bridge the gap between Cobalt-containing and Cobalt-free materials.

Above you can see the Cobalt-free materials in green. These materials represent the ultimate goal in our Cobalt exit strategy, providing effective alternatives without compromising performance or regulatory compliance. We remain committed to supporting our partners in making informed choices that align with their specific needs and regulatory requirements.

Driers that perform after storage

One of the crucial testing results pertains to the loss of dry effect over time. When a painted surface is left on the shelf, the driers in the formulation tend to absorb onto pigments. Interestingly, this absorption is particularly prominent with Titanium dioxide and inorganic pigments. This leads to a noticeable decline in drier performance.

Through our testing, we observed a significant disparity in drier performance between freshly produced paints and those subjected to elevated temperature storage for three weeks. This discrepancy underscores the importance of having robust drier performance in paints that have been stored for extended periods.

The specific test we conducted involved a modern high solid, long oil white alkyd paint (a paint system known for its unique characteristics). The paint was co-accelerated with Calcium and Zirconium, demonstrating the effect of our new driers on enhancing performance and maintaining efficacy even after prolonged shelf storage. By addressing the issue of dry effect loss, we strive to provide our partners with driers that deliver consistent and reliable performance throughout the paint’s lifespan.

A better rounded performance

When it comes to air drying alkyd paints, there are several crucial conditions and requirements for the drier. Here is a summary of the key factors:

Set dry and through drying: The drier must exhibit excellent performance in the initial set drying stage and the subsequent through drying stage, which is responsible for the paint’s hardness development.

Discoloration: As white exterior paint systems are commonly used, it is important for the drier to minimize any discoloration effects, ensuring the desired color stability over time.

Skin formation: Prevention of skin formation is crucial, as it eliminates the need for anti-skin additives. Future formulations should aim to minimize or eliminate this issue.

Compatibility: The drier should be compatible with both waterborne and solvent-based systems, allowing for versatile application across various paint formulations.

Anti-corrosion effect: Considering that many paint systems are applied directly to metals, the drier should provide effective anti-corrosion properties, enhancing the durability and protection of the surface.

Post-storage performance: The drier should maintain its effectiveness, ensuring that the paint retains its quality and performance even after being stored for an extended period.

Modern requirements: Meeting industry standards, such as low VOC or near-zero VOC content and a high flashpoint for safe handling, is essential. These aspects cater to environmentally friendly practices and ensure safe usage, particularly in applications like inks.

By utilizing the new Cobalt free driers with specific ligands, most of the requirements and conditions can be effectively addressed. These innovative developments offer superior performance compared to existing technologies. They provide a reliable and sustainable solution for air drying alkyd paints.

Guiding formulations

The following formulations have been tested and validated with the collaboration of our in-house research partners.

SPF based on SETAL® 312 SM 88

Cobalt-free

Brushable glossy white topcoat

Mill base 17.00 SETAL® 312 SM-88
3.34  Shellsol D40
0.38  Ca 10%(auxillary drier)
33.04 Kronos 2190
53.76
Let down 30.55 SETAL® 312 SM-88
54.05 Millbase
4.00  Shellsol D40
1.66 Zr 12% (auxillary drier)
0.42 ADDITOL® dry CF100 (C0-free drier)
0.25  ADDITOL® XL 297 (anti-skinning additive)
9.07 Shellsol D40
100.00

SPF based on RESYDROL® AY 615w/45WA

Cobalt-free

DTM glossy anti-corrosion monolayer

Mill base 66.10 RESYDROL® 615w/45WA
0.30 Ammonia 25%
0.10 AMP-90(multipurpose additive)
0.42 ADDITOL® dry CF100 (C0-free drier)
0.42 ADDITOL® dry CF200 (auxillary drier)
0.30 ADDITOL® XL 297 (anti-skinning additive)
0.70 ADDITOL® XL dry CF200(auxillary drier)
18.20 Kronos 2190
2.90 Nubriox 102
1.90 Blanc fixe micro
0.50 ADDITOL® VXW 6387 (anti-setting additive)
0.70 ADDITOL® VXW 6208 (dispersing additive)
0.40 ADDITOL® XW 376 (foam control additive)
Let down 5.75 water, deionized
1.05 Acrysol RM 6000/Fod. (thickener)
100.00

To the left is a brushable glossy white paint formulation. This formulation utilizes Additol CF-100 as the primary paint drier, along with a small amount of calcium as a sacrificial drier, which is a commonly used practice in the industry.

To the right is a modern direct-to-metal glossy anti-corrosion monolayer paint formulation. For this formulation, Additol CF-100 serves as the primary paint drier. To accommodate higher film thickness, we incorporate a Lithium/Zirconium mix as a secondary drier.

These guiding formulations demonstrate the effectiveness of the new Cobalt-free paint driers in achieving desired paint properties and performance. By incorporating these, formulators can create high-quality coatings that meet specific requirements.

Using ADDITOL® dry CF

Dosages play a crucial role for our customers, and the good news is that our Cobalt-free driers can be added at any stage of the process. The dosage requirements are remarkably low compared to Cobalt or Cobalt polymer driers. In fact, our Cobalt free paint driers require approximately 10 times lower dosages.

Formulators can achieve multiple benefits. Firstly, it helps reduce the overall cost of the formulation. Additionally, these driers contribute to improved paint performance. The use of Cobalt-free driers is particularly advantageous in the world of alkyd high-solid formulations, where slightly higher dosages may be required.

  • Can be added directly to the paint system at any stage of production

  • Dosages of 0.3 to 0.9% (calculated on solid binder) recommended for medium solids and waterborne alkyds

  • Ultra-high solids alkyds require up to 1.3%

  • When directly replacing Cobalt in existing formulations, ADDITOL® dry CF100 can be used in FOD at same levels as Cobalt 10%

  • The new products can be combined with all common secondary and auxiliary driers eg. Aluminium, Barium, Calcium, Lithium, Potassium, Strontium, Zinc or Zirconium

  • Sometimes the auxiliary driers have to be adjusted when switching from Cobalt to our new products – experience shows higher requirements of zirconium

The ECOWISE™ CHOICE range of Cobalt-free driers offer a cost-effective solution, while maintaining excellent performance. The ADDITOL® dry CF range from allnex is the ideal choice for formulators seeking to optimize their formulations.

FAQ

Let’s answer some of the most frequently asked questions about drying metals:

The latest information indicates that Cobalt is currently categorized as a reprotoxic material under Category 2. While its usage is still permitted at dosages up to 0.1 percent, reclassification to Category 1 is expected in certain European countries in the near future, which may lead to a phase-out. We closely monitor these developments and stay in contact with regulatory authorities such as the ECHA and REACH.

No. Barium and Lithium serve as auxiliary driers with a specific function in air drying paints. Their role is to transport oxygen into lower film layers. Unlike primary drying metals which can create a tightly sealed film, auxiliary driers ensure Oxygen penetration. Barium, Lithium and other secondary driers are critical for maintaining effective drying throughout the paint film.

Initially, Manganese driers had a more significant impact on discoloration compared to Cobalt. However, through extensive research and development, specific compositions of ligands and organic accelerators have been formulated to compensate for Manganese’s discoloration tendencies. These formulations have been approved for use in bright white applications, even for exterior facade paints, with minimal impact on whiteness.

Our approach focuses on providing the best options for our customers. While we offer a primary Manganese dryer (CF100) for formulation purposes, the use of auxiliary driers (such as Calcium, Barium and Lithium) is optional. The combination of primary and auxiliary driers can enhance through drying, hardness development and oxygen transfer if required for specific resin systems. However, the use of auxiliary driers is not mandatory.

Dosages of Cobalt-free driers are significantly lower than traditional Cobalt or Cobalt polymer driers. In most cases, Cobalt free paint driers require approximately 10 times lower dosages. This reduces formulation costs while maintaining performance. In high solids alkyd systems, slightly higher dosages may be needed. Overall, Cobalt free driers offer cost-effective and high-performance alternatives.

Cobalt free paint driers, such as ADDITOL® dry CF100 and CF103, have been specifically designed to maintain performance even after prolonged storage. These driers feature patented multi-acceleration systems that compensate for any performance loss over time. They are also compatible with both waterborne and solventborne systems, making them versatile options for various formulations.

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The following article is from a webinar between Daria Samonova (Corporate communications manager, allnex) and Bernard Hirschmann (Director of Global Business Development & Innovation for Additives, allnex). For the full presentation, please see the video above.