Techniques to Enhance UV Stability of Dye Anodized Coatings

The ultraviolet stability of anodic coatings is assessed based on lightfastness/colorfastness testing.

Tej PatelThe tests predict any color change that can occur to the coating after exposure to UV light. In most cases, this color “shift” results in the fading of a color; however, in some cases, organic dyes can become darker when tested.

Research has shown that UV stability has some correlation with heat stability, particularly that of dyed anodic coatings; therefore, these two properties can be interrelated.

Applications where UV and Heat stability are critical requirements include architectural, automotive, medical, and epoxy bonding to anodized components

The factors that contribute to the UV stability of anodic coatings:

Alloy Type
Coating density/thickness
Coloring Method Organic vs. Inorganic
Organic Dye Chemistry
Seal Chemistry

In addition to managing these factors, differing methods of testing UV stability are available, each with its own varying degrees of difficulty, particularly as it relates to the interpretation of the results.

A clear understanding of the pros/cons of each testing method will help anodizers determine what conditions are best suited for their applications.

Since weathering behavior is a critical factor for various applications when compared to paints, anodized coatings have several benefits increased corrosion resistance, particularly filiform corrosion; superior optical appearance/look; more environmentally friendly (no VOCs); and superior durability.

As seen in the images above, when a painted part is compared to an anodized part that has been dyed and sealed correctly, the results are drastically different. The anodized coating retains much better color and visual appearance.

Heat Fastness and UV Stability

UV and heat stability are correlated when assessing the quality of an anodized coating that is dyed. A heat-fast dye has a much higher likelihood of being UV stable. However, this does not mean all UV-stable dyes are heated fast.

Dye 1, when tested in the Anocaliber UV24, showed no color shift. When tested for heat fastness showed a minimal color shift after 2 hours of exposure at 500°F. However, Dye 2 showed significant fading when heated. When Dye 2 was tested for UV stability, there was less color shift in the Xenon chamber compared to the Anocaliber UV24

Various industry applications require UV stability to ensure anodize products do not fail in the field:

Automotive – External applications must have minimal dE shift
Architectural – AAMA specifications have tight guidance on dE color shift
Military/Defense – MIL-A-8625

Furthermore, the new MILSPEC updates are requiring anodizers to develop boundary samples to ensure their color finishes maintain a certain quality standard that historically was not required.

Alloy Type

Certain alloys, when anodized, behave differently due to their alloying constituents. In general, Al alloys that tend to produce more porous coatings will be more UV stable than alloys that produce less porous coatings.

Therefore, in general, the 6000 and 7000 series alloys will take color better, which can result in a more UV-stable finish.

Anodizing Conditions

When optimizing the anodizing conditions for UV and heat stability, there are two primary parameters to manage: coating thickness and coating density.

In general, the thicker the coating, the more amount of dye can be filled in the pore resulting in a more durable color. Furthermore, the lower the current density of the coating, the more dye can enter the pore since the pore structure is larger, resulting in a deeper color that has more dye.

The images to the left validate this phenomenon

Dye Chemistry: Dye Buffering

Dye buffers are additives for the dye tank that enhance the quality of the product by

Improving the dye strike rate and activity
Improving dye coloring uniformity
Maintaining pH
Reducing dye bleed
Increasing lightfastness of dyes
Enhancing the depth of dye color
Reducing the effects of dye contamination

Based on the images included, the use of a dye buffer increased lightfastness. Additionally, The type of Dye Buffer and concentration used also helps in enhancing the lightfastness of the organic dye.

Dye Chemistry: Type of Dye Mixture

When optimizing the UV stability for an organically dyed coating, the type of dye that is selected is critical.

Single Component
Multicomponent/mixtures containing at least two dyes

Typically, single-component dyes perform much better than mixtures due to the fact they can saturate the coating faster and more efficiently.

Color management in a single component is much easier than that of a multicomponent because single-component dyes do not contain other dyes that may not be lightfast or have different rates of adsorption.

Among the single-component dyes, there are various types based on their molecular structure. Typically, the 1:1 molecule single-component dyes are the most stable and provide the most consistent color.

If using a multicomponent dye, there are several factors to consider:

The number of total components (the more components, the more inconsistency)
The type of dye components (Is each component lightfast?)
Component compatibility (Do they have the same pH, dye coloring rate, etc.?)

Dye Chemistry: Dye Metallization

In addition to determining the type of dye to use, single vs. mixture, it is important to understand the effects of metallization when selecting a dye

During the dye manufacturing process, a metallization step is required, where Cr, Co, Ni, or Cu are attached to the dye molecule in order to improve dye color, dyeability, and lightfastness.

Metalized Dyes: Based on the images to the left, dyes that are metalized with Ni or Cu appear to give the best UV stability. This is likely due to the fact that Ni and Cu act as UV filters in anodic coatings.

Non-Metallized: On the panel’s test to the left, results indicate that some non-metalized dyes can be lightfast; however, there is a significant variance in UV stability when using Non-Metallized dyes.

Another factor to consider with Non-Metallized dyes is that when sealed with Ni, they tend to shift much more in color because they are metalizing during the sealing process, which results in a color change.

Seal Chemistry: Nickel Vs. Non-Nickel

Based on these results, the non-nickel seal did not perform well after only 5 hours of accelerated UV light exposure. In general, non-nickel sealing does not provide good results with organic dyeing.

Since UV light is filtered by Ni, dyes sealed in Non-Nickel seals tend to break down much faster through UV degradation.

Seal Chemistry: Mid Temp Nickel

The most common type of sealing uses hot nickel acetate to hydrate the anodic coating and incorporate nickel into the anodic pore

In the panels on the left, different concentrations of nickel were tested in order to determine how to improve UV stability.

Based on these images running the Nickel Acetate seal at an elevated concentration significantly improves color durability. The more nickel that can be incorporated into the anodic coating, the more lightfast the part

Seal Chemistry: Cold Sealing

Another very popular method of sealing is cold sealing which occurs at room temp between 80°F – 95°F. In this process, sealing is divided into two steps: nickel incorporation and hydration in hot DI.

Since sealing is separated into two steps, more nickel can be incorporated into the coating resulting in almost four times more nickel in the anodized finish depending on the type of cold seal chemistry.

Based on these results, cold sealing is one of the best sealing methods for organic dyes when trying to achieve maximum light fastness.

Type Of Seal: Hot/Cold Nickel Seals

The type of nickel seal used can play a role in UV stability, particularly depending on the type of dye used. In the images above, we have identified three different dyes, each with unique results, using a cold Ni seal and a hot Ni seal.

The dyes that perform with the most consistent results along with the best color remain single components, as seen with the Black HCB and Black HP-5, which sealed well in both hot and cold Ni Seals.

UV Testing Methods: Summary

Various types of testing equipment are used in order to determine the UV stability of coatings. However, each test has its own limitations and shortfalls, which makes it critical to understand the application of a part prior to testing.

One major difference is that most UV testing methods do not include a “heat” stage/element. Therefore, the test chamber does not truly match external conditions as it relates to deserts or other warm weather locations.

Some of the specifications require a 1,500-hour test which can take from 60 to 90+ days, depending on the lab. This can mean a proliferation of products in the market before a problem is identified.

Weatherometer testing is also expensive, and financial considerations may discourage frequent testing, which can result in potential failures of components in the field.

Test Type	Characteristics
Carbon Arc Test	One of the first machines used to check the light fastness of textiles, dyes, paints, and resins. The light source was generated by energizing carbon rods which would glow and produce a UV light. The disadvantage of this machine was that the rods had to be changed every 24 hours.
QUV Accelerated Weathering Test	Simulates sunlight, rain, and dew but uses a UV source from fluorescent bulbs. The disadvantage is that this unit is temperature-controlled and does not create atmospheric heat
Xenon Weathering Test	This machine is the most popular and cycles to simulate the actual behavior of daylight, night rain, and dew The disadvantage is that this unit is temperature-controlled and does not create atmospheric heat
Techevon UV ANOCALIBER 24	A high-intensity UV source that produces a spectral band very similar to daylight. The chamber temperature can reach up to 120 F, simulating external temperatures and ensuring accurate and representative testing for exterior applications.

UV Testing Methods: Challenges and Issues

Most UV Testing methods are designed to be multipurpose (for all coatings), therefore as it relates to anodizing, there are several issues:

Less effective in assessing anodic coating durability
Extended testing time period to predict failure
Not representative of organically dyed anodic coatings

The integrity of organic coatings is a function of the coating chemistry (acrylic, urethane, epoxy, etc.) as well as the pigment that is dispersed.

Organic coatings fail because of the organic used, and the color degrades largely because of the failure of the organic system. Degradation shows up as a chalky (velvety) appearance on the surface as an atmospheric attack, as seen below

Anodic coatings do not chalk, and so the weatherometer test, as it relates to anodizing, is only checking the weathering/light fastness, but this test is done at a controlled temperature, not representative of true outdoor exposure.

For example, automotive trim has been recorded to have a temperature of 120 °F on a hot sunny day. An organic dye will have different resistance to UV if the substrate temperature is 80°F vs. 120°F.

We feel that testing the anodic coating under controlled conditions, such as that in a weatherometer, might qualify a dye that has good light fastness but poor heat fastness.

Final Thoughts and Considerations

When determining how to enhance the UV stability of an anodized coating, there are five primary factors to consider

Application of the anodized part (outdoor/indoor, heat exposure)
Type of testing method/procedure
Type of Colorant - Organic vs. Inorganic
Type of Organic dye (molecule type 1:1 vs 1:2, single component vs multicomponent)
Type of Seal Chemistry (Ni vs. Ni Free, Hot/Mid Temp Ni vs. Cold Ni)

A thorough understanding of how the anodized component will be used is vital to determining the appropriate method of both testing and chemistry.

Does the application require heat stability because not all dyes that are UV stable are heat stable?
If a specific color is required, can it be achieved using a single-component dye? Is the color saturated to ensure the maximum amount of dye is in the coating?
When sealing, can cold sealing be used instead of a Hot Nickel Acetate to ensure more nickel is incorporated into the coating?

Overall, UV-stable organically colored anodized coatings are achievable when the appropriate anodizing conditions, dye, and sealing methods are used. Therefore, when working with your anodizing chemistry supplier, we urge that you ask these questions:

Is my dye a single component or a mixture? If a single component, is it also heat fast in addition to lightfast?
Does my seal chemistry provide the best results for the application? What data is there to validate this?

If the correct anodizing parameters, dyes, and seals are used, it is possible for organic dyes to sustain years or more of outdoor exposure with limited color shift.

Tej Patel is a Director at Techevon Metal Finishing Solutions, and received the top presentation award at the Aluminum Anodizers Council conference in October. Please visit https://www.techevon.com.

Note: This paper received the Donald I. Johnson Award of Excellence for Best Presentation at the 2021 Aluminum Anodizers Council conference. Patel received the award at the AAC conference in Kansas City this year.

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