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The Joy of Tidying Up Your Painting Process with DTM Coatings

Wallace Kesler – Sep 24, 2020

TAGS:  Industrial Coatings      Automotive Coatings    

The Joy of Tidying Up Your Painting Process with DTM Coatings Simplification is a recent lifestyle mega-trend popularized by authors such as Marie Kondo in her best-selling books. Minimalism, as a consumer choice, is characterized by clean design and multi-functionality for goods, such as appliances and furniture. These principles applied to manufacturing processes enable the conservation of resources and cost reductions by eliminating steps.

Arguably the most complex coating process is OEM automotive finishing. The automotive painting uses the majority of the energy consumed in vehicle manufacturing. Industrial and construction finishing doesn’t involve the complexity of automotive manufacturing, but the advantages of process simplification still apply.

Eliminating the primer coating of metal surfaces by using Direct-to-Metal (DTM) coatings is an excellent way to simplify the painting process. Applying DTM coatings can simplify, accelerate and economize the finishing process in the following ways.

Let's begin by understanding the simplification process...

How DTM Coatings Simplify the Finishing Process

DTM coatings simplify the finishing process by:

  • Eliminating the coating steps (application/flashing/curing) associated with priming metal substrates.
  • Reduced emissions from fewer coating applications often facilitating a less restrictive permitting process.
  • Cost savings through labor elimination and coatings inventory reduction.

Although it is a small portion of the total global coatings market, the advantages of DTM coatings have made it one of the fastest-growing. DTM coatings are expected to grow at 1.5 to 2.5 times the rate of other market segments.

The benefits can only be realized if the DTM coating is capable of performing the functions of primer and topcoat. These functions include the adhesion and corrosion resistance provided by a primer coupled with the weathering protection and cosmetic appearance expected of a topcoat.

DTM Development Goals and Challenges

Substrate Wetting

Substrate wetting is the foundation on which all properties are built and is critically important for DTM coatings. Wetting is a function of many formulation components but is largely dependent on the binder system.

Improved Substrate Wetting with Blue Paint Droplets by Chemical Dynamics
Improved Substrate Wetting with Blue Paint Droplets
Source: Chemical Dynamics

Solvents and additives, such as surfactants can improve substrate wetting, but the backbone polymer is the primary driver of proper wetting. Combinations of binder chemistries are often utilized in DTM coatings to achieve this property.

Adhesion to Multiple Metal Substrates

There are multitudes of metals that could be finished with DTM coatings. The most common are:

  • Cast iron
  • Various steels, and
  • Aluminum alloys

Of these, aluminum alloys are traditionally thought of as more difficult to achieve adequate adhesion.

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Adhesion is closely related to wetting and is also a primary attribute of the coating binder system. Screening a starting point formulation for adhesion to the target substrates is the first step to developing a DTM coating formulation. Material selectors, supplier recommendations and experience with similar applications are all good methods determine candidates.

Application Attributes to Meet End-User Expectations

Tailoring the application properties to meet the end-user expectations is especially important for DTM coatings. Since one of the primary reasons favoring the implementation of a DTM system is process simplification, uncomplicated application methods are critical.

DTM coatings might be applied by methods ranging from brush or roller to electrostatic turbine bells. Each application method requires an optimal set of properties in order to be successful, such as:

For example, the balance between sag resistance and the ability to build an adequate wet film thickness is a key application performance indicator for DTM coatings.

Corrosion Resistance

A mission-critical function of all metal coatings is preventing corrosion of the substrate. Often the end-users perceptions are that DTM coatings will fall short of expectations in this area. While that might be true for harsh conditions, such as C4 or C5 a properly formulated and tested DTM coating can provide protection from corrosion under service conditions as severe as C3.

The important properties the formulator can tailor to deliver corrosion resistance are:

A caution sign in the road to achieving corrosion resistance in DTM coatings is to take care and not to compromise other properties in the process.

Corrosion Resistance of Polychem 7775-EB-70 by OPC Polymers
Corrosion Resistance of Polychem 7775-EB-70 at (A) 168 hrs (B) 336 hrs
Source: OPC Polymers

UV Durability

Depending on the end-use, UV durability/weathering resistance can be critical or trivial for a DTM coating. Interior or automotive under-the-hood applications require little to no UV durability. This is one of the properties that might be compromised by striving for maximum corrosion resistance. For example, an epoxy coating probably shows exceptional corrosion resistance in a DTM application, but the UV durability would be poor.

When formulating DTM coatings it is important that you, “Begin with the end in mind”, in the words of author Stephen Covey. 

The contrast between the demands on DTM coatings and primer/topcoat systems is very clear when considering the balance of corrosion resistance and UV durability. Two coat systems can rely on the primer for corrosion resistance while the topcoat provides exterior weathering ability. The DTM coating must accomplish both with one formulation.

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 Gloss and Appearance

Gloss and appearance of the finish are the first two things a consumer notices when considering a product purchase. Performance can meet or exceed all requirements, but if the appearance doesn’t meet expectations, the coating is not fulfilling the intended purpose.

Satisfactory appearance must be achieved in a DTM coating without the help of a primer coat to fill a porous or uneven surface. Wetting and application properties discussed above are important factors in how the DTM finish appears after curing.

1K for 2K Performance

DTM coatings don’t have to be 1K systems, but it adds value to the finishing simplification equation. Industry suppliers are devoting significant resources to developing polymers that can be formulated into coatings achieving two-part performance in a single package.

Related Read: Decoding Coil Coatings - Basics to Applications

Wide Range of Chemistries Available for DTM Coatings

The coatings formulator has allies in the industry suppliers conducting innovative, focused research to develop raw material targeted to the DTM market. Resin manufacturers regularly introduce new products geared to this market segment. Additive suppliers are also developing new products for the formulation of DTM coatings.

Wide Range of Chemistries Available for DTM Coatings

1K Solvent-borne Systems

  • Solvent-borne Alkyds are still the most common ambient curing single package coatings used worldwide. Although this is a mature technology, in regions where regulations allow alkyds can perform well as DTM coatings. View alkyd grades for solvent-based systems »

  • Melamine/Urea Cross-linked Bake Enamels is another mature technology with applications for DTM coatings. While less popular than they once were due to VOC and formaldehyde restrictions in manufacturing, these coatings still have a place in industrial finishing.

  • Blocked Isocyanate Urethane Baking Systems can deliver two-component urethane performance without the drawbacks of mixing and pot-life limitations. They are also low VOC capable and formaldehyde-free.

2K Solvent-borne Systems

  • Acrylic-Urethane coatings are the preferred binder system for ambient cure systems requiring UV resistance and high gloss, such as ACE coatings. The perception is that direct-to-metal adhesion of acrylic urethane paints is often sub-par. See urethane acrylic resins available for coatings»

    Several resin manufactures are supplying acrylic resins specially developed for 2K solvent-borne DTM urethane coatings. allnex markets SETALUX® 17-2319 for VOC compliant DTM acrylic-urethane coatings. This HAPS-free acrylic polyol offers:

    • Adhesion to variety of metals
    • Humidity and corrosion resistance
    • Long pot life, and 
    • Good hardness/flexibility balance

  • Polyester-Urethane coatings offer improved adhesion to metals over acrylic-urethane coatings, favoring them in DTM applications. As a general rule, polyester-urethane coatings do not have the UV resistance of acrylic urethane coatings. Often polyester and acrylic polyols are combined to improve a specific property of the resulting coating. For example, a polyester polyol-based coating can be modified with a compatible acrylic polyol to increase the UV stability.

  • Epoxy coatings provide excellent adhesion to metal and outstanding corrosion resistance. The UV stability is unsatisfactory for DTM coatings used in exterior applications. They can perform to the highest standards for applications, such as:

    • Under-hood automotive components
    • Structural building applications, and 
    • Industrial machinery

Related Read: The Secret to Formulate Waterborne Metal Epoxy Coatings That Work

1K Water-borne Systems

OPC Polymers Direct to Metal Coating Series
  • Self-Cross-linking Acrylic Latex coatings have many advantages for DTM applications. They can be formulated to very low VOC levels ensuring regulatory compliance in many regions. Acrylic polymers exhibit good to excellent UV durability, and they are generally very easy to apply.

    Concentrated development of self-cross-linking acrylic latexes means there are frequent product introductions.

    • BASF offers Acronal® S 760 na for the DTM coatings market featuring long-term durability and corrosion resistance. 
    • allnex markets SETAQUA® DTM 6850 which contributes excellent adhesion, corrosion resistance and high gloss capability to direct to metal coatings.

  • Water-borne Alkyd Dispersions can provide very low VOC DTM coatings with excellent application properties. Another advantage is the high percentage of plant-based content possible for WB alkyds. A possible disadvantage is the slow speed of drying.

  • Polynt offers Beckosol AQ® 405 Short Oil Alkyd Emulsion developed to formulate into DTM coatings with good corrosion resistance and high gloss. Sustainability advantages include the fact that:

    • The emulsion does not contain solvent as supplied.
    • It can be formulated without any added volatile coalescent.

    The certified bio-based content of Beckosol AQ® 405 is 36%.

2K Water-borne Systems

2K Water-borne coatings include:

  • Isocyanate
  • Carbodiimide and
  • Aziridine Cross-linked chemistries

These might be based on acrylic or polyester polymers with functional groups tailored to the cross-linking chemistry. The cross-link density can vary greatly depending on chemistry affecting performance in parameters, like:

  • Corrosion and humidity resistance
  • Hardness and flexibility

The highest performance is realized when hydrophilic isocyanates are used in combination with hydroxy functional polymers.

Carbodiimide and Aziridine cross-linkers react with carboxylic acid groups to give coatings with a longer pot-life than urethanes, but with power performance.

The Future of DTM Coatings

The future is bright for DTM Coatings. They are already growing faster than the global coatings market. As end-user acceptance of DTM coatings in the decorative, industrial and protective coatings markets increases their growth rate will accelerate.

As innovation in polymers for DTM coatings progresses, present shortcomings will overcome. Improvements in corrosion resistance and exterior durability will expand current markets and open up new ones, such as marine protective coatings.

Sustainability will drive the future growth of DTM coatings. Plant-based raw materials used in novel binder systems like Polyaspartic coatings will ensure an important place for DTM coatings for the foreseeable future.

Additives, Resins & Pigments Suitable for Metal Coatings

View a wide range of additives, pigments and resins available today for metal coatings, analyze technical data of each product, get technical assistance or request samples.

Decoding Coil Coatings: Basics to ApplicationsWant to Preserve Your Coil Coating Margins?

Learn how to eliminate edge-to-edge & head-to-tail variations by leading expert Michael R. Bonner so that your coil coating process is stable over the entire run and repeatable from run-to-run, achieving color consistency & film uniformity.

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