Direct-to-metal (DTM) coatings are a relatively small portion of the larger metal coatings market, but they are one of the fastest growing segments. Why? DTM coatings can deliver significant gains in production efficiency and cost savings by replacing multi-coat systems, which often include a pretreatment, primer and top coating, all applied separately. At a minimum, they usually have a primer and top coat layer.
In traditional metal coatings, the primer is needed for adhesion and primary corrosion protection. The top coat provides weather and UV protection, plus aesthetic properties like color and gloss. Primers are typically epoxies and topcoats are usually polyurethanes.
A DTM coating includes paint and primer in one. Aptly named, a DTM coating is applied directly to the metal, ideally after only minimal surface preparation because it is designed to be applied in a single application. Adhesion, corrosion and resistance to weathering are all provided in one coating, which saves steps and reduces drying time.
Historically, DTM coatings were used in light duty industrial maintenance applications including factory machinery and farm equipment. However, as the technology advances, applications with higher performance requirements such as bridges, infrastructure, and even marine applications can benefit from DTM coatings as well.
Evaluating DTM Coatings
If you’re thinking about DTM coatings, there are several factors to consider when choosing one, including:
Type of End Use (durability)
What environmental exposure will the object have to endure? How harsh will it be? These can be broken down into different categories based on the expected lifetime of the coated item. Minimum durability might be specified by the architect, designer or government agency. Or, it can be based on historical data, such as what systems were used previously and how long they lasted. Durability could also be based on preference or price point. For some applications, the coating is not expected to last the lifetime of the object being coated (like a bridge). It becomes a matter of how often it needs to be repainted.
The environment where the DTM coating is being applied and application constraints are important. If it’s being applied in the rain or in high heat, a solvent-based coating may be preferable. If drying time can be longer, a water-based coating may be preferred.
The viscosity of the DTM coating will be a primary factor in determining whether to use a sprayer or roller. Spray coatings are thinner because they must be aerosolized. Thicker coatings are ideal for rolling or brushing so they won’t run.
Length of Service/Cost
Cost and durability/performance are major factors to consider. The longer a coating will last, generally the more it will cost (the “good, better, best” model). When the coating is “used up,” it will start to peel and lose its protective properties. Peeling will happen regardless, but there can be a wide range of useful life among coatings. In harsher cold-weather climates, cheaper coatings might only last a year, whereas higher-end coatings should last for 5-10 years.
There are three core chemistries often used in DTM coatings, each offering different price points and performance:
- Alkyd-type coatings are the lowest priced and offer performance to match
- Acrylics are in the middle of both price and performance.
- Polyurethane chemistries are priced higher and offer higher performance—primarily hardness (scratch resistance) and chemical resistance—for industrial settings.
Alkyds and acrylics are the most common chemistry types. In the DTM coatings market, Lubrizol has focused on polyurethanes. When looking at polyurethanes for DTM, there are typically three categories: Polyester, polyether and polycarbonate. Each has different strengths and weaknesses. The most durable is polycarbonate, but it’s also the highest priced.
Lubrizol also introduces a fourth class of polyurethane known as a polyamide polyurethane, which has amides in the backbone.
Aptalon™ polyamide polyurethanes are exclusive to Lubrizol and provide excellent hydrolytic stability, hardness and chemical resistance. These properties all play into the durability of the coating, so the coating can have a high-performing top coat and primer element for adhesion. Lubrizol has had tremendous success with polyamide polyurethane technology and is working on developing DTM coatings using this proprietary technology.
Commercially available polyurethane DTM coatings can be difficult to find, which is why we’re excited to break new ground with DTM water-borne coatings using the science behind Aptalon polyamide polyurethane technology. When we fully launch these revolutionary DTM coatings, they will offer a new way to save time and money by switching from a two-coat (or three-coat) system to a one-coat system with exceptional performance properties.
For more information, visit go.lubrizol.com/aptalon or contact your Lubrizol account manager.