If your application requires high performance, it’s time to shake off the TGIC polyester mindset and start exploring the world of thermoplastic powder coatings.
As many articles on powder coatings start out, there are two broad chemical categories: thermoset and thermoplastic. Before devoting the rest of the information to the much larger world of thermosets, such articles usually give a two sentence nod to thermoplastic powder coatings, acknowledging that they do exist, and then provide a glorious description such as “a wax-type chemistry that melts when heated and solidifies when cooled.” Wow – wax! Really sparks the imagination and makes you want to learn more, doesn’t it?
So why give thermoplastic powder coatings further consideration when they play the “I get no respect” bit (apologies to Rodney) to the more prevalent thermoset chemistries like TGIC polyester (polyester cured with triglycidyl isocyanurate)? One word: performance. Thermoplastic powder coatings have performance properties unattainable with thermoset chemistries. If your application requires 5000 hours salt spray protection, 10 years UV resistance, 500 in./lb impact resistance, better than 0T flexibility, high lubricity or tack, it’s time to shake off the TGIC polyester mindset and start exploring the brave new world of thermoplastic powder coatings. Come on in, the water’s fine.
Let’s dispense with the wax concept once and for all. (As you can see, this one has bothered me for some time.) Many high-performance thermoplastic powder coating chemistries are engineered systems with functional chemistries designed to provide specific attributes. Properties such as adhesion to metal, lubricity of the coating, flexibility and elongation of the coating film, and many more can be optimized for a given application. Being thermoplastic in nature, the powder exhibits these properties inherently without requiring further curing or crosslinking once on the part. The molecular weight or length does not increase with a crosslinking reaction. Thermoplastic powder coatings generally only require enough heat and time to melt the powder into a smooth, continuous film.
Herein lies the definitive difference between thermoset and thermoplastic chemistry. Conventional thermoset powder coatings start with relatively short molecular lengths, which are subsequently knitted together into a matrix with a crosslinking reaction once on the part. This is the “cure” that is required for thermoset coatings. This difference between the longer chain molecules of thermoplastics and the crosslinked shorter molecules of thermosets is where many of the property differences are derived.
Using thermoplastic powder coatings requires a new mindset. As you put the TGIC polyester mindset aside, you’re leaving the idea of a relatively thin 3.5 mils of a hard, rigid, tightly cross linked coating to perform the job. As you begin to accept the thermoplastics powder coating mindset, you’re embracing the idea of a true plastic film that is thicker, more flexible and more ductile, and can bring with it properties more closely associated with plastic films. Coating film thicknesses around 8 mils are common, but thicknesses of 100 mils and higher are routinely used. Chemistries like nylon, polyethylene, polypropylene and polyvinyl chloride come into play. Put away the idea of solvent rubs and pencil hardness, and start thinking in terms of barrier properties, tensile strength and percent elongation. Many thermoplastic powder coating users think of the technology as a method of applying a plastic film to substrates that do not lend themselves to film lamination or direct film extrusion due to the geometry of the part.
A good example to start getting your arms around thermoplastic powder coatings and where they fit (and don’t) is pickup truck grill guards. In this application, the amount of road spray and stone impact varies a great deal between pavement and gravel road driving. Both TGIC polyester (a thermoset) and copolymer polyethylene (CoPE, a thermoplastic) are used in this application by the same manufacturer. The TGIC polyester provides a very hard coating that has excellent scratch and mar resistance, but lower impact resistance. The CoPE provides a thicker, more ductile film that has extremely high impact resistance, but lower scratch and mar resistance. Grill guards that are intended more for pavement driving purposes use TGIC polyester since the scratch resistance is more important than the impact resistance. The grill guards intended for off-road use CoPE since the chip resistance is extremely high, even though the coating will mar with the impacts from the rocks.
To the manufacturer, the decision boils down to which is more important for each application – the ability to retain a scratch-free surface after light stone impacts with the potential to chip off in heavier use, or the ability for the coating to remain on the part protecting the metal after severe rock impacts, even if the coating is scratched. The mechanism by which these two coating systems take impacts is often the driving force behind thermoplastic powder coating applications. When film ductility is combined with high adhesion levels, the impact resistance is extremely high. The illustration in Figure 1 demonstrates CoPE’s ability move with the metal substrate during impact damage compared to the less flexible thin film TGIC polyester.
Chemistries and Properties
We’ve been discussing thermoplastic powder coatings in general terms thus far. Now it’s time to start breaking down some of the more common chemistries, as listed in Table 1. Also listed are some generally accepted descriptions and common applications for each chemistry.
The most frequently asked questions regarding thermoplastic powder coatings – following “What are they?” and “Can they help me?” – are: How are the coatings applied? Can I use my existing application equipment? Will they work in my plant?
Most thermoplastic powder coatings can be applied with conventional electrostatic powder coating equipment, but the thermoplastic nature of the coatings also allows some additional coating options, such as fluidized bed coating and the use of mini coaters.
Electrostatic spray is one of the most common methods of applying thermoplastic powder coatings that have good electrostatic characteristics, such as CoPE and nylon. Since the particle size of these coatings is generally larger than conventional thermoset coatings, and a greater coating thickness is generally applied, adjustments in airflow and charging voltage may be necessary when compared to thermosets; however, these adjustments are usually minimal.
Fluidized bed coating is another common application method and is somewhat exclusive to thermoplastic powder coatings. Without the worry of starting a crosslinking reaction in the fluidized bed, ultra-thick films in excess of 100 mils can be achieved; however, 10 to 20 mils is more the norm. The basic principle is that the part is preheated and subsequently dipped in the fluidized powder. The heat in the part melts the coating, and film thickness builds. Fluidized bed coating is common in welded wire products as it eliminates the faraday cage problems that arise from cross-welded wires. It also used for outdoor furniture, where coating thicknesses of 30 mils are desirable.
Mini coaters are excellent for smaller parts coated in larger numbers. In a continuous process, small parts are preheated and dropped in a vibrating bed of powder. As the heat sink of the part runs out, the coated film thickness stops building. The parts are then run through a secondary heating process that completes the flow-out of the coating. Since the film building process is temperature driven, crystalline polymers such as nylon are generally used due to their crisp melt point.
The coating material costs of thermoplastic powder coatings range from below $1/lb to well over $10/lb, depending on the chemistry. But in completing the mindset change from thermosets to thermoplastics, you need to go well beyond the cost per pound and look at the total cost of application. When comparing thermoplastics to thermosets, issues such as specific gravity, average coating thickness and heating requirements can swing the applied cost calculations as much as, if not more than, the cost per pound of the coating material.
Specific gravity differences will generally favor the thermoplastics. Since most thermoplastic powder coatings are lighter than thermosets, you will use fewer pounds of powder to achieve the same coating thickness. However, this reduction is often offset by the higher film thicknesses used with thermoplastic powder coatings to achieve the desired performance properties.
One of the biggest opportunities to think outside the cost equation box involves the coating’s heating requirements. Since the coating only needs to be melted and flowed out, and extended times at “cure” temperatures are not needed, many new options become available. Some processes only use a preheat prior to the coating application and allow the residual heat left in the part to flow out the coating. Alternatively, some individuals using conventional post-heat methods have found that they can turn their conventional ovens down significantly, while others have even been able to shorten their ovens.
This article is not an attempt to devalue thermoset chemistries. TGIC polyester and its various blends are truly remarkable coating systems capable of providing an outstanding price/value ratio in many applications. The tendency, however, has been to try to shoehorn TGIC polyester into applications well beyond its capabilities. Barely a month goes by that I don’t encounter someone attempting to achieve 5000 hours salt protection by applying 7 mils of TGIC polyester, almost always with poor results; or someone with welded wire parts failing in the field due to faraday cage issues, who never considered using thermoplastic powder coatings in a fluidized bed coating process. The objective is to raise the visibility of thermoplastic powder coatings and get them into the mix of options for solving coating problems. They do not fit everywhere, but when you need them, you really need them.
Credit: PCI Magazine August 4th, 2006 Tim Brinner, Author