Recent technical advancements in powder coatings have been influenced by the introduction of new raw materials and the desire to expand the powder coating market. On the surface, it appears that technical progress has been limited. However, upon closer examination, the technology of every powder classification has advanced as formulators work to meet the demands of end users.

To understand the advancements in powder technology, the two fundamental types of powder coatings—thermoplastic and thermoset—will be mapped out. These maps will be used to show the basic advantages of each powder chemistry as well as the drivers influencing the direction of powder coating technology today.

Thermoplastic Powder Coatings
Thermoplastic powder coatings are based on polymers that flow when heated to their melting point to form a smooth, continuous coating. Since thermoplastic resins lack functionality, the fusion process is strictly a melt-flow physical change and not a chemical one.

Figure 1. Thermoplastic Powder Technology

The physical properties of a thermoplastic powder depend upon the type of resin and its initial molecular weight. Thermoplastic powders have had some limitations in the past, including high fusion temperatures, poor adhesion to metal (necessitating the need for a primer) and high applied cost. Yet, thermoplastic powders are still relatively easy to make, use commodity polymers and are easy to apply.

In response to rapid growth of thermoset powder coatings, changes in thermoplastic powder coatings have emerged. The direction of thermoplastic powder technology is mapped out in Figure 1. In this diagram, the core technologies of thermoplastic powder coatings make up the center of the map.

Thermoplastic coatings are traditionally divided into three categories: nylon; vinyl (PVC and fluoropolymers); and polyolefins (polyethylene and polyester). Nylon and PVC are the two major chemistries with more than 95% of the market share. (Thermoplastic powder coatings only account for 10-20% of the entire powder coating market.) As indicated in Figure 1, thinner films, corona application, waterborne primers, primerless powders and polymer alloying are some of the technical developments that are helping thermoplastic technology expand its reach.

Nylon. Practically all of the nylon powder coating presently used in North America is based on type 11 or type 12 nylon resins. Powder coatings produced from nylon resins have several key characteristics: toughness; excellent abrasion and impact resistance; a low coefficient of friction; good solvent and chemical resistance, except to acids and low toxicity. Many nylon formulations also carry USDA approval.

Formulation of nylon coatings typically includes about 80% resin, with the balance consisting of pigments and additives. Nylon coatings withstand higher temperatures than other thermoplastic finishes. They are recommended for continuous operation up to 180°F in air and can be used up to 280°F in the absence of air, such as under oil.

There are several interesting uses for nylon powder coatings. A metal part with a thin nylon coating will provide the excellent friction, wear and abuse characteristics of nylon with the torque and load bearing properties of the metal. The unique combination of low friction and good lubricity make nylon coatings ideal for sliding and rotating bearing applications such as automotive spline shafts, relay plungers, shift forks and other bearing surfaces on appliances, farm equipment and textile machinery. Hospital equipment coated with nylon can be repeatedly sterilized without deleterious effects. Additional applications for nylon powders include truck drive splines, surgical instruments, hooks, meat smoking baskets and racks, vapor degreaser baskets, printing press rolls, valves, pumps, shopping carts, appliance door hinges and file sliders.

One of the most recent developments is the introduction of thermoplastic epoxies to compete with nylons. Interest is high in thermoplastic epoxies because they are potentially cost-effective replacements for some traditional nylon applications.

Vinyl Polymers. Polyvinyl chloride (PVC) powders have excellent corrosion, chemical resistance and dielectric strength. Due to the porosity of PVC, it can be easily heated and blended (alloyed) with other chemicals to modify UV durability, hardness, tensile strength and temperature resistance. For example, PVC can be formulated to withstand the stress of metal fabrication operations such as bending, embossing and drawing. Vinyls have a long history of successful performance for outdoor durable applications. PVC-resin-based formulations resist chemical attack from dilute mineral acids and alkalies at temperatures up to about 160°F. They will withstand most inorganic acids, edible oils and aliphatic organic compounds. PVC coatings are not suitable for contact with solvents such as ketones, esters and halogenated hydrocarbons.

Typical applications for PVC powders include dishwasher baskets, conduits, fan guards, air conditioner grills, bus bars, lawn furniture, cable trays, fence posts, shelving, chain link fencing and a variety of wire goods.

Fluoropolymers are another type of vinyl polymer. PVDF (polyvinylidene fluoride) has been copolymerized with monomers to increase flow and flexibility. ETFE (ethylene-tetrafluoroethylene) copolymer and FEP (fluorinated ethylene propyl) copolymer are other fluoropolymer alternatives. PVDF is known for its outstanding exterior durability. High heat, dielectric and chemical resistance can also be achieved.

Polyolefins. Polyethylene coatings provide excellent protection against corrosion, demonstrate high chemical resistance and have zero water absorption. Polyethylene coatings are resistant to most acids, alkalis, salts and alcohols up to 200°F and to many solvents at room temperature. Polyethylene coatings are softened by a number of oils and greases. A variety of properties can be achieved with polyethylene depending on the density. Variation in melting point and resistance to acid and bases is generally increased as the density increases.

For optimum adhesion, metal surfaces should be cleaned and roughened prior to coating. Blast cleaning is an effective means of achieving good results. Pigmented formulations of polyethylene coatings are suitable for use in exterior applications, but the natural clear type will degrade under UV light. Poor adhesion has always been the problem, but improvements have been made and in some cases polyethylene is challenging PVC as a potential competitor. Shrinkage still remains an obstacle for many applications.

Typical applications for polyethylene powder include test tube racks, plating racks, laboratory tools, fume exhaust ducts, glass bottles, filter plates, pump impellers brake covers, chemical tank lids and pipe hangers.

Figure 2. Thermoset Powder Coatings

Thermoset Powder Coatings
Thermoset resin systems are low-molecular-weight solids that go through a melt fusion and chemical reaction to form higher-molecular-weight polymers. Thermosets are characterized as heat cured and will not remelt when heat is applied. Figure 2 details the foundation and growth of thermoset powder coatings.

Epoxy. Some of the very first thermoset powder coatings were based on epoxy resins. This group of materials offers the best chemical, corrosion and heat resistance. Typical applications include oil and gas pipe. Epoxy has always been limited by poor ultraviolet (UV) resistance. But, this weakness has been a driving force behind several advances in this chemistry:

• Mixtures of epoxies with other resins
• Lower temperature cure epoxy backbones
• UV-durable epoxies
• UV-curable epoxies.

Epoxy resins have typically been cured by a number of different curing agents, including DICY (dicyandiamide), OTB (orthotoly biguanadine) and phenolics. Although these crosslinkers don’t detract from the favorable properties of epoxy, they don’t solve the problem of poor UV resistance.

To address this problem, epoxies were first mixed with polyester, creating what are commonly known as polyester-epoxy hybrids. Some improvements in durability were noted, but only a few people consider polyester-epoxies suitable for outdoor applications. In any event, by varying the ratio of polyester to epoxy, a variety of properties can be achieved. This unique formulation flexibility helped polyester-epoxies quickly establish themselves as a dominant chemistry in the powder market.

Recently, influenced by the growth and wider acceptance of acrylics in the automotive market, another type of epoxy hybrid has emerged, commonly known as acrylic hybrids. By combining acrylics with epoxy in various ratios, an interesting balance of properties can be obtained. These include UV, chemical, hardness and scratch resistance superior to polyester hybrids. Interestingly, this chemistry is growing at the expense of polyester hybrids and even polyester urethanes when extreme UV resistance is not needed.

The desire to expand powder coating into new markets has been the other driving force behind new epoxy technology. During the last several years, UV durable epoxies have entered into the market. This type of epoxy has been backbone modified to increase durability. Concerns remain regarding its storage stability but commercial systems are now available. Interest in using epoxy technology on wood has led to lower-temperature cure epoxies now available from several epoxy suppliers. In addition, the need to balance cure and storage stability for wood has dictated tighter specifications for some epoxy resins.

The final direction of epoxy technology has been the expansion of epoxy powders into UV cure. This has allowed the production of powders that can cure as low as 100°C. In UV coating, the melting and curing of the powder are separated into two phases. First, the epoxy formulation is heated to achieve a melt. Once melted, it is then cured using UV or electron beam (EB) radiation. Curing epoxy functional resins is achieved via cationic photo polymerization. It is expected that expansion of UV curing epoxy materials will continue to move this fundamental class of powder coatings into the future.

Polyester. Polyester-epoxy hybrids are one of the dominant chemistries used in powder coating. Advantages of this class of powder coating include improved yellowing resistance and flexibility compared to epoxy. However, decreased hardness, chemical resistance and response to cure are typical. Typical hybrid applications include file cabinets, farm equipment, drawer slides, water heaters, shelving, oil filters, hospital furniture and fire extinguishers.

Improvements in color stability and reduction in cure temperature have resulted in hybrids that provide color resistance that often rivals exterior durable polyester counterparts. Cure latitude has also been extended with some polyesters capable of curing down to 300°F.

In addition to polyester-epoxy combinations, polyesters are typically combined with a number of durable crosslinkers, most notably TGIC and urethane. Advancements in these chemistries have focused on four areas:

• Reduction in toxicity,
• Improvement in durability,
• Internally blocked urethane cross linkers,
• UV curable polyesters.

Polyester-TGIC powder demonstrates an excellent mixture of flexibility and impact, chemical and UV resistance. It is often considered the most universal thermoset powder chemistry.

Unfortunately, TGIC has battled toxicity problems over the years because of mutagenic concerns. This has led to the nearly wholesale replacement of TGIC in Europe. Replacements include hydroxy-alkylamide, digyclidal epoxy and a methylated version of TGIC. Yet many parts of the world have stuck with TGIC based on its historic track record for safe use.

Durability improvements have been achieved by the addition of super-durable polyesters. This class has always been suspect for limited flexibility, but through resin and formulation development, flexibility has been dramatically improved. Commonly, this class has become known as “super-durable polyesters.” Combination with TGIC or its offsets are common. One common application is the architectural market, where AAMA specifications for five-yr durability can be obtained.

The final class of polyester compounds is polyester urethane. This class of powders demonstrates excellent flow over a wide gloss range and is known for its excellent chemical resistance and hardness. Super durable counterparts are also available.

One significant advancement in this chemistry has been the introduction of internally blocked urethanes with improved flow. Improvements in flow have made this a viable option when the hardest UV-resistant coatings are needed and an acrylic is not desirable or too expensive. Yet, high cost and relatively high-cure temperatures compared to other durable polyesters detract from the positive features of this chemistry.

Finally, UV-curable polyesters have emerged by the desire to move powder into the wood market. The most common type is unsaturated polyesters. These example compounds are cured by free radical polymerization and have become a dominant technology in the UV market. Applications include heat-sensitive parts, wood and some plastics.

Acrylics
The attributes of acrylic-epoxy hybrids were detailed in the epoxy section. The main influence that has opened the door to acrylic technology has been the introduction of glycidal acrylic powders for clear automotive topcoats. The stringent demands of the automotive markets have stretched powder formulators and resin makers alike.

However, improvements in flow, color and compatibility with liquid basecoats have brought this to the automotive market as a replacement for high-VOC liquid topcoats.

The added advantage of low-temperature cure, down to 275°F, may allow further expansion of this chemistry for application on temperature-sensitive substrates and alternate markets.

The Choices Expand
Clearly, new technology is being developed to move powder coatings into new markets. Advancements in durability, cure potential, application and cost/ performance abound.

The extensive use of GMA acrylics and acrylic-epoxy hybrids has opened up new possibilities in UV durability, clarity, gloss control and the wide array of properties well associated with the acrylic backbone. Several new low-cure polyesters and epoxies have been developed from work on wood and alternate substrates. Equally important thermoplastic technologies have improved in adhesion, film build and cost, helping them compete in new markets and those dominated by thermosets.

In step with technical formulation changes, application advancements are moving forward as well. Overall, application
advancements can be categorized into advancements that make the powder easier to apply and advances in the application equipment itself. As application systems become more sophisticated, the need to understand the physics and dynamics of the electrostatic application of powders is critical.

In the 1990s, application was listed as an area that needed attention. As a result, materials are available today that are significantly more user-friendly and which further enhance the desirability of powder coatings. To aid in application, powder coating companies are manipulating particle size and particle fluidization. Minimization of fines and chemical modifications to influence electrostatics are two typical methods.

Overall, this science has been shown to dramatically affect film thickness and uniformity, enabling an overall reduction in powder consumption. PFD