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Tuesday, 26 September 2017
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Raw materials & technologies, Technologies, Powder coatings

Powder Coatings - Back on the fast track

Tuesday, 3 March 2015

A long period of innovation in powder coatings came to a halt at the turn of the century, primarily due to economic and market changes. Promising new market areas now require developments in formulation or application. These include the use of plastic substrates and large objects which cannot be cured in ovens. Several ‘smart’ technologies have now been implemented as powder coatings.

Quelle: fotolia, wotanxx

Quelle: fotolia, wotanxx

Powder coatings market history

Although the original invention of powder coating occurred in the 1950s, the industry made its real debut in the 1960s. The spectacular growth of this environmentally-friendly coating technology in the 1980s and 1990s was spurred by concentrated and relentless innovation. Progress in powder coating technology ground to a halt after the turn of the century. Many factors are responsible for the recent dearth of innovation in powder coatings. Double-digit growth in the 1990s spurred numerous industrial paint companies to make a foray into powder coating technology. This created a glut of producers, which evolved into manufacturing overcapacity. The 2000-2001 recession shrank the powder coating market in Europe and North America and with that put the brakes on R&D budgets. At the same time the Asian market heated up, which encouraged multi-national powder manufacturers to redirect their R&D expenditure to where the action was. Even though the Asian market was expanding at record rates, its technology demands were modest, as existing technology was being applied to meet standard-grade industrial requirements.
The great recession of 2007-2009 further curtailed R&D spending. Consequently, the first decade of the 2000s saw very little progress in powder coating technology. In addition the traditional sources of innovation, mainly raw material suppliers, experienced the same trends and as a result limited their powder technology R&D.
 

Today’s innovations come from new sources
The past few years have ushered in a correction in the powder coating industry, as evidenced by the failure of poor performing powder producers and the consolidation of many others. Essentially, the stronger players have survived.
So what has this done to the prospects of innovation? Large multi-national powder producers remain cautious with their R&D budgets. The same is observed with resin and additives suppliers. In spite of these trends, innovation is finally back on track. This recent rebirth of innovation is being driven by non-traditional enterprises. Instead of raw material companies developing the next generation of binder technology, advanced material scientists, powder coating formulators and application engineers are pushing back the frontiers of technology.

The importance and problems of  plastic substrates
Innovation is being driven by a number of external forces. Most prominent are new generations of fabricated goods. In the past, manufacturing engineers were content to design and fabricate goods based mainly on metal substrates. As the transport industry strives to continually improve fuel economy by incorporating weight reductions into its products, more and more manufactured goods are being designed as moulded plastics. These moulded articles are replacing traditional sheet metal forming products and cast metal parts.
Conventional powder coatings are typically applied electrostatically to an earthed conductive substrate, but as these moulded parts are non-conductive, powder application is problematic. Options to overcome this conductivity issue include:

  • Incorporate conductivity into the plastic substrate
  • Apply conductive agents to the plastic surface
  • Apply molten powder via thermal spray

Of these options, the application of a conductive agent is the most consistent and economical. However, great strides are being made in optimising thermal spray techniques. The incorporation of conductive material into the plastic polymer is expensive but very effective. This technique may have promise in the future if the economics improve.
One of the challenges in finishing plastic substrates is establishing adhesion between the coating and the plastic. High HDT (heat deflection temperature) plastics tend to provide a good surface for adhesion; though the presence of mould-release compounds can compromise adhesion and therefore must be carefully assessed.
With lower HDT plastics, especially those based on polyolefins, it is difficult to achieve a good bond between the powder coating and substrate. These materials possess a high surface tension which inhibits the wetting of the plastic by the molten powder coating. Hence pre-treatment or conditioning schemes are sometimes required to reduce the surface tension. Techniques to overcome this include chemical agents, flame treatment and plasma.

Table 1: Thermosetting powder coatings technologies for high HDT plastics
CHEMISTRY TYPICAL STOVING SCHEDULE APPLICATIONS
Epoxy 25 min at 110°C Primers, interior
Hybrid (epoxy-polyester) 25 min at 125 °C Primers, interior
Polyester-TGIC 25 min at 130 °C Exterior, general purpose
Acrylic 30 min at 125 °C Exterior, automotive

Plastic substrates can be based on a number of polymers and polymer alloys. Some possess thermal stability high enough to use conventional thermosetting powder coating formulas. These include relatively expensive materials such as nylon 6, acetal copolymer, PEEK and polyamide/polyphenylene ether.
As can be seen in Table 1, the stoving times required to cure low temperature powder coatings are rather long (25 to 30 minutes). However, cure times can be reduced through the use of infrared curing techniques.
Caution must then be employed when curing different colours as the heat which infrared delivers to a colour is dependent upon the spectral properties of the pigments in the powder coating formula. Whites, pastels and bright metallics are somewhat reflective and therefore require higher doses of infrared to effect cure than their darker coloured counterparts.
 

UV curing is the solution for low HDT plastics
The more common and less expensive thermoplastics such as ABS, polycarbonate, PVC and polyolefins dominate the moulded parts industry. These materials possess heat deflection temperatures below those used in thermoset powder coating cure regimes and therefore cannot be finished with conventional powder coatings.
Here UV curable powder coatings are the materials of choice. UV curable powders utilise a cure mechanism driven by photoinitiators (PIs) that accelerate the free radical polymerisation of unsaturated vinylic and acrylated oligomers. This process can be accomplished at relatively low temperatures (95 to 120 °C) thus rendering the low HDT plastics undamaged. Table 2 summarises the preferred choice of powder coating technology for different plastics.

Table 2: Common plastic materials and recommended powder coating technology
Substrate (common name) Composition

HDT

(046 MPa load)

Powder cure mechanism
ABS Acrylonitrile butadiene styrene 98 °C UV
Acetal copolymer Polyoxymethylene (ethylene) 160 °C Thermoset
Acrylic Acrylic 95 °C TUV
Nylon 6 Polyamide 160 °C Thermoset
PC Polycarbonate 140 °C Thermoset
PC/ABS Polycarbonate/ABS blend 80-100 °C UV
HDPE High density polyethylene 85 °C UV
PMMA Polymethylmethacrylate 105 °C UV
PP Polypropylene 100 °C UV
PS Polystyrene 95 °C UV
PVC Polyvinyl chloride 90 °C UV
Noryl GTX Polyamide/polyphenylene ether 190 °C Thermoset
PEEK Polyetheretherketone 160 °C Thermoset

Recent advances have led to the development of UV curable powder systems that perform well over PC/ABS alloys and some polyolefins. The process used to finish moulded plastics involves these steps:

  1.  Clean the plastic surface with deionised compressed air
  2.  Apply conductive agent to the surface
  3.  Electrostatically apply a 50 to 60 micrometre film of specially formulated powder coating;
  4.  Melt (convection, infrared or a combination of both)
  5.  Expose the molten coating to ultraviolet light

The entire process can take as little as five minutes to produce finished parts. It is important to note that UV cure operates  on line-of-sight, that is, coating cure will only be achieved where the coating is exposed to a sufficient dose of ultraviolet light. Not only is the dose critical but so is the wavelength of the light.
Powder coating formulators incorporate photoinitiators to initiate the free radical cure of oligomers. These PIs are very wavelength-specific and require specific lamps to function effectively (see Table 3). The coating is completely cured after exposure to sufficient UV energy. With a few specific exceptions, no further cure occurs after the surface is removed from the UV light.

Table 3: Common UV sure lamp types
UV lamp Wavelength range (nm) Powder coatings type
Standard mercury 240-320 Clear coats
Iron doped mercury 320-400 Clear coats and metallics
Gallium doped mercury 410-440 Pigmented and thick film

Robotic systems provide UV cure for large objects
UV curable powder coatings have been commercialised in factory settings for applications as diverse as polyvinyl sheet flooring, medium density fibreboard, automotive radiators and electric motors. Recently, efforts have been made to cure UV powder coatings on large objects in the field.
This work, conducted by SAIC (Science Applications International Corporation) under a US government grant, has investigated the use of robotics to melt and cure the powder coating after deposition onto a surface. The powder is applied conventionally to the substrate using an electrostatic method.
The powder is then melted by robotically passing an infrared emitter over the surface. Cure is then achieved by swiping UV light over the molten film. Both the IR and UV devices can be affixed to the same articulated robot arm (see Figure 1).
The UV curable powder coatings developed for this robotic curing technique are designed to meet United States Naval coating performance including outdoor durability, resistance to hydraulic fluids and corrosion resistance (see Table 4 for details).

BB_ECJ_2013_12_Biller_fig 01

Figure 1: Robotic UV curable powder coating scheme with infrared emitter (photo courtesy of PCR Group)

Thermal spray technologies for field application
The field application and curing of powder coatings has always been an elusive goal for technologists. The development of this technology would allow the powder coating of large structures such as bridges, ocean-going ships and aircraft. Flame spray has been tried with intermittent success.
The biggest challenge is film build control and adhesion to inconsistent substrates. Inconsistency emanates from surface conditions (oxidation, contaminants, etc.) and environmental factors such as moisture and temperature. A number of approaches are currently being perfected and hold promise to advance powder technology into field applications.

Resodyn Corporation (USA) has developed a thermal spray technique that relies on a combination of electric resistance (or propane) heat and hot process gas to soften polymeric powder as it exits a specially designed application gun. The powder is propelled by compressed process gas and reaches the substrate as a molten fluid. The fluid wets the substrate surface and cools to a hard film.
The Resodyn equipment, dubbed the "PTS” System can be used with both thermoplastic and thermosetting types of powder coatings. Targeted end-uses include heavy equipment, architectural infrastructure, concrete swimming pools, highway barriers, thermal insulation and naval radar hardware.

Smart technologies applied to powder coatings
In other arenas scientists are developing so-called ‘smart’ powder coatings, that is, materials that respond to environmental stimuli. These stimuli can simply come from changes in temperature, pressure and light or from more complex scenarios such as colour shifting, self-repair, corrosion resistance, electrical conductivity, hydrophobicity and lubricity.
The key to these technologies is the incorporation of particles or compounds that ‘recognise’ and react to the stimuli then transform either themselves or the coating composition to confront the task at hand. Adapta Powder Coatings in Spain has developed a very interesting collection of powder coatings that respond to the environment (see Table 5).
Self-cleaning powder coatings destroy organic contaminants by photocatalytic activity. Photocatalytic compounds in the coating are excited by the UV energy in sunlight and in their excited state they decompose organic molecules. Another approach is to make the powder coating surface hydrophobic in order to repel the deposition of aqueous contaminants.
Proprietary dyes can be incorporated into powder coating formulas to respond to changes in temperature. These may find use in fire detection systems, electrical processes and food and beverage handling equipment.
Powder coatings that change in colour when exposed to UV light can be used in anti-counterfeiting, batch identification and decorative applications.
One of the more novel smart technologies absorbs polluting airborne nitrous oxides and renders them harmless to the environment. These systems can be very useful in controlling air pollution in urban areas.

Future progress will see renewed expansion
Innovation in powder coating technology has ebbed and flowed since its inception as an industrial coating technology decades ago. The turn of the century heralded a dearth of new developments because of overcapacity, globalisation and commodification. In addition R&D groups were decimated or shipped overseas.
However, hope is not lost as recent times have produced a rebirth in innovation, much of it emanating from non-traditional sources. Novel materials and inventive processes are positioning powder coating technology for introduction into new markets and for new end uses. 

This article has been published in European Coatings JOURNAL, 2013, 12, p74-79 Author: Kevin Biller, The Powder Research Group

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