Curing Powders- -How Low Can You Go? by Karl Rijkse Tiger Drylac, Guelph, Ont.

p owder coatings have earned a fixed position in industrial application over recent years and now account for roughly 10% of the total

amount of the industrial coatings market. Every year, powder coatings grow in sales by about 5 to 15% depending on the region. The sale of wet liquid paint coatings remains stable and shows almost no growth; however, the growth in powder coating sales has started to level off in recent years, especially in Europe and North America.

One of the major reasons to use powder coatings is the environmental benefit. The total absence of solvents and the low level of VOCs emitted during cure makes powder coatings one of the most envi- ronmentally friendly paint systems. There are more benefits to powder coatings: application possible in one coat resulting in a tough, hard, and scratch resistance film; easily automated with very high utilization rates; corrosion resistance of all powder coatings is excellent; available outdoor systems based on polyester and polyurethane have a good weathering performance; and safety in the work- place due to the absence of flammable liquids.

Next to pros there are still several cons to powder coatings that prevent a broader application: repaint- ing of parts is difficult; powder coating application relies mostly on electric charging; smoothness of powder coating is still not as good as wet paint; color change in the application is difficult and time con- suming; and curing temperature is high and most of the time above 320°F.

This article focuses on one of the big cons that will open new markets for powder coatings when it is turned into a pro. Low-temperature-cure powder coatings will find their way into new markets and boost the sales again at the expense of wet paints.

Examples of new applications that open up for low-cure powder coatings are: application of MDF for kitchen cabinets, ready-to-assemble furniture, or office furniture; plastics; preassembled components with temperature sensitive parts; dense materials with tremendous heat sink characteristics; and coil coating lines.

W H A T IS POSSIBLE? Looking into the technology of powder coatings high- lights the technical difficulties that are part of the

development for a low cure system. Applicators of powder coating want a product that works at their plant without the need to take any specific precau- tions to work with the provided product. Next to that the production plant of powder coatings should be able to make the product. What is the use of a development without the possibility to use or make the developed product?

The powder coating production process starts with the weighing and mixing of the solid raw ma- terials. The blended raw materials are extruded at temperatures at around 210°F. In this process, ad- ditives, pigments, and fillers are melted and dis- persed in the resin system. The cooled and crushed extruded product is then ground to powder with a mean particle size of 55 microns.

There are two critical steps in the production process that limit the range of low-cure resin sys- tems:

1. The resin has to be solid because of the blending procedure. This fact sets limits to the melting point of the resin and glass transition tempera- ture (Tg). Tg is a temperature parameter that shows when polymers are transforming from the solid state into a more rubber state. This param- eter is an indication for the stability of the pow- der coating system for storage. So if this Tg is too low the shelf life of the powder coating is limited. To have a proper shelf life the Tg of the powder coating should be at least l15°F.

2. The extrusion temperature is related to the melt- ing temperature and viscosity (the flow) of the resin system. In a normal operation the extrusion temperature is around 210°F. If the low-cure powder coating has a curing temperature in the same range then curing will partly occur in the extruder. This blocks the process and the proper- ties of the coating will be bad, especially the flow. Using adapted extrusion techniques can elimi- nate this problem.

In the application process there are some limiting factors, too. As already mentioned the storage sta- bility and, therefore, shelf life of the powder coating should be at a practical level. The next obvious limitation in how low can you go is the melting temperature of the powder coating itself. Wet paint

April 2001 47

~ o I ~ e s R1 0 0

R2Frr~O " ~ R 3 ~ ~ 2 400.

Glycidyl-fct Polyanhyddde of Polyacrylate Dicarboxyli¢ acids "~ 300-

Figure 1. Reaction mechanism for GMA acrylic coatings. 200-

is already in a liquid phase. Powder has to be melted to get there. The normal melting temperature of powder coating is in the range of 160 to 180°F. This is the real borderline!

I want to make clear that new developments where powder coatings are dispersed in water to become powder slurry make it possible to apply the powder like wet paint. The final film has still to be cured above the melting temperature of the dis- persed powder system.

THE POWDER COATING CHEMISTRY Three different chemistries that are in market in- troduction right now and have all the advantages of lower cure temperatures compared to existing sys- tems will be discussed.

GMA-Based Acrylic Powder Coatings The development of acrylic clear coatings started with the demand of the automotive industry for a more environmental friendly topcoat system. BMW is now one of the car suppliers that apply acrylic clear coat based on GMA acrylics as their topcoat. Due to the R&D effort by raw material suppliers and application equipment suppliers the acrylic technol- ogy is available with new opportunities.

The reaction mechanism is shown in Figure 1. The benefits of the acrylics are superior flow,

hardness, and t ransparency in clear compared to polyester powders; cure temperatures as low as 280°F; better outdoor resistance; and better corro- sion resistance, especially against filiform corrosion.

The problems are high contamination with exist- ing technologies; no tribo application possible; and tubes that are used for powder transport in the application line have to be grounded.

Some of the problems have their root in the chem- istry of the acrylics. Due to very good charge ability the powder coating sticks in the tribo gun during application. The powder gets charged quickly in the handling system, which makes grounding of the tubes during powder transport for application also necessary. This is the first step into acrylic technol- ogy. Still several hurdles have to be overcome, but the possibility the technology offers has yet to be explored. Nearly 15% in the outdoor market in Asia is based on acrylic systems.

B

A

IO0

SO0 "

Time (sec)

Figure 2. The viscosity of a number of powder coatings relative to temperature of time.

Epoxy or Polyester Low-Bake Powder Coatings The cure temperature of the new developed low- bake powder coatings based on epoxy systems can go as low as 230°F. In the same range polyester sys- tems are operating, only here flow is not as good as the epoxy-based systems. The reason for this is the difference in viscosity, being the flow behavior of the resin at different temperatures. Figure 2 clearly shows that difference.

Generally speaking the low bake systems are straightforward. Increased reactivity at a lower tem- perature can be reached by adding a catalyst or use of more reactive functional groups in the binder system. Don't forget that the flow of the powder coating is related directly to the reactivity and vis- cosity of the system used. So there is a delicate balance between these parameters as can be seen from Figure 2. This limits the cure speed that can be used and generally speaking the cure times at 230°F are between 15 and 25 minutes when we use con- ventional ovens. Although the cure temperatures are low still the shelf life of these products is around three months at 86°F.

The reaction mechanisms of an epoxy system and of a polyester/TGIC cure is given in Figures 3 and 4.

The reaction mechanisms in Figures 3 and 4 are polyaddition reactions like the GMA acrylic system. Applications possible with these systems are appli- cation of MDF for kitchen cabinets or ready-to-as- semble furniture; plastics; preassembled compo- nents with temperature sensitive parts; and dense

- E - O - C H 2 - CH- CH2+ HN (R1-- '~- E - O - CH2- CH- CH2 - N(R1

0 OH

Figure 3. Reaction mechanisms for an expoxy system.

48 Metal Finishing

P e ~ C ~ o H * c-c-c-N'c"? - c - c - c - . / , , . ~ pe-....~A.~c-o-c-~-c~ ,~-c-c-c,,

polyester res,n I addfllOn reaction I contammg C--C--C C--C--/C carboxyl groups trlglycldyl ~socyanufate ~0 / o

Figure 4. Reaction mechanism of a polyester/TGIC cure.

materials with tremendous heat sink characteris- tics.

The flow and surface hardness is not on the same level as wet paints. Nevertheless, a one-coat appli- cation of, for instance, MDF saves a t remendous amount of labor. Based on the epoxy technology very nice one-layer powder coatings can be applied as primer. The topcoat could still be a wet paint. This saves at least 2 or 3 layers of wet paint and all the labor involved in sanding and drying the wet paint. The sandabili ty of the epoxy-powder coating is ex- cellent.

Due to the inferior flow compared to the epoxy system the polyester system is particularly suitable to make textured finishes for MDF. The application where this finds its way is shelving.

As already mentioned the cure time is around 15 to 25 minutes. The goal should be not only lower cure temperatures but also shorter times. This would save time and energy with an immediate impact on coating cost. It also doesn't make a lot of sense to heat a MDF part in a convection oven. Wood being a bad heat conductor takes a long time to reach the desired temperature. Next to that evapo- ration of water and, therefore, cracking of the wood will probably occur. Radiation cure is the solution to overcome these problems. Compared to convection curing there is a much more efficient energy trans- fer. The radiation is mainly absorbed by the binder system of the powder coating instead of the sub- strate. This also allows shorter cure times. A medi- um-wave infrared cure of low-bake powder coatings shortens the cure time to 5 to 8 minutes. This is a remarkable saving in time compared to convection heating.

All the benefits you have of s tandard powder coatings in comparison to wet paint also are appli- cable to low-bake powder coatings like corrosion resistance and mechanical properties on metal do not differ compared to s tandard powder coatings.

Infrared/UV Curable P o w d e r The ult imate product at this moment available in radiation curing in powder coating is the infrared/ UV-curable powder coating. The technology of UV curing has existed for a long time in wet paint. The technology in powder is relatively new and world wide there are only a few commercial installations

at this moment. The reaction mechanism of UV- curable powders is cationic photo polymerization mainly used with epoxy or vinyl ethers or free rad- ical photo polymerization of unsa tura ted com- pounds, for example methacrylated polyesters. In the formulation a photo initiator absorbs the UV light at a specific wavelength and triggers the reac- tion. The reaction mechanism is as follows:

Initiation: X-Y + hv --~ X + Y Start: X" + M -* X-M" Propagation: X-M" + n M --~ X-M'~+ I Termination: 2 X-M --~ X-M-M-X

The great advantage of the use of this combina- tion of radiation cure is:

I. During the production process there is no danger of a prereaction. Due to the fact that UV initiates the curing and not heat there occurs simply no reaction.

2. The split of melting (by IR) and cure (by UV) brings also the advantage of a flow of the powder coating that is not disturbed by a cure reaction that is going on at the same time.

3. Heat is only necessary to melt the powder coating not to cure the product; therefore, lower temper- atures can be used, as low as 190°F.

The UV powder coating can be applied with con- ventional application equipment. The storage stabil- ity is at 86°F for several months not a problem.

The IR/UV powder coating has several pros and cons.

The pros are very smooth flow due to the separa- tion of flow and cure; high surface hardness; short melt and cure time; and no cooling down of the cured substrate.

The cons are investment in equipment is high; layer thickness maximum 4 mils due to through cure of the coating; limited color range; yellow colors are very difficult; and 3D shapes are difficult, although not impossible.

IR/UV powder coatings can be used in the follow- ing applications: application of MDF for kitchen cabinets, ready-to-assemble furniture or office furni- ture; plastics; preassembled components with tem- perature sensitive parts; dense materials with tre- mendous heat sink characteristics; and coil coatings.

Here you see an overlap with the thermoharden- ing low-bake powder coatings. The main difference is the flow, chemical resistance, and surface hard- ness properties between these coatings. Applica- tions that require only a primer system have no need for an application of IR/UV paint. In those applica-

April 2001 49

Table I. Comparison of Powder Coating Properties

Standard Polyester/TGIC IR/UV Reverse impac t in.fib T-bend Pencil ha rdness Acid sa l t sp ray res is tance QUV-A exposure 50% gloss re ten t ion F lor ida exposure 45 ° angle 50% gloss re ten t ion

160 80 1-2 1-2 2H 2H

1,000 hr < 1 mm 1,000 hr < 1ram 3,000 hr 2,000 hr

22 months 18 months. Base metal aluminum Cr 6 pretreated.

Table II. Time for Various Powder Curing Methods

Curing Process Temperature in °F Time in Minutes Standard powder coating by convection >320 10-20 Low-cure powder coating by convection >230 15-25 Low-cure powder coating by infrared >230 5-8 IR/LW powder coating Melting at 190 Melting and curing: 1-4

tions where you need surface hardness and a smooth surface there IR]LW is the right choice.

The new generation of IR/UV powder coatings can be used on metal, too. Flexibility of the resin system is good enough to have a good adhesion on metal. In the past, due to shrinkage of the paint film, adhesion on metal was not always a success. It is curious that although people are focused to apply low-cure sys- terns on wood the main applications running now with UV powder coating are on metal.

The formulation and final properties of IR/LrV powder coating are comparable to the standard out- door polyester/TGIC systems. Formulations of IR/UV powder coatings differ in that respect that you need photo initiators to start the chain polymer- ization reaction. Here we must remember that the UV light absorbed by pigments shouldn't interfere with the UV light necessary for the photo initiators to start the reaction. It is because of this interaction that the yellow colors are still a problem to develop.

In Table I there is a comparison of the properties of ItULW powder coatings to standard polyester/ TGIC powder coatings.

The IR/UV powder coatings can keep up very nicely in performance with standard polyester/TGIC powder coatings. It is also possible to coat metal for outdoor use with IR/UV powder coatings and, as in standard products, better super durable systems are also available.

CONCLUSION Thanks to recent developments in resin chemistry and the effort of the powder coating industry in general new systems have become available. The systems are capable of taking away one of the cons that still existed in powder coatings. Low-cure tern- perature powder coatings are capable of further ex- pending the application possibilities for powder coatings. This will give new opportunities for grow- ing the market for powder coating use.

The benefits of the mainly shorter curing process are outlined in Table II.

BIOGRAPHY Karl Rijkse's experience includes university study in organic and analytical chemistry. He worked at Sigma Coatings in Europe as an R&D Manager and Product Manager from 1985 to 1999 and is currently at Tiger Drylac Canada as the R&D Manager. Rijkse is the owner of 5 world patents and has published 20 technical papers.

BIBLIOGRAPHY Buysens, K., European Coating Journal, May 2000 Blatter, K., European Powder Show, January

2000 Udding-Louwrier, S., New developments in radi-

ation curable powder coatings, February 1999 Drummond, C., Powder Coating on MDF, Novem-

ber 1999

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