General information about UV powder coating

Powder coating
Powder coating is the technique of applying dry paint particles to a substrate. The powder used is a mixture of finely ground particles of resin and pigments. After spraying the particles onto the substrate, the part is placed in an oven where the powder particles melt and coalesce to form a continuous film. The powder coating technique has several advantages. Since thick layers can be achieved easily with powder coating one layer is frequently enough to give a good result. Moreover, powder coating produces a coated surface with very good properties like hardness, durability, abrasion resistance and toughness. Also important, powder coatings are very environmentally friendly. No solvents are used in these coatings, so there is no contribution to the emission of volatile organic compounds. Because of these advantages, powder coatings continue to grow with the main markets in: appliances, metal products, automotive and the architecture/building industry. The application of these conventional thermo curing powder coatings is, however, limited to substrates that can resist the curing conditions of 10 min at 180-200°C (high temperatures). In practice this means that only metal substrates can now be powder coated.

Liquid UV coating
The chemical components used in UV curing react very rapidly upon exposure to high intensity UV light resulting in a crosslinked polymer network i.e. the liquid film will form a solid coating. UV coatings have the advantage of a high cure speed at a low process temperature. Because of this low temperature it is possible to coat heat sensitive substrates like wood, paper and plastic. Application of one layer of liquid UV coating gives a thin layer of typically 50µm.

There are different types of UV coatings: 100% reactive liquid and waterbased dispersions. The properties of these coatings depend very much on the specific formulation.

The 100% reactive liquid is the most important group. The properties of these coatings are very good and the curing is fast. The solid content of this coating is usually near 100%, although some formulations may contain organic solvents for specific application properties. A disadvantage of these coatings is the use of monomers in order to reduce the viscosity. These acrylate monomers can cause irritation and sensitization. Therefore it is not recommended to apply the coating with hand spray techniques, the most favorite application method.

A UV curable waterborne coating, by contrast, usually contains no monomers, but is thinned for application with water. This results in a product with excellent physical properties, but more time, and effort, is required in order to evaporate the water before curing.

UV curable powder coating
Curing of powder coatings with UV light is a new technology and can be described as "the best of two worlds". This new method makes it possible to benefit from the advantages of high cure speed and low cure temperature as well as environmental friendliness. The handling and preparation of UV powder coatings is similar to the methods used for conventional powder coatings. During application the UV powder coating is generally melted by exposure to IR light. This process takes only 1-2 minutes with a required final temperature of 90-140°C, depending on the selected UV powder system. Because of this relatively low temperature it is possible to cure heat sensitive substrates with UV powder such as MDF, plastic, glass and pre-assembled parts which combine metal and heat sensitive materials. UV powder can also be applied on metal, with the main advantage that the metal does not have to be heated to such high temperatures, or for as much time as required with thermal curing powder coatings. Crosslinking of the melted UV powder occurs by exposure to UV light. The curing by UV radiation is only a few seconds which gives a total curing time of 1-3 minutes (figure 1).

The cured coating has properties of adhesion and hardness that compare very well to liquid UV coating and the chemical resistance is generally even better.

Figure 1: The process of UV powder

UV curable powder coatings are very environmentally friendly for several reasons. First, there is hardly any emission of volatile organic compounds. In the future it is expected that the emission of VOC´s from industrial processes will be decreased and controlled by governmental regulations. As a result many companies will have to decrease their emission of VOC´s drastically. The use of UV curable powder is a good solution. Another factor affecting the environmental friendliness is the much shorter heating period compared to conventional powder, as well as the lower heating temperature. UV curable powder coating saves energy and reduces CO2 emissions. Compared to solvent based systems the waste of UV powder coating is much lower because overspray is easily reclaimed and reused.

The total costs of a coating system depend very much on the costs of the material used. In general, UV powder has the advantage of high material utilisation. 95% of the powder can be used, compared to 60% for most sprayed wet paints. As mentioned above, the overspray of UV powder coating can be recycled , whereas overspray of liquid paints is generally scrapped. Therefore, the price per liter or kg of paint cannot be fairly compared. Comparisons can better be based on the price per coated square meter. The price per coated square meter is also dependent on the number of coating layers. A good result with UV powder coating is often obtained with one layer. With other coating systems, such as UV liquid or nitrocellulose lacquers, multiple layers are necessary, which increase the costs per square meter. For example, solvent based nitrocellulose lacquers cost EUR 1.36 / l (15% solids and 60% usage) compared to UV powder coating ± EUR 15 / kg (100% solids and 95% usage). With a layer thickness of 50µm you would need several layers of lacquers equivalent to 11 l of nitrocellulose lacquers compared to 1 kg UV powder. Consequently, the price per square meter of both systems has the same magnitude.

The main advantages of UV curable powder systems are:

• Low system costs
• Excellent coating properties
• High cure speed
• Low cure temperature
• Hardly any emission of volatile organic compounds
• Maximum powder use with overspray recycling
• Application of thicker coating layers (depending on pigmentation level)
• Application of one layer

General
The handling and processing of the powder is much like the handling and processing of conventional powder and a standard extruder can be used. The extrusion conditions will be discussed in section 3.

For the grinding a mill of 0.5-1.0 mm is used followed by sieving (90 or 125µm). Depending on the substrate the Corona or Tribo spray gun may be used to apply the powder particles. The spray system can be manual or automatic.

Figure 2: IR/UV curing equipment

The applied powder is cured in an installation that consists of two sections: the IR section (melting and flow) and the UV section (cure). The IR section is equipped with medium-wave IR lamps (600W). With a pyrometer coupled to the power supply of the IR section a good control of the melting temperature can be achieved.

In general, the H, D and V bulbs of an electrodeless Fusion F600 UV-lamp (240W/cm) are recommended for the UV section. The lamps can have a variable power supply. It is important that the lamps are suitable to cure the coating with an optimal result. Not only the lamps are of importance, but also the photoinitiators and the pigments. These must all be compatible to each other (see for information about UV absorbance of photoinitiator and pigments section 4). For clear coatings an H-bulb (Hg-lamp) is used at 120W/cm which is 50% of the maximum power. For the white pigmented coatings, the V-bulb (Gallium lamp) which has longer wavelength radiation gives the best result. For a white coating the power used is 144W/cm (60% of the maximum power).

In the following table (table 1) an overview is given of the different coatings techniques. The most important characteristics and properties of a coating are mentioned in this table. Each property is given a grade from - to +. A positive point will receive a +.

Table 1: Comparising of the technics

The most favorable application method for many applications is the spray system. UV liquid systems can often not be sprayed because of the irritating acrylates and VOC´s.

Only powder systems give a good result in one layer; UV powder and thermosetting powder both give layers of approximately 60 µm and thick layers of >120 µm can be achieved. Properties of hardness, flexibility and chemical resistance depend very much on the specific formulation. In general UV powder coatings give very good properties compared to other coating techniques. The chemical resistance is better than most other techniques.

Pigmentation of all UV systems requires careful choice of pigmentation and pigment concentration. Most colors can easily be achieved with UV systems but yellow pigments should be minimized as they absorb UV light and slow the speed of cure.

UV powder coating formulations consist of:

• UV powder resin
• Photoinitiator
• Additives
• Pigment / extenders
• UV powder resin
  

DSM has developed two binder systems based on a blend of a maleic / fumaric unsaturated polyester and a vinyl ether urethane component. The UV crosslinking reaction is based on free radical polymerization. It proceeds firstly via the formation of a donor-acceptor complex of the vinyl ether and the maleic ester, followed by a homopolymerization of this complex (fig. 3).

Formation of Donor Acceptor complex:

MA + VE ->[MA-VE]

Homopolymerization of Donor Acceptor complex:

PI ->R· + [MA-VE] ->R-MA-VE· + [MA-VE] ->R-MA-VE-MA-VE·

Figure 3: Mechanism of maleate - vinyl ether alternating copolymerisation

Photoinitiator

The photoinitiators must be suitable for the system. In clear coatings the hydroxy-acetophenone photoinitiators like Irgacure™ 184 or 2959 (CIBA) can be used. For the curing of pigmented formulations it is even more important that the combination of the photoinitiator, the pigment and the UV lamp are optimised. Firstly, since the pigment absorbs UV light, a photoinitiator should be chosen which absorbs at different wavelengths than the pigment. Secondly, to obtain good curing, a UV lamp should then be chosen that emits UV light at wavelengths where the photoinitiator absorbs. In general good results with white pigmented coatings have been obtained with the bisacylphosphinoxide (BAPO) photoinitiators such as Irgacure™ 819 or 1800, which absorb at longer wavelengths. The longer wavelength UV light penetrates deeper into the coating because the BAPO derivatives are photobleaching, thereby facilitating the through cure. For price reasons mixtures of the BAPO photoinitiators and hydroxyketone photoinitiators are used, although it is possible to cure a coating with Irgacure™ 819 alone.

Additives

Flow additives are used in the coating to increase flow properties e.g. 0.7 wt % Byk 361 (Byk-Cera) for a clear coating or Resiflow PV5 (Worlee) for a white pigmented coating. For textured coatings Ceraflour 969 (Byk-Cera) 4%(wt%) can be used.

Pigments

A standard pigment for white powder coatings, such as Kronos™ 2160, TiO2 (Kronos) can be used. For other colors many pigments are available. Yellow pigments can only be used with low color strength since these pigments generally absorb strongly in the (near) UV region.

Introduction

Different UV-powder coating colors can be formulated for textured and other finishes. The coating can be applied as a primer, or a one layer system or topcoat.

Since each application requires its own characteristics, DSM has developed a UV curable powder coating system that easily can be adjusted in its performance. The Uracross P3125 base resin is combined with either the Uracross P3307 resin or the P3898 resin or a combination of both. For application on wood it is important that the coating is hard and scratch and chemical resistant. These properties are obtained with the blend of Uracross P3125 and Uracross P3307. For application on pre-assembled metal parts the coating must be more flexible. For these applications the combination of 3125 and 3898 has the appropriate properties. Combining the three components allows the formulator to adjust the properties between these extremes.

Application, products and main properties

• Composite wood (MDF)
  -Uracross P3125 and Uracross P3307
  -Hard coating with high scratch resistance and high solvent resistance
• Metal (pre-assembled)
  -Uracross P3125 and Uracross P3898
  -Flexible coating with good solvent resistance