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UV-curable coatings for plastics  #1-2(119-120), 2013

UV-curable coatings represent now the most dynamically developing segment of the paint-and-lacquer materials’ market. The more and more actively UV paints are applied for coloring of 3D parts of metal and plastic. These paints have both environmental and economic benefits. Thus, for instance, shortening of applying and drying time leads to reduction of energy consumption and increase in performance. Besides, they can be applied onto heat-sensitive substrates

Anton SCHABELSKI,
representative of Lankwitzer Lackfabrik GmbH

The advantages of UV coatings
As a rule, the modern UV coatings do not contain volatile organic compounds (VOCs), and therefore meet the strict requirements for limiting VOC. Due to the shortest paint curing time – it takes just few seconds, the UV coatings are well-suited for industrial production: staining stage can be easily integrated into a rapid manufacturing process – such advantage cannot be achieved via any other painting technique.

These advantages became possible due to ingenious research engineering in the labs of manufacturers of raw stock and paint-and-lacquer materials. As result the main issues were resolved related to poor adhesion, caused in turn by rapid curing and shrinkage of the coating in polymerization process. Due to the use of UV paints of double curing or being cured in an atmosphere with reduced amount of oxygen, dyeing of such complex parts, as the car’s steel axles, aluminum wheel disks, shock absorbers, engines – became possible.

90 percent of the new paint equipment is supplied with circulating-air dryer that is suitable for all types of paint-and-lacquer materials. At the same time for drying varnish only 2-5 percent of the consumed energy is used.

In the age of total rationalization the great importance for profitability of paint line has the proper choice of both paint-and-lacquer materials and coating technology. While using the UV paints the times of painting and drying are essentially reduced, the same concerns other production expenses – hence, profitability of entire manufacture goes up.

Types of UV curing
UV curing is distinguished between radical and cationic, and the latter is most often used in plastics processing and metal-working industry for dyeing three-dimensional parts.

From technical point of view the UV paints of radical curing are better suitable for dyeing metals and plastics, as soon as in most cases they have better reactivity and therefore faster curing. Radical UV paints, both transparent and colored, allow to obtain more thick dried color layer which is often simply impossible for cationic systems. Besides, the cost of radical UV systems is much lower than that of cationic.

Multiple researches have shown that without additional drying the cationic UV coatings are disposed to appearance of bubbles on the metal. Despite this, cationic systems have intrinsic advantages - perfect adhesion to various substrates, pre-coated surfaces and even such problem metals as nickel. Radical systems can not provide such adhesion. Cured layer (film) of cationic UV coating does not contain low-molecular compounds and is completely odorless.

Composition of UV paints
UV coatings consist of film-forming components, reactive diluents, photoinitiators and additives. In color systems pigments and fillers are used. In industrial scale in the most cases the UV coatings of radical cure are used.

The basic component of any polymer is the film-forming one. The preference in case of UV paints is given to epoxy-acrylate, polyester-acrylate, urethane-acrylate oligomers, which determine the basic properties of the cured coating. These film-forming components are sometimes combined with each other to get the desired coating properties.

Epoxy-acrylates are characterized by a high hardness, abrasion resistance, chemical resistance and fast curing.

Combination of different low-viscous polyester-acrylates allows to obtain maximal reactivity without applying monomers. Urethane-acrylates are the universal film-forming agents for UV systems. They are highly resistant to impact, scratches, abrasion, are notable for chemical resistance and hardness.

Reactive diluents are used to reduce viscosity of material – as an additive to obtain coating without applying dilutants and water. They take part in the coating’s polymerization process and affect it – depending on diluents’ functionality. Thus, single-functional products become more versatile – high-functional reactive solvents improve their chemical stability, reactivity and density.

Photoinitiators can be of the short- and long-wave types. The first are mainly used for transparent varnishes though can also be applied in pigmented colors, whilst the long-wave absorbing photoinitiators are used solely for the deep curing of color coat. Depending on photoinitiator used some features are affected directly or implicitly: reaction speed, yellow tint of layer immediately after UV curing, hardness and shrinkage of the cured coating. The content of additional components in the UV coatings is small enough and they all are mainly destined for improvements in applying process. They are intended to stabilize the UV coating, prevent foaming, improve adhesion with substrate and flowability, as well as to obtain some special features – like light resistance, easy-to-clean, etc.

Properties of UV paints and varnishes
Apart from reduction of VOC emission, there’re a number of other arguments in favour of UV coatings. The following properties are the distinctive features of UV paints, which are gaining the more and more industries and find different application scopes:
 - instant drying;
 - high profitability;
 - reusability;
 - operating safety;
 - environmental sustainability.

Due to rapid drying it becomes possible to achieve high production speed (up to 120 m/min) of coating by colorless varnish. Articles colored with UV paints are having high covering’s chemical resistance together with resistance to mechanical damage and can immediately after curing be stored and packaged.

The unique properties of UV paints and varnishes provide possibilities to organize a high-speed coating process, being a prerequisite to build it in high-performance assembly-line production. In the latter case the essential savings on the sizes of painting line and on transport system are looking obvious. Instant drying at a normal temperature allows to paint such heat-sensitive surfaces, as plastics. Painted articles retain their elasticity, and the resistance of UV coatings to scratches is much higher than that of coating systems on water-soluble or solvent-borne base.

Drying rate depends on the emitting power, wavelength, the number of emitting sources (lamps) and their distance from the product. UV lamps should be adjusted to (compliant with) the type of photoinitiators used in paints (or vice versa).

Parameters of UV radiation
Not only thermal radiation emitted by UV lamps but also the high temperature of an article being coated help to cure the UV coating. As was shown in practice, for painting of three-dimensional parts it is reasonable to use three UV emitters to provide uniform irradiation and curing of the coating applied onto volumetric product.

UV rays – is a “narrow” part of electromagnetic radiation ranging from 200 to 380 nm. The wave spectrum of UV radiation is conditionally divided into three areas.

Long waves deeper penetrate into the coating layer than short ones. Therefore, the long-wave UV-A radiation due to better penetration ability is used for deep curing, whilst the short-wave UV-C - primarily for the top layer of the coating.
 
UV-C radiation is almost completely absorbed by the painted surface, causing the rapid curing of coating. But for more efficient curing of UV coatings one can combine two different types of radiation. For instance, by means of UV-B radiation the energy transfer is carried out to maintain polymerization reaction. The combined effect is particularly noticeable in comparison of curing processes of colorless and black UV varnishes. The matter is that transparent coverings with the same thickness of dry layer might have the curing rate 5 times higher than black ones (in case only UV-B is applied). The difference can be severely diminished in case UV-A is added, which waves are penetrating almost to the very substrate.

To cure transparent UV coatings generally high-pressure mercury lamps are used, while the colored coatings are better suited by high-pressure long-wave lamps with reflective metal additives. Emitting power of lamps used for curing of UV coatings, is ranging from 120 to 240 watts per arc centimeter.

High profitability
For applying of UV paints the compact painting sets are used, taking 90 percent less space in comparison with paint lines used for conventional coloring systems.

This becomes possible because such painting sets do not require soaking zone, where evaporation of solvents happens, as well as zones of heating, drying and cooling. At that the greatest cost savings are achieved by eliminating just the need of heating and cooling. UV paints are solvent-free, so there is no emission of solvents. This eliminates the risk of explosion, as soon as explosive mixtures do not appear in the air. Hence the UV painting set does not require implosion protection, which reduces the entire cost of the set. UV paints have also simplified storage conditions, ‘cause they do not require special safety measures, and the use of VOC-containing cleaners is not assumed for sets’ cleaning. In case conveyor stop or failure in coating process, the power of lamps is automatically reduced by 25 percent in order to save energy and avoid overheating. Instead the lamps are switched on having power from 80 to 200 W/cm.

Depending on construction of the object being painted various emitters are used, equipped with reflectors and mirrors required to achieve optimal energy consumption.

The need to organize painting process out of production facilities disappears, as soon as UV paints are not explosive. Inclusion of UV painting set(s) into production line essentially increases flexibility of the latter – articles are painted immediately after manufacturing and are prepared to be sent to the customer. In this case are reduced both storage and logistical expenses.

Application of UV systems is more efficient due to the high speed of coating curing, thus increasing production capacity without additional investments. If however, UV painting sets are not used at full capacity (as in the case of multi-layer continuous dyeing) they can be turned off and activated again only on demand, thus also providing additional cost savings.

Since the pre-heating is not necessary, even small lots of articles can be painted without considerable costs. The benefits of such flexibility and efficiency of production process have been particularly appreciated by manufacturers of brake discs. The compact design of painting and drying equipment and rapid drying process let in case of equipment readjustment, i.e. when articles’ lot is changed, accomplish coloring new details within just few minutes. Enterprise engaged in painting no longer has to wait for 30-40 minutes or more until the party of painted parts is completely dried in an oven, which is typical for production run paint equipment. And there is no guarantee that the parts will be well painted. You will need to set up equipment for as long as you receive the required quality. But! – Via UV coating technology the coating reject can be immediately identified and eliminated.

Due to 100-percent solid residue and the possibility of re-applying overspayed UV paints, the efficiency of such coatings’ application oscillates within 95-98 percent. Therefore the transition from solvent-borne coatings to UV-curable paint systems leads to reduction of consumed paint material by 25-30 percent.

And if at the first sight the price of 1 kg of UV paint seems to be high enough, after calculation of paint consumption per 1 m2 of UV coating it becomes obvious that such coatings are quite cost-effective as compared with the modern water- and solvent-borne, one- and two-component paint systems (Table 3). Such efficiency is achieved due to reusability and high solid residue in UV materials.

Reusability
Equipment for applying UV coatings can be used in two modes: in in-line production with a given clock cycle and in continuous production with constantly moving conveyor.

It is of prime importance that ultraviolet radiation is not getting into spraying zone – this can limit reusability of the paint material. Therefore on existing paint facilities this zone must be protected from UV rays lower 440 nm. This can achieved via the use of specially designed UV protective film that can be applied both to viewing windows and lamps.

When applied, the UV coatings with 100 percent solid residue practically do not get any loss in properties, therefore the picked oversprayed material can be directed into repeated cycle. At that there is no threat of drying up, as organic solvents do not evaporate. Oversprayed UV coating is picked by recirculation system: paint-and-varnish mist is drawn by exhaust system into reservoir and through the filter is fed to the fresh paint material. With this painting efficiency reaches 95 percent.

Labour safety issues
In order to get the uniform covering thickness, UV coatings are mostly applied by the paint equipment operating in automatic mode. This allows to avoid the problem of excessive applying of paint-and-lacquer materials; with manual treatment coating may have a non-uniform thickness, which in turn leads to insufficient curing and at last – to coating reject.

UV-curable paints are usually marked by “XI, irritating”. Theoretically their contact with the skin is only possible while stirring up, filling spare capacity or cleaning equipment. But in case of appropriate safety precautions, such as using special gloves and protective clothing, irritation of skin can be easily avoided. Cleaning of equipment is done with special cleaning agents that may contain solvents or be entirely free of them.

Only in a few cases it is possible to change formulation of UV coating and use raw materials that do not contain skin irritating ingredients while retaining the coating properties. The reason lies in limited availability of the necessary components on the market, namely monomers and oligomers.

UV lamps and reflectors are placed in a drying tunnel in a way that the UV rays cannot go outside. Clear that if the tunnel should be opened - for cleaning or replacement of lamps or repair work – for this time the lamps are switched off.

Ozone, which is generated from oxygen when exposed to UV radiation, in a short time reverts back to oxygen. Measurements carried out near the UV curing camera have shown that at a distance of 2 meters from the fan the ozone content already equals to maximum allowable concentration.

High level of consumer properties
If necessity emerges in a coating with high corrosion resistance, which is of particular relevance for automotive industry, then nothing better can be found than UV paints with 100 percent solid residue. Water-base UV paints do not have such corrosion resistance and therefore in most cases are used for decorative purposes.

UV coatings are also characterized by high resistance to mechanical impacts, especially to abrasion. In average their hardness by Buchholz is 80, at that they are remaining sufficiently ductile. And resistance of UV-curable coatings to oil, fuel and brake fluid meets all standards and requirements of automotive industry.

Despite provided high crosslink density of polymers, the UV paints still yield in some aspects to conventional coatings.

Say, until recently the problem was the lack of adhesion of UV paints to the already painted substrates with a thickness of 50 to 100 microns. But today Lankwitzer Lackfabrik GmbH already has at disposal the paints that have good adhesion to the base with already applied ground prime coat of synthetic polymers, epoxy resins and two-component polyurethane materials. Using curing in inert gases medium it is possible to paint and cure more complex parts, like automobile axles, shock absorbers, aluminum disks.

For painting plastic products there’re generally used the water-base UV paints and UV coatings with 100 percent solid residue. At that preference is given to water-soluble UV paints, because they afford an opportunity to obtain a good mat surface and also coverings with metallic effect. However, some hues of metallic coating series cannot be yet obtained with UV coatings, the same concerns getting of perfect gloss. Both issues are resolved in a two-layer system, comprising a water deluting base and transparent UV varnish. Thus, the 24-hour test by ethanol and 6-hour Loreal-Test showed that such coating has sufficient resistance to liquids.

Today for coloring plastics the most regularly used are the water-base highly transparent UV paints and UV coatings of double curing. Coating systems of the latter type have very high resistance to chemicals.

The most widespread plastic types for manufacturing products of plastics are ABS, SAN (styrene acrylonitrile), PS and PC. Products of them have a good adhesion of substrate and do not require pretreatment. For other polymer types, as a rule, the surface is treated by a burner flame, as in case of PP. In the case of LDPE metallization (vacuum metal sputtering) good adhesion is achieved using UV paints with 100 percent solid residue.

The color hues palette of UV-curable coatings is yet restricted for technical reasons – light, glossy, orange and red hues and metallics can be obtained, but with some limitations: to simultaneously get both good covering properties and sufficient curing is still a complicated problem. Therefore nowadays this issue is resolved by applying the base water deluting coating and transparent finish UV varnish.

Thus, due to introduction of the modern UV systems enterprises have got an excellent opportunity to use the up-to-date technology corresponding to all nowaday environmental requirements and implement it in production. The obtainable via this technology coatings are of the high consumer characteristics and at an attractive cost. And despite the seemingly high price of one kilogram of UV paints, recalculation to the cost of the finish covering’s square meter already shows substantial cost advantage of UV-curable coatings.


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