Formulating to good effect

Transcription

1 Seite/Page: 1 Formulating to good effect Some newly developed resins which lend themselves to the formulation of UV-cured effect coatings for plastic substrates are described These include UV- PUD dispersions which provide excellent metallic effects Another UV-PUD readily gives a smooth "mirror finish" An oligomer may be used to obtain tactile finishes ranging from silky/peach to tacky/rubber-like Designing UV curable coatings for metallic, mirror and haptic finishes Philippe De Groote* Gaia Franzolin Wendy Yu Indrajit Bhattacharya Xavier Deruyttere Due to the increasing use of plastics in the automotive, electronic and telecommunication fields, the coatability of plastics is becoming a more and more significant issue, and there is a growing demand for high performance, high efficiency and value added coatings for various plastic substrates The combination of low surface energies and sensitivity to temperature and solvents makes many plastics difficult to coat Therefore, formulators and raw material suppliers face daily challenges in developing coating systems High volatile organci compounds (VOC) emissions, the need for thermal drying tunnels and significant space requirements as well as the limited curing speed of solvent based products have been the key drivers in the search for alternative products and technologies In that respect, energy curable coating technology is well established for surface protection and decoration in many applications, including furniture, wood flooring, construction, graphic arts and electronics, and its use on plastic substrates is growing Energy curing has already shown it can overcome the limitations of conventional coatings, and it also offers other advantages such as the ability to coat heat-sensitive substrates as well as higher performance of the cured coating (scratch, abrasion and stain resistance) However, challenges can be encountered in formulating for particular types of finishes Formulation of radiation curable coatings Energy curable coatings are composed of acrylate functionalised resins (oligomers and monomers) which polymerise or cure instantaneously on exposure to electron beam or when formulated with a photoinitiator (PI) on exposure to UV light This leads to higher production efficiency, energy cost saving and more compact, space saving industrial lines The resins are either 100 % solids materials (possibly diluted with solvents for ease of application) or water-based Among these water-based (WB) systems, energy-curable polyurethane dispersions (UV-PUDs) allow the development of low viscosity, high performance formulations For both 100 % UV and WB UV systems, final properties can be fine-tuned with the proper additives The present report will show how new UV resins can be formulated to obtain special effects which are becoming increasingly popular: One-coat WB UV metallic finishes; WB UV high gloss mirror effect coatings; UV haptic finishes Metallic finishes pose problems with UV cure Metallic colours, especially silver, are popular and give added market value to office and home appliances based on plastics (eg, casings of audio equipment, video equipment, mobile phones, cameras, etc) Metallic paints typically make use of solvent-based conventional thermoplastic acrylics (TPA) or two-component polyurethane (2K PU) metallic base coats (containing, for example, aluminium flakes) In some cases, a clear topcoat is applied in order to improve the durability, scratch resistance and chemical resistance Solvent-based (SB) UV formulations have been developed for this type of application and must deal with the following challenges: - Obtaining adhesion on the substrate and also limiting the coating shrinkage during cure by carefully selecting oligomers and monomers; at the same time the coating should have enough crosslinking density to provide chemical and mechanical resistance for critical applications Ensuring complete UV curing of the bottom layer (metallic pigments can strongly interfere with short wavelength UV light) by adding photoinitiators such as monoacyl phosphine (MAPO) and bisacyl phosphine (BAPO) which react under longer wavelengths of light A surface curing photoinitiator also needs to be added in order to guarantee surface cure and reduce oxygen inhibition The metallic pigment orientation effect of UV oligomers and monomers is limited due to their low molar mass; monomers especially should be limited in quantity as their ability to wet the pigments and stabalize their dispersion will be limited Also, aggressive monomers should be avoided as they may attack the metallic pigments and affect the color tone The stability of formulations containing metallic pigments is low and requires good in-can stabilisers (radical scavengers) Metallic coatings can show limited abrasion resistance Low solids coatings (requiring solvent recovery) and the hazardous nature of some solvents can constitute serious problems, depending on economics and local legislation New dispersions perform well in metallic finishes One-coat formulations based on UV-PUDs are attractive alternatives to solventborne systems Not only is formulation simpler than for SB UV systems, but the initial physical drying before UV cure releases water, with no hazardous solvent Shrinkage is also limited thanks to the high molar mass of the polyurethane acrylate; this is beneficial for both adhesion and metallic pigment orientation Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

2 Seite/Page: 2 However, there are processing aspects specific to waterborne systems Due to the high surface tension of water (even within dispersions), a wetting agent is necessary in order to obtain good wetting on plastic substrates An appropriate defoaming agent will also have to be selected, especially for spray applicationtable 1 summarises the formulation of a one-coat UV-PUD formulation for metallic effects UV PUD1 and UV PUD2 have a high hardness that makes them both suitable as protective coatings An addition of about 5 % UV PUD3, a flexible coating, to the formulation has been shown to improve adhesion (if required) on the plastic substrate without affecting coating performance Table 2 summarises the characteristics of these coatings Spray application conditions were as follows: 10 nozzle spray gun application; Target dry film thickness around 20 to 25 µm; Physical drying (water flash-off) in oven at 60 to 80 C for 5 to 10 min; UV curing under 1 Ga lamp 120 W/cm and 1 Hg lamp 120 W/cm, line speed 10 m/min Figure 1shows the comparison between polycarbonate panels sprayed with a metallic coating based on a UV-PUD and a reference 1K PUD The metallic orientation of the UV-PUD based formulation is excellent and comparable to the one obtained in 1K PUD systems, while displaying the improved chemical and water resistance, abrasion resistance and hardness typical of energy curable coatings The main properties of this metallic coating include good levelling and metallic orientation, 100 % cross hatch tape adhesion, F to H pencil hardness, a high level of resistance to both solvents and abrasion The conclusion is that radiation curable polyurethane dispersions UV PUD1 and UV PUD2 allow a one coat metallic finish to be formulated with excellent metallic orientation and a good balance of properties High gloss mirror effect coatings High gloss mirror effect finishes are generally obtained with solvent-based formulations (based on unsaturated polyesters, for instance) while the required finish might involve a time-consuming labour-intensive polishing step The growing trend for no/low VOC environmentally friendly coatings is driving the development of water-based alternatives for these mirror image finishes, preferably without need for a polishing step A new solvent-free radiation curable PU dispersion, UV PUD4, combines all of these features Its main characteristics are given in Table 2 Persoz hardness (measured on a 50 µm dry film on glass, EB cured 220 kev, 5 Mrad) can attain 360 sectable 3 gives a starting point formulation which was applied on PVC and PET panel substrates under the following application conditions: 30 µm bar coater application giving around 12 g/m² dry coat weight; Physical drying: 5 min at 50 C (or 2 min at 80 C); UV curing: 2 x 120 W/cm Hg lamp at 12 m/min line speed; 24 h conditioning before testing The final coating provided 100 % crosshatch adhesion on PVC and PET panels, with a gloss at 60 in excess of 90 as well as a mirror image effect on both types of panelfigure 2illustrates its performance: a standard UV-PUD topcoat applied on a dark base-coated panel will give a blurry reflection, in spite of a 60 gloss close to or even above 90 Comparatively, UV PUD4 gives a high gloss surface with a nice reflected image, showing that the coating can provide an exceptionally smooth surface during film formation Complementary trials showed that the coat weight required for an optimised mirror effect is dependent on the substrate surface roughness While 10 g/m² dry might be enough on flat panels, this may have to be increased up to 50 g/m² for surfaces showing some degree of roughness Hence, the new dispersion can deliver a high gloss and mirror image effect on plastic substrates as well as other surfaces like wood A touching performance The use of haptic coatings, ie coatings providing special tactile effects, has seen growing use in many coating applications, such as consumer goods, electronics and flexible substrates Haptic properties are very subjective and not easy to define; and in addition their perception might vary from person to person However, Figure 3gives an example of a haptic scale Depending on the final application, the required tactile effect can vary, so coatings must be developed for different applications (low, medium and high haptic properties) Consequently there is a clear interest from industry in having a product which can deliver various haptic effects by blending and formulation Conventional haptic coatings typically make use of 2K SB or WB systems and, as with the two topics previously discussed, energy curing makes it possible to obtain a broad range of haptic finishes with one-component formulations, meaning easier handling and low/limited VOC content ALUA1 is a new energy curable oligomer which allows this kind of versatile formulation work This is a 100 % solids aliphatic urethane acrylate (ALUA) melting at 53 C Molecular weight is around 2400, with a Höppler viscosity at 60 C of 2400 mpas and a theoretical functionality of three Other radiation curable resins were selected in order to combine ALUA1 s haptic properties with other coating properties such as flexibility, elongation, chemical and scratch resistance Physical properties of these various resins are summarised in Table 4 Three different haptic finishes were obtained using the starting point formulations detailed in Table 5 In all three cases, the formulation was applied on a polycarbonate panel at 50 C using a 200 µm doctor blade (with formulation, substrate and doctor blade conditioned at 50 C beforehand) A layer approximately 100 µm thick was obtained and subsequently UV cured with 4 passes at 5 m/ min, using a 120 W/cm Hg lamp It follows that formulating ALUA1 with other oligomers, monomers and matting agents allows the user to obtain various types of haptic finishes, ranging from rubber-tacky to peach-silky Further fine-tuning of these formulations can involve the incorporation of elastomeric particles for enhanced flexibility and haptic finish as well as dilution in solvent for spray application on tri-dimensional objects Disclaimer Cytec Industries and its affiliated companies decline any liability with respect to the use made by anyone of the information provided here This information represents Cytec"s best knowledge, without constituting any express Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

3 Seite/Page: 3 or implied guarantee or warranty of any kind as to accuracy, completeness of the data or other considerations No license or right under any patent or other intellectual property rights is implied, nor is any guarantee or warranty regarding the use or accuracy of the information The final choice of use of a product and/or information as well as the investigation of any possible violation of any intellectual property rights remains the sole responsibility of the user Results at a glance Radiation curable systems have established themselves in various markets as offering high-performance with very low VOC emissions However, challenges may be encountered in formulating for specific applications or types of finishes Some newly developed resins for the formulation of UV-cured effect coatings are described Two UV curable PUD dispersions were shown to provide excellent metallic pigment orientation with high levels of physical properties and solvent resistance Another radiation curable PUD provides an exceptionally smooth 'mirror finish' without any final polishing Haptic or 'soft-feel' coatings may be required to give a range of touch sensations A new aliphatic urethane acrylate oligomer may be formulated to obtain UV cured finishes ranging from silky/peach to tacky/rubber-like * Corresponding author: Philippe De Groote Cytec Industries Inc T philippedegroote@cyteccom Vincentz Network +++ Plathnerstr 4c +++ D Hannover +++ Tel:+49(511)

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