Annals of Warsaw University of Life Sciences - SGGW Forestry and Wood Technology № 92, 2015: 49-54 (Ann. WULS - SGGW, For. and Wood Technol. 92, 2015)
Comparison of selected properties of varnish coatings curing with the use of UV and UV-LED approach PIOTR BORYSIUK, MATEUSZ DERDA, RADOSŁAW AURIGA, PIOTR BORUSZEWSKI, SŁAWOMIR MONDER Faculty of Wood Technology, Warsaw University of Life Sciences, Poland Abstract: Comparison of selected properties of varnish coatings curing with the use of UV and UV-LED approach. The tests were made on samples of the coating of pine and beech wood using acrylic varnish curing classic method of UV and hybrid UV-LED method. For test coatings were determined thickness, hardness Bucholz, resistance to scratching, abrasion resistance, resistance to aggressive agents and resistance to UV radiation. Studies have shown that, in general there are no significant differences in the properties of the coatings curing using the method of UV and UV-LED. Coatings curing by UV-LED exhibited better resistance to abrasion. Keywords: coating properties, curing by UV, curing by UV-LED
INTRODUCTION Curing of lacquer coatings is one of the indispensable stage of surface finishing. The proper process of curing directly determines quality of coating. Currently, a popular method of curing coatings in furniture industry is method using ultraviolet irradiation (Kijas 2011, Rudnicki 2011, Wnuk 2011, Kien 2013). It can be used for curing of paint systems based on acrylic or polyester having a high dry matter content. In this method, UV radiation activates molecules of photoinitiators contained in the coating product by breaking the double bond. The results of the process are chemical reactions of photoinitiators with resin which lead to the crosslinking (Jabłoński et al. 2009, Wnuk 2011). UV technology makes allows the curing of paint systems products on solid wood and wood-based panels (MDF, HDF) (Jabłoński et al. 2009). UV radiation is generated in the cells by special radiators, mostly by gala and mercury vapor lamps which emit 3 types of radiation: UV-C, UV-B and UV-V. UV-C (100280 nm) and UV-B (280-315 nm) are used for surface curing, however UV-V (400-450 nm) is applied to deep curing pigment products. Mercury lamps are characterized by the wavelength of 200-400nm and work time around 2000 hours. While gallium lamps radiate in the range of 400-450 nm and their working time is determined at 3000 hours (Akzo Nobel 2013). Mercury-vapor lamps are generally used to curing colorless coatings. While Gallium lamps are usually used to curing the thick pigmented coatings. The main advantages of curing paint systems by using UV radiation include (Rudnicki 2011, Wnuk 2011, Kien et al. 2013,): the rate of curing (curing time 15 s), reducing of solvents use, and coatings with the high quality. The main disadvantages of this process include primarily (Wnuk 2013): generating of hazardous substances (ozone), intensive heating of coated elements (which effects in the resin flows), high energy costs due to low efficiency of radiators, fire hazards or rapid wear of the radiators. Some disadvantages of the method can be eliminated by the use of LED to emit electromagnetic radiation (Bekas 2012, Wnuk 2013). LEDs operate in a very narrow spectrum of radiation 365-395 nm (+/-20 nm), and at the same time almost all of the generated energy is used to curing lacquer (Bekas 2012). In this case, there are only insignificant heat generation. Nevertheless, it is recently the most commonly used solution is hybrid UV-LED: LED emitters are used for crosslinking and deep curing of coating, however final curing effect
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occurs under UV lamps. Table 1 shows a comparison of curing technology using conventional UV lamps and LED radiant. Table 1. Comparison of curing technology using conventional UV lamps and LED radiant (Bekas 2012, Wnuk 2013, Kien et al. 2013)
Criterion
Energy consumption
UV lamp High
Durability of radiator ok. 2 thous. man-hours Resin outflows due to the material Yes heating up by the radiator Ozone emission Yes The necessity of lifts Yes Noise yes Require heating and Intermittent work cooling Painting products containing Yes irritants The generation of hazardous yes substances The problem with coating Varnish cost Radiators availability UV spectral width Paint products availability Annual operating costs
Gloss reduction low wide 200 - 400 nm wide high
LED radiant 45% lower than in case of UV technology ok. 15 thous. man-hours No No No No Yes Yes No Varnish yellowing after 30-90 min. final color (transparent varnish) high limited 365 - 395 nm limited low
The curing of coatings with LED radiant besides of its undoubted advantages also has disadvantages. A major difficulty is the coating yellowing effect immediately after curing . This effect disappears after a certain time (30-90 min) (Bekas 2012) , but ultimately prevents the possibility of hue correction during the process. An important limitation is also the need of maintaining a precise distance between the curing surface and radiant light. This prevents curing of the line elements of different thicknesses at one time. Mentioned problems will certainly be solved with the development of technology. Referring to the possibility of varnish curing by UV and a UV -LED it should be noted that besides general statements considering only evaluation of the methods there is no detailed information on the comparison of the quality of the coatings obtained using the different methods of cure. The present paper is aimed at examining the mechanical properties and resistance to UV radiation coatings curing with two methods : the classical method using UV and hybrid UV –LED. MATERIAL AND METHODS Within the framework of the study on the pine and beech wood, a coatings based on acrylic paints were applied and curing in two ways: UV and UV-LED. The characteristics of paint materials included in the individual coatings shows Table 2. All paint materials were applied on the sanded surface of the wood samples by rolls. The amount of applied material and way of curing the respective materials presents Table 3.
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Table 2. Features of the paint materials used in the tests.
Coating ingredients Solids content density viscosity VOC content Solids content density viscosity VOC content
Lightweight filler Primer Coating curing by UV 99 % 99 % 1,45 g/cm3 1,45 g/cm3 22000-24000 mPas 22000-24000 mPas 3g/l (0,24%) 3g/l (0,24%) Coating curing by UV-LED 99 % 99 % 1,45 g/cm3 1,45 g/cm3 22000-24000 mPas 22000-24000 mPas 3g/l (0,24%) 3g/l (0,24%)
Topcoat 99 % 1,45 g/cm3 22000-24000 mPas 3g/l (0,24%) 99 % 1,45 g/cm3 22000-24000 mPas 3g/l (0,24%)
Table 3. The order and the application and way of curing various materials incorporated in the coating.
Treatment
Application Coating curing by UV lightweight filler 20-25 g/m2 primer 15 g/m2 grinding (P 400) primer 15 g/m2 topcoat 5-6 g/m2 Coating curing by UV-LED lightweight filler 20 g/m2 primer 15 g/m2 grinding (P 400) primer 15 g/m2 topcoat 5-6 g/m2 * Ga – gala radiator, Hg –mercury radiator , LED – radiator LED
Curing* Ga+Hg Ga+Hg Ga+Hg Ga+Hg
LED LED + Hg LED LED + Hg
After processing the wood samples were subjected to an air-conditioning under the laboratory circumstances, then the samples for further testing were prepared. For the obtained coatings were examined to: thickness – using the ultrasonic thickness gauge POSITECTOR 200; scratch resistance – according to CEN/TS 15186:2007 Furniture. Assessment of the surface resistance to scratching, circular method ( method B); hardness – by Buchholz method; abrasion – using the Taber device, used wheels with a hardness of H10 and load 1000g; resistance to aggressive agents – according to PN-EN ISO 2812-4:2008 Paints and varnishes - Determination of resistance to liquids - Part 4: Spotting methods, the statement aggressive factors used in the study are shown in Table 4; resistant to UV radiation – according to EN 15187:2006 Furniture - Assessment of the effect of light exposure. UV irradiation was applied in the chamber SOLARBOX 1600E. The total exposure time of a sample of 50 h, BST temperature of 55 °C , the radiation intensity of 550 W/m2. Measurement of colour samples before and after irradiation was carried out using a spherical spectrophotometer X-Rite. The colour change was determined from the relation:
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E L2 a 2 b 2 ΔE – change colour; ΔL = L1 – L2 – change of the colour brightness; Δa = a2 – a1 – colour change in a range from green to red ; Δb = b2 – b1 – colour change in range from blue to yellow ; 1 – coordinate value prior to irradiation 2 – coordinate value after irradiation Table 4. Summary of aggressive agents used in the study of coatings resistance to aggressive media
Agressive media Destilated water Ethanol 50% Blue ink Caffee Acethon 95,5% Red wine Bleach (chlorine)
Exposure time 24±1h 24±1h 24±1h 24±1h 120±10s 24±1h 24±1h
Initial temperature ±5 ºC 20 ºC 20 ºC 20 ºC 80 ºC 20 ºC 20 ºC 20 ºC
RESEARCH RESULTS Test results of coatings curing by UV and UV-LED are shown collectively in Table 5. All tested coatings curing with UV and UV-LED approach, had a similar average thickness (approx. 40 - 46 microns) in both cases of pine and beech panels. The differences in coating thickness for each tested variants are statistically insignificant. Likewise, no effect of the curing methods (UV and UV-LED) was reported with reference to the hardness of the coatings tested by Buchholz. On the basis of studies it can be concluded that with a thickness of approx. 40 microns hardness of obtained coatings was dependent only on the type of substrate to which it is applied (pine or beech ). Table 5. Results of coating examination
Curing Coating Hardness Resistance Resistance to Resistance Resistance to UV approach / thickness 1 in Buholza to scratch 2 abrasion 3 to agressive radiation – colour substrate [µm] scale [-] [N] [cycles] media 4 change ΔE [-] UV beech 46 (10) 59 3 1200 + 5,70 UV pine 46 (10) <59 1,5 1500 + 6,04 UV-LED 40 (9) 59 3,5 2000 + 7,75 beech UV-LED 40 (9) <59 1,5 2000 + 8,21 pine 1 in parentheses are standard deviations [%] 2 minimum weight at which there was a complete circular scratch coating 3 cycles ( rotation of the sample) were recorded for the local (point) abrasion coating 4 + - full resistance to aggressive agents investigated Considering the resistance of the coatings to the scratch, it can be seen that as in the case of hardness, it is highly dependent on the substrate on which the coating was formed. In the case of pine there was no effect of curing method (UV and UV-LED) on coating
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resistance to the scratching. On the other hand, in case of a substrate made of beech wood a higher scratch resistance of the coating was observed for UV - light. An effect of curing the varnish (UV and UV-LED) was stated in case of resistance to abrasion test. In case of coatings curing by UV-LED, regardless of the type of substrate, a local (point) abrasion of coating was obtained after 2000 cycles of abrasion. When under the same test conditions, the curing by UV was characterized by (depending on substrate) 25 % to 40 % lower abrasion resistance. Higher abrasion resistance of coatings curing by UV-LED may be associated with its more uniform internal structure after the curing. Coatings curing by UV as well as UV-LED, exhibited a similar resistance to aggressive agents test. In both cases there were no noticeable changes in the coatings, caused by used chemicals. Considering the resistance of coatings to UV radiation it can be concluded that in all variants there was reported a noticeable change in colour of the wood samples (ΔE ˃5). Based on the obtained results it can be observed that the curing of coatings had no significant effect on the protection of refined material from UV rays .However, it was noted, that under the same conditions of exposure the coating curing by UV-LED showed greater average of change colour in relation to coatings curing by UV light. CONCLUSIONS On the basis of studies on coatings formed on pine and beech wood, with the use of using acrylic paints curing by UV and UV-LED you can draw the following conclusions: 1. Method of curing the coatings (UV and UV-LED) does not affect unambiguously hardness and resistance to scratching. A significant impact on these properties shows the type of ground surface - coating applied to the beech have a higher mechanical strength compared to coatings formed on pine wood. 2. The coating curing by UV-LEDs have a higher resistance to abrasion with respect to the coating curing by UV light. 3. Method of varnish curing by UV and UV-LED does not change the resistance of coatings to aggressive agents (ink, ethanol, coffee, distilled water, acetone) 4. Method of varnish curing by UV and a UV-LED does not change the resistance of coatings to UV radiation. These conclusions apply to varnishes used in research. Dependencies shown in the study, should not be regarded as generally applicable to other finishing materials curing by UV and UV-LED. REFERENCES 1. Akzo Nobel, 2013: Przemysłowe lakierowanie powierzchni drzewnych, https://www.akzonobel.com/wood/no/system/images/AkzoNobel_FF_pl_tcm8619049.pdf (10.09.2015 r.). 2. Bekas J., 2012: Pierwsza linia lakiernicza z diodą LED, http://gpd24.pl/1.2-297Pie3sza_linia_lakiernicza_z_dioda_LED.html (10.09.2015 r.). 3. Bekas J., 2013: Skuteczniej utwardzają, a nie podgrzewają. Gazeta Przemysłu Drzewnego, 9 (200), 45. 4. CEN/TS 15186:2007 Furniture. Assessment of the surface resistance to scratching, circular method. 5. EN 15187:2006 Furniture - Assessment of the effect of light exposure. 6. Jabłoński M., Świetliczny M., Ružińska E., 2009: Polimery syntetyczne i materiały malarsko-lakiernicze w przemyśle drzewnym. Wydawnictwo SGGW, Warszawa.
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7. Kien W., Osajda M., Boratyński P., 2013 Linie lakiernicze z utwardzaniem powłok promieniami UV - dobierz odpowiednie urządzenie. Lakiernictwo przemysłowe. Uszlachetnianie Powierzchni Drzewnych cz. V, 40-47. 8. Kijas B., 2011: Cały rok promienie UV możesz mieć u siebie w lakierni- nie czekaj na słońce. Lakiernictwo przemysłowe. Uszlachetnianie Powierzchni Drzewnych cz. II, 35. 9. PN-EN ISO 2812-4:2008 Paints and varnishes - Determination of resistance to liquids Part 4: Spotting methods. 10. Rudnicki A., 2011: Lakiery utwardzane promieniami UV - wiele zalet. Lakiernictwo przemysłowe. Uszlachetnianie Powierzchni Drzewnych cz. III, 16-17. 11. Wnuk R., 2011: Wodne materiały utwardzane promieniowaniem UV- przyszłość w lakierowaniu drewna. Lakiernictwo przemysłowe. Uszlachetnianie Powierzchni Drzewnych cz. II, 16-18. 12. Wnuk R., 2013: Utwardzanie powłok lakierniczych przy użyciu LED. Oszczędne i "pożerające" mniej energii. Lakiernictwo przemysłowe. Uszlachetnianie Powierzchni Drzewnych cz. V, 34-35. Streszczenie: Porównanie wybranych właściwości powłok lakierniczych utwardzonych metodami UV i UV-LED. Badaniom poddano powłoki wytworzone na próbkach z drewna sosnowego i bukowego z wykorzystaniem bezbarwnego lakieru akrylowego utwardzonego klasyczną metodą UV i hybrydową metodą UV-LED. Dla badanych powłok oznaczono ich grubość, twardość Bucholza, wytrzymałość na zarysowanie, wytrzymałość na ścieranie, odporność na działanie czynników agresywnych oraz odporność na działanie promieniowania UV. Badania wykazały, że na ogół nie ma istotnych różnic we właściwościach powłok utwardzanych przy użyciu metody UV i UV-LED. Powłoki utwardzone metodą UV-LED charakteryzowały się wyższą odpornością na ścieranie. Corresponding authors: Piotr Borysiuk, Mateusz Derda, Radosław Auriga, Piotr Boruszewski, Sławomir Monder Warsaw University of Life Sciences, Faculty of Wood Technology 159/34 Nowoursynowska Str., 02-787 Warsaw, Poland e-mail:
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