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UV Curing in the Plastic Components Industry

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UV Curing in the Plastic Components Industry
  JULY/AUGUST 2003 RADTECH REPORT 19        F     e     a     t     u      r     e U  V is rapidly becoming thetechnique of choice for curingcoatings in the plasticcomponents industry, but it’s not beenall plain sailing. Complex part shapes,temperature sensitive substrates anddemanding finish quality are just someof the challenges faced by chemistrysuppliers and equipment manufactur-ers alike. This article reviews thedevelopment of UV-curing chemistriesand techniques for plastic components,their current uses and potential. UV Curing in the Plastic Components Industry By Dawn Skinner enhance the potential for coatedplastic components.UV curing was an obvious contender.Introduced commercially in the 1970sfor flat, web-type applications, thetechnology was already proven topractically eliminate all VOC emissions,offer energy and space savings incomparison with large curing ovens,provide faster curing times and eventhe ability to cure on heat sensitivesubstrates with UV-resistant coatings.However, could UV-curable coatingsprovide the quality and range of visualand textural finishes required, espe-cially for the modern, fashion-consciousteletronics industry? In addition, wouldit work for the complex 3-D shapesfrequently found in plastic components? The Development Process The principle of UV curing is basedon initiating a chemical polymerizationinside a liquid coating using direct UV irradiation to create a cross-linked,dry, solid coating. There are twoelements to a successful UV-curingsystem—the UV-curable coating andthe UV-curing lamps with theirsupport systems.The finished properties of aUV-cured coating—hardness, scuff andscratch resistance, gloss or mattappearance, UV stability, chemicalresistance, etc.—are dictated by theinitial chemical composition of thecoating, the “match” of the UV lamp’sspectral output, intensity and dosewith that chemistry and the optimumphysical sitting of the UV lamps.In order to develop UV curing into afeasible and reliable production In order to develop UV curing into a feasible andreliable production process for the plastics industry,chemistry suppliers and equipment manufacturershave had to work together to create effective,workable solutions to these and other issues. Then and Now Until recently, coaters and decora-tors of plastic components had nooption but to paint or coat piecesusing water or solvent-based productsand then to dry them all thermally inlarge curing ovens. Though effective,the water-based coatings required very high energy input to drive off thewater content, the VOC emissionsfrom drying solvent-based formula-tions required expensive containmentor treatment equipment and thedrying ovens simply took up a hugeamount of space.Identifying and developing adrying technology that could elimi-nate some or all of these problemswould obviously revolutionize theproduction process and massively  20   RADTECH REPORT JULY/AUGUST 2003        F     e     a     t     u      r     e process for the plastics industry,chemistry suppliers and equipmentmanufacturers have had to worktogether to create effective, workablesolutions to these and other issues. Chemistry While the concept of a coating that isfree of organic solvent carrier, butwhich retains the performance proper-ties of a solvent-based coating, is veryattractive, designing such a product hasproved to be a challenge.UV coatings are very different fromconventional solvent or water-basedliquid and paste coatings. They are upto 100% solids, containing little or nosolvent or water carrier, but formulatedfrom monomers, oligomers, pigments,additives and photoinitiators. Whenexposed to high-intensity UV light thephotoinitiators start a chain reaction topolymerize the formulation, producing adry, solid coating. Challenges and Solutions For the chemistry formulators, therewere two initial challenges when creatingUV-curable coatings for plastics:•To create coatings which could beapplied using standard applicationmethods.•To compensate for the lack of solvent content which, in conven-tional coatings, aids adhesion tothe substrate.The commonly used applicationmethods, such as spray coating, dipcoating and flow coating, all presentedproblems for 100% solids UV formula-tions. Hence, UV-curable coatings forthe plastics industry are likely tocontain some solvent or water content,to allow the coating to be applied bythe chosen method.There are some 100% solidssprayable UV systems available commer-cially, but these are very limited. Theinclusion of low-viscosity cross-linkablematerials to enable effective applicationmay compromise the final properties of the cured coating. Thus, these 100% UV systems tend to be used in “lessdemanding” applications—as decorativecoatings for the cosmetic containerindustry, for example, where highdurability or scratch resistance are notcritical factors.However, the exception isUV-curable powder coatings. Thoughstill requiring further research anddevelopment work to bring these 100%solids formulations within the reach of many of today’s plastic coating opera-tions, current UV-curable powders arebringing the possibility of powdercoating plastics a little nearer.Unlike traditional powder coatings,UV-powder coatings only require theapplication of infrared (IR) or convec-tion heat for the melt/flow process, andare then cured by low temperatureUV-radiation curing. This reduces the  Figure 1 Coatings comparison for plastics Property Water-Based Solvent-Based UV CurableSolids content 40-60%60% (clear coatings)40-95%20-30% (pigmented)Few at 100% Soft feel Easy to apply andEasy to apply andDifficult to achieve becauseachieveachieve UV tends to produce hard surface cure. Visuals All gloss rangeAll gloss rangeAll gloss range possible. Very(gloss/matt etc.)possiblepossiblelow matt can be more difficult. Durability ModerateHighHigh Advantages •“Easy” switch from•Known technology•Small compact plantssolvent-based•Wide range of•High production ratessystems to reducesuppliers•Reduced reject ratesVOC emissions•Lower cost capital•Range of textures/equipmentappearances possible Disadvantages •Energy/time costs•High VOC levels•Some solvents toto remove waterbe removedprior to thermal cure•New technology•New capital equipmentrequired  JULY/AUGUST 2003 RADTECH REPORT 21        F     e     a     t     u      r     e heat required from a typical range of 160-200 ° C to 80-120 ° C and the dwelltime at these temperatures from20-60 minutes to only 2-10 minutes—depending on part size and shape.Melt temperatures will have to bereduced further for many plasticsapplications and the advantages willneed to offset the costs of specialstorage and handling. However,UV-curable powder coatings can beapplied using the same methods as forconventional powder coatings andprovide exceptionally high-qualityfinished coatings.Therefore, UV-curable coatingshave been formulated to meet standardapplication requirements, but they alsoneed to adhere to the substrate.The wetting (spread) of the coatingon the surface and the adhesion of thedried (cured) coating to the surface of the substrate depend on the tension atthe boundary surface. For sufficientwetting and adhesion of a coating, thesurface tension of the substrate mustbe higher than that of the coating. Inmost cases, the surface tension of thecoating can’t be changed so that of the substrate must be increased,which is usually achieved with anoxidizing flame, corona discharge orUV pre-treatment.The surface treatment results in aphysically and chemically modifiedsurface with increased surface tension,enhancing the wetting of the coatingand producing better adhesion.The solvent used in conventionalcoatings to aid application can also beused to enhance wetting and chemi-cally attack the surface of the sub-strate to increase the adhesion of thecoating. UV coatings do not necessarilycontain such solvents and may nothave the opportunity to physically“anchor” themselves in this way.Therefore, it is important to checkthe surface tension, and modify if necessary, when using UV coatings. An additional challenge to coatingsformulators has been the achievementof specific finished properties such assoft feels or low matt finish, both of which are in demand for modernconsumer teletronics products andwhich can be difficult to achieve withUV-curable coatings.Figure 1 (on page 20) shows anoverview comparison of the three maincoating technologies. Solvent-basedcoatings are the known, “tried andtested” systems, which have served theindustry well for many years. However,the drive to reduce the use and emissionof solvents leads manufacturers to lookat water-based coatings. This is seenas an “easy” technology switch, butwater-based coatings do require specialconsideration when it comes to thedrying operation to ensure the coatingis fully cured.No one technology is the perfectsolution for all applications and coatingrequirements, but UV-curable coatingscertainly have many important featuresand benefits. The high durability of thecoatings coupled with the significantimprovements in production speeds andreduction of finished product reject ratesare the key driving forces for implement-ing UV-curing technology in many plasticcoating operations. Development workcontinues to increase the range of UV-curable coatings with new soft feel, low matt and one-coat high gloss, pigmentedsystems coming into the market.With all these factors and options,plastics coaters are making theirchoice of coating on an “application-by-application” basis. No one coatingis sweeping across the board; rather,the choice of coating is usuallydriven by economics, with coaterschoosing the most cost-effectivesystem that will deliver their requiredperformance characteristics. UV-Curing Equipment Existing UV-curing technologycould be easily adapted for flat plasticcomponent applications, with installa-tions incorporating the latest in-linemodular UV-curing units, variablepower supply and electrodeless bulbs.However, creating UV systems to curecoatings on 3-D plastic componentspresented far more of a challenge. 3-D Curing Many plastic components arecomplex 3-D structures containingfrequent undercuts that create“shadow” areas. If UV curing was to be  Figure 2 Process window Scratch resistancehardnessgloss Energy  22   RADTECH REPORT JULY/AUGUST 2003        F     e     a     t     u      r     e successfully applied to plasticcomponents, then the problem of achieving optimal cure in these“shadow” areas had to be solved.The optimal characteristic profileof a coating can only be achieved if the parameters of the individualmanufacturing steps are situatedwithin certain boundaries—the processwindow (Figure 2).For two-dimensional parts the UV dose can be adjusted simply bychanging the power of the UV lamp orthe line speed; however, for 3-D parts,due to their complexity, it is morecritical to achieve an even energydistribution over the whole surface.Intensity, dose and UV spectrum mustbe combined to provide a curedcoating with the optimum combinationof properties required.Following intensive R&D involvinglaboratory experimentation and pilottesting, a variety of solution conceptshave been developed to eliminate“shadow.” These are often combined inproduction to obtain optimal results:•A programmed robot can moveeither the lamp over the surface of the part, or in reverse, the part infront of the UV lamp.•Short and modular UV lamps can beplaced at different angles aroundthe geometry of the part.•A variable power supply alters thepower of the lamp, and therefore,the dose supplied.•Additional installation of reflectorsensures optimal utilization of theirradiation.•Distance between the lamps and thepart to be cured can be altered.•Installations can be made with a UV lamp on a flexible arm.•The installation of ellipticalreflectors rather than parabolicreflectors improves the curing of former shadow areas (Figure 3).Of all these solutions, robotics is theprobably the most exciting, but also theone that presented the most problems. A robot used for UV curing must bedesigned to meet specific requirements.Depending upon the UV irradiation unitused, the robot must have a carryingcapacity between 8-16kg. Precision andspeed are not necessarily primarycharacteristics, depending upon thegeometry and size of the part to becured; however, many degrees of freedom and a large work radius isimportant. UV stable supply cables andgaskets must be used.The UV lamps also have to beadapted for robotic use. Traditionally,UV lamps were designed to bemounted almost exclusively in a fixedposition. To cure 3-D plastic compo-nents from a moving robot, the lampshad to be adapted to cope with thenew environment.Internal components insensitive to vibrations were developed to overcomeproblems caused by rapid movements,sudden changes in direction andsudden stopping. The construction andcooling of the UV lamps also had to bere-designed to ensure that the lampswould not overheat.Robot mounted UV-curing lamps arenow being used successfully in a varietyof production installations throughoutEurope by companies such as DIAM of France. In production, using robots forUV curing has been found to provideprocess flexibility and high productivity.Optimized programming of robots haseven made the UV curing of extremegeometries possible. However, there isstill much development that can be doneto refine and improve the technique. The Future UV-curable coatings are now commonly used for plastics applica-tions that experience very harshenvironments where high levels of scratch/scuff resistance are required,such as consumer teletronics productsand automotive polycarbonateheadlamp lenses.Despite this success and towiden the usage of UV-curablecoatings in the plastics industry,there are some technical issues stillto be resolved. Environmental,legislative and economic pressuresnecessitate ongoing research anddevelopment programs.Customers are beginning to lookbeyond the simple “solids content” or VOC issues, to evaluate the coating  Figure 3 Different types of reflectors Elliptical reflector Parabolic reflector  JULY/AUGUST 2003 RADTECH REPORT 23        F     e     a     t     u      r     e process “in the round”—taking intoaccount performance and cost driversand the implications for their plantprocesses and procedures whenmaking a choice of coating system.The constant drive for reducedsolvent content in coatings formula-tions will remain a major influencingfactor; however, progress in this area islikely to be limited by the viscosityrheology requirements of the applica-tion process. Will this lead to new application techniques in the future?  Acknowledgments This article, with some additions,srcinally appeared in the September2002 issue  Polymer, Paint andCoatings Journal.   ◗  —Dawn Skinner is a processdevelopment manager for  Fusion UV Systems Inc., Alton, UK. Properties RAHN USA Corp.1005 North Commons DriveAurora, Illinois 60504 Tel.001 (630) 851- 4220 Fax001(630) Speciality Resin 01-554 RAHNAGDörflistrasse120CH-8050 Zürich Tel.++41 1 315 4200 Fax++41 1315 For very low gloss coatings The material combines: Surface cure and hardnessLow gloss (gloss < 10%)Low viscosityGood flexibility Formulations based on the material show: Low glossMedium reactivityGood hardness and non-yellowingGood adhesion to various substratesSuitable for screen and flexo-printing Excellence in UV Curing BOMAR SPECIALTIES COMPANY 210 Holabird Avenue •  Winsted, CT 06098 Phone: 860-379-7748 •  Fax: 860-379-1528Web: B omar Specialties Co. U    V    S   O  L U   T   I   O   N    S INNOVATION…INNOVATION… in UV/EB Oligomers Our customers look to Bomar for problem-solving oligomers not available from other industry sources.Put Bomar innovation to work for you.Call us to talk about solutions for:    ADHESIVES    INKS    COATINGS    LAMINATES 
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