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Benefits of UV-curable coatings
Typically flexible fluoropolymer PUR coatings are sprayapplied directly after vulcanisation of the rubber during thecontinuous extrusion process and then thermally cured in aseries of conventional, microwave or infrared ovens. Thisconventional cure process is costly, inefficient and results inhigh overhead costs. In the age of continual demands forcost reduction, improved efficiency and reduction in VOClevels, a range of waterborne 1-pack UV curable coatingsmeeting market and technical requirements have beendeveloped. Realizing the benefits of a UV curable coatingsystem in a continual extrusion process means at least anenormous increase in speed of cure, waste reduction, lowersolvent emissions, lower energy costs and huge spacesavings. In some cases it is even the only way to implementthe application of a coating into to the manufacturingprocess due to limitations in line speed and/or in themechanical properties of the rubber substrate.Kevin Bruen, Kurt Davidson, Daniel F. E. Sydes, Peter M.Siemens.Nowadays more and more flexible substrates for industrialapplications, e.g. in the automotive industry or the buildingindustry, are equipped with flexible fluoropolymer PUcoatings to improve or modify the end-use properties of agiven substrate (most often EPDM). Typical applications areweatherstrip seals and glass run channels of a car, wherelow friction, non-stick, freeze release, noise reduction aswell as chemical or abrasion resistance are important. Morerecently coloured or pearlescent flexible fluoropolymercoatings became design elements in the automotive andbuilding industry, where UV-resistance and easy-cleanproperties are important.Rubber seals play an important role both in the constructionof vehicles and buildings. Without them it is currentlyimpossible to make a windows wind and water tight. Sowithout rubber we have no dry, calm and pleasant interiors,unless we want to abandon sunlight from our everyday life.Modern automotive rubber door and boot seals for example,including primary and secondary seals for weatherstrip andglass run applications, are commonly manufactured fromextruded EPDM. There is a requirement to apply a coatingto the seal to improve the end-use properties. These includelow friction, non-stick, freeze release, noise reduction,chemical and abrasion resistance properties specified by theautomotive manufacturers. A complete range of flexiblecoating systems is produced by Whitford. These principallyfluoropolymer-based coatings have been used extensivelyto coat all kinds of seals for automotive applications forseveral years.A new field of application are architectural sealing systems.Modern office buildings and houses often have facades withmany windows or are even nearly completely made ofwindow panes. All these windows in principle consist ofwindow glass fitted into a frame made of metal (typicallyaluminium), plastic (e. g. PVC) or wood using multiplerubber seals of various shapes (see Figure 1). Most of theserubber seals are invisible to the beholder when the windowis closed, but some parts of them are visible as a blackborder between the glass and the window frame. In modernarchitecture windows and window frames became a designelement several years ago. Nowadays these visible rubberseals get more in focus. The aim is to have coloured windowseals, that either match the colour of the window frame orthe facade, or form a pleasant contrast that catches the eye.
On top of acting as a design element flexible fluoropolymercoatings are used as an assembly aid for window profiles.The rubber seals have to be lubricated before they can befitted into the window frame. Currently silicone oils are usedfor this, but this solution suffers from disadvantages. Assilicone oils are liquids they tend to contaminate the windowglass during the assembly. This leads to extra costs forextensive cleaning. In the case of modern self-cleaningfacades the use of silicone oils is not possible at all as theydestroy the self-cleaning property of the glass by attackingthe coating on the glass.
Before the curing step the coating has no abrasion orchemical resistance and is often tackyTypically conventional polyurethane dispersion (PUD) basedcoatings contain a lubricant and are spray applied directlyafter vulcanisation of the rubber during the extrusionprocess. After application of the coating a secondary(coating) cure cycle is required and usually involves a seriesof convection 'tunnel', microwave or IR ovens (see Figure 2).Before the curing step the coating has no abrasion orchemical resistance and is often tacky, even when thesolvent (either water or organic solvents) has evaporated.The heat driven cure facilitates the reaction of thecrosslinker with free OH-groups to form a fully crosslinkedpolyurethane.Although the heat-cured coating systems show very goodperformance and are therefore well accepted in the industryand often specified by the automotive manufacturers, thecure process itself implies a number of significantdisadvantages:Facilitating the cure process is a costly and inefficientprocess overhead. The tunnel ovens for the curing processcause high energy costs and high investment in machinery.They also occupy a significant amount of expensive spacein production: Typically at least 30 % of the space for anextrusion line with online coating application is occupied bythe ovens facilitating the heat cure of the fluoropolymercoating.
UV-curable coating systems cure in less than a secondUV-curable coatings are well known within the industry. Asignificant amount of the coatings used e. g. in the furnitureor window industry is cured by UV radiation. One of themost important advantages of UV-curable coatings overconventional air-drying or heat-cured coatings is the speedof cure. Typically a UV-curable coating is cured in less thana second. A conventional air-drying wood-coating needsseveral minutes or even hours to dry completely. Thisdifference in speed of cure allows much higher line speeds,an enormous economic advantage.The different cure mechanism also gives the cured film amuch higher chemical resistance as a rigid network ofcovalent bonds is formed. Typically these UV-curable woodcoatings are 100% monomer systems. That means theycontain no additional solvents, but only UV-curablemonomers and/or oligomers and solids like pigments, fillersetc. (see Figure 3).
UV-technology for continuous extrusion processescoating systemsExisting heat-cured fluoropolymer coatings for rubbersubstrates are usually chemically cross-linked. Therefore
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the reason for implementing UV-technology into thisbusiness area is not improved chemical resistance. The aimis to develop a product that offers the benefits of the wellestablished heat-cured coatings, but with the short, almostimmediate, curing time of a UV-curable coating.Transferring these benefits of UV curable coating systems toa continuous extrusion process leads to:
Space savingThe UV drying system requirements are in the region of 5min length, the equivalent to the space occupied by thespray-booth configuration in contrast to typically 30 m for aconventional hot air oven. The heat source used in thecoating process is a short set of IR-lamps. This is necessaryto ensure complete water removal before the UV-cure takesplace (see Figure 4), as all presented coating systems arewater-based.
Easy installation of UV curing equipmentThe required UV curing equipment can easily be installed onto the existing extrusion/coating lines, at the same timefreeing up expensive manufacturing space occupied by thedefunct 'tunnel' oven confi- guration.
Lower energy costsAs stated previously the use of 'tunnel' ovens requires thepre-heating of the oven before the coating of the extrusionprofile can commence and maintaining this required'working' temperature when the extrusion line is idle. UVlamp systems allow for standby, shuttered operation at lowpower when not in use and incorporate rapid, de factoinstantaneous, lamp warm-up when needed.
Reduced wasteAs with any continuous process there is always a degree ofwastage until the required running conditions are achieved.In this process obtaining the correct wet film/dry filmthickness of the coating is required. Presently for the heatcured PUD coatings dry film measurements are obtainedafter the 1,5 - 3 min cure cycle, combined with a typicalextrusion speed of 15 m/min this equates to waste coatedprofile until the correct coating thickness is obtained andmay require several spray gun adjustments. For the UVcoating process, since the cure cycle is greatly reduced thecorresponding amount of scrap profile is also greatlyreduced.
Increased line speedDuring the last couple of years many modern extrusionprocesses have been developed. State of the art extrusionlines for EPDM profiles, especially those types of profilesused for windows, can run at up to 50 m/min. With thecurrent PUD coating systems it is not possible to cure thecoating at the high line speeds because- The required oven capacity, approximately 100 m inlength, would require too much investment in machinery,consume huge amounts of energy for heating and wouldrequire too much space or would not fit in typical productionfacilities.- As PUD coatings are typically tacky till they are fully cured,the extruded profile must run though the 'tunnel' ovenwithout touching any parts of it with a coated surface. Thiscan be achieved at the current oven lengths, but can not bemanaged at oven lengths needed for these high line speeds.
Coatings are based on different types of waterbornePUR-dispersionsThe coatings used in this set of experiments, code-namedas "Xylan 2525", are based on different types of
water-based polyurethane dispersions. A wide range ofUV-curable PUD's is available from various manufacturers.One that has been used in this special application is"Neorad R440" (NeoResins), a UV-curable aliphaticurethane oligomer dispersion with 40 % solids. As theseUV-curable PUD's are not flexible enough for this kind ofapplication (car manufacturers usually specify 100-150 %elongation) the UV-curable PUD is blended with anon-reacting aliphatic polyurethane dispersion, e.g. "NeorezR600" (NeoResins). This gives the resulting PUR-coatingenhanced adhesion to the rubber substrate and betterelongation. The fluoropolymer component is eitherPolytetrafluoroethylene (PTFE) or a perfluoropolyether. Toimprove the dispersibility of the fluoropolymer component afluorosurfactant has been added. Other additives aresilicone polyester acrylates and polyether siloxanecopolymers which have been used as wetting agents andslip and flow additives. A mixture of two α-Hydro-xyketonesand Bisacylphosphine (BAPO) is used as a photoinitiatorblend. The coating has been pigmented to give a matt blackfinish. See Table 1 for an example of a standard formulationused.
UV-curing equipment with very stable UV output overlife timeA standard lab-scale conveyor UV-oven (Fusion UVSystems) is used for curing the coatings (see Figure 5).Different types of microwave powered lamps have beenchosen as the UV-source. The reason for this is thatmicrowave powered lamps have a very stable UV outputover their life time (>6000 h). The UV-spectra of the differentbulb types used in the experiments are shown in Figure 6.As only one slot for UV-lamps was available, longerexposure times were simulated by multiple passes troughthe oven. In case of curing schedules with multiple lamptypes the bulbs were changed in between the passes.
Parameters tested: solvent resistance, abrasionresistance and flexibilityThe wet coating has been spray-applied on both EPDMswatches from automotive weatherstrip seals as well asaluminium panels. Before the application of the coatingmaterial both types of substrates have been treated with aprimer for adhesion promotion ("Xylan 4016"). The primerhas been spray-applied and then been flashed-off for 2 minat 150°C (if not stated differently) to simulate the IR flash-offwhich is typically much faster. Immediately afterwards thecoating is sprayed onto the substrate, flashed-off 2 min at150°C and then cured in the UV-oven at a conveyor speedof 15 m/min. The different curing schedules used are 2D(two passes under a D-lamp), 2V, 2H, 2V+2H (two passesunder a V-lamp followed by two passes under a H-lamp)and 2D+2H.The parameters tested for the cured coating are solventresistance, abrasion resis-tance and flexibility. Solventresistance was determined by a solvent rub test. For thistest a cotton swab is immersed in the specified solvent. Inmost cases isopropanol, toluene or N-methylpyrrolidone(NMP) are used, but any other liquid is possible, dependingon the end use of the coated parts. Immediately afterwardsyou begin rubbing the test panel in a back and forth motionin a straight line with a stroke at least 8 cm long usingmoderate pressure. Approximate weight of pressure shouldbe around 50g. The rate of rubs is approximately 100 doublerubs per minute. The rubs are counted (one forward and onebackward to be counted as one double rub) and continueduntil the coating film is attacked. The resulting number ofdouble rubs is the measuring unit for the solvent resistanceof a coating on a given substrate. If the solvent used is very
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volatile (e. g. toluene) the cotton swab is immersed in thesolvent every 50 double rubs to avoid abrading the coatingwith a dry swab.Abrasion resistance is measured by a modified Crock test.The standard Crock test as specified by Ford (Ford BN107-01) demands a given result for the discolouration of thecloth after a specified number of cycles (e.g. 500 cycles)with a specified load (900 g). We have modified thestandard wet crock test with soapy eater (Ford specifies anaddition of a mixture of soap : water 1:5 (w/w) inspecification WSB M2D 49-A2, sector 3.8) to make it moresevere by adding 1 mL of a mixture of liquid detergent(standard washingup liquid) and water 1:2 (w/w) onto thecloth. Instead of running the test for a given number ofcycles the test is continued until the coating fails (i.e. thesubstrate is visible).For elongation tests flat EPDM samples are elongated tillcracks appear in the coating film. At this point the elongationin percent (%) is recorded.
Water removal stage is essentialTo evaluate the impact of the water removal stage on theperformance of the cured coating several different time andtemperature pairs have been examined. As a means tomeasure the quality of the cure solvent double rubs withtoluene have been carried out. It can be seen (see Table 2)that the water removal stage is essential for the cure of thecoating. If water is still present during the curing step, it getsentrapped and the network formed by the cured PUR's ismost likely to loose to give the coating the necessary solventresistance. It can also be seen that too extensive flash-offtimes or temperatures have a negative effect on solventresis-tance, too. This can be attributed to thermaldegradation of the photoinitiators used.
Combining UV-output of a V- and a D-bulb respectivelyof a H and a D-bulb results in superior solventresistanceThe experiments show that the curing conditions have ahuge impact on the solvent resistance of the finished coatingon aluminium panels (see Table 3).Using only a single UV source results in very limited solventresistance, especially in the case of the H-bulb. D- andV-bulbs show no significant difference solvent resistance.This is quite surprising as V-bulbs are usually recommendedfor white coatings, not for black ones. Even when a whitecoating formulation for architectural sealing systems wascured, this correlation was confirmed (see Table 4).Combining the UV-output of a V- and a D-bulb respectivelyof a H- and a D-bulb results in superior solvent resistance ofthe cured coating. As the combination of D- and H-UV givesbetter results in all experiments done, this combination isrecommended for the curing of these types of black "Xylan2525" coatings and will be used in all future experimentswith these materials.The coating passed 5000 cycles in the modified crock testmentioned above. This means the abrasion resistance is farbetter than usually specified for flexible coatings. Otherresults for standard tests as specified by the automotiveindustry are summarized in Table 5.
ACKNOWLEDGMENTSWe would like to thank Fusion UV Systems GmbH for theirtechnical assistance and advice and Dätwyler AG for theircooperation.
Result at a glance- In the age of continual demands for cost reduction,improving efficiency and reduction in VOC levels we have
approached these problems and developed UV curablecoatings, "Xylan 2525", for automotive applications. Thesecoatings meet major car ma-nufacturers' specifications, areenvironmentally friendly and satisfy VOC legislation as wellas all other present legislation.- For the first time UV-curable flexible coatings forarchitectural sealing systems are presented. These coatingscan act both as coloured design elements as well astechnical assembly aids.- Applying UV-curing technology to continuous extrusionprocesses gives major economic benefits. It also makescoating technology for flexible materials applicable to awider variety of substrates. In the course of the conversionof the materials used to build cars to increase theirrecyclability other kinds of rubber substrates have becomemore and more important.- At the moment EPDM is widely used, but it lacks anyrecyclability. Therefore more and more thermoplasticelastomers (TPE's) and thermoplastic vulcanisates (TPV's)are used for automotive applications as they can berecycled more easily. But as they are thermoplastic they cannot be coated with the existing, heat-cured coating systemsdue to the high cure temperature. The presented UV-curablecoatings overcome this problem by eliminating the need forhigh temperatures from the curing process.
The authors:> Kevin Bruen, project chemist for flexible finishes and aChartered Chemist, is responsible for coatings applicationand technical support in the Flexible Finishes industry. Hehas 25 years experi ence in formulating and applyingspeciality coatings, including a spell in the extrusionindustry.> Kurt Davidson, B.S. in Chemistry from the University ofPittsburgh (1994) and M.S. in Chemistry from LehighUniversity (2002), worked from 1995-1999 at Air Productsand Chemicals, Inc. as a research technician mainlyperforming emulsion polymerization and formulatingpolyurethane coatings for the automotive industry. Thenfrom 2000-present he is working as a development chemistfluoropolymer coatings for various applications for WhitfordCorp.> Daniel Sydes, a graduate engineer, joined Whitford in1993 and has worked in the coatings industry for 15 years.As the business manager for Flexible Finishes he isresponsible for his division of Whitford Worldwide.> Peter Siemens, Dr. rer. nat. in Physical Chemistry fromUniversity of Bielefeld (2000), joined Whitford as TechnicalManager for Whitford GmbH (Germany) 2 years ago. Thedevelopment part of his work is mainly focussed on FlexibleFinishes and Textile Coatings.This paper was presented at the European CoatingsConference in Berlin, March 17-18 2004
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Figure 1: Modern facades built with state of the art architectural design consist almostonly of glass and rubber seals. Today these architectural sealing systems get into the
designer's focus. (Source: Dätwyler AG).
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Figure 2: Simplified representation of an EPDM-extrusion line with online coatingapplication and heat curing (not to scale).
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Figure 3: Comparison of curing mechanisms of physically drying coatings versusconventional UV curable coatings..(Source: Fusion UV Systems GmbH).
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Figure 4: Simplified representation of an EPDM-extrusion line with online coatingapplication and UV-curing (not to scale).
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Figure 5: UV-oven used in the experiments. The UV-source is mounted on top of anadjustable conveyor system.
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Figure 6: Spectral output of microwave-powered D-bulbs, H-bulbs and V-bulbs in theUV-region of the spectrum.
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