UV Coatings for Plastics

Radiation curing

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Stefan SmeetsEgbert BoerrigterStephan Peeters

Rapid curing, no or very low VOC content ,high performance properties - UV-tech-nology offers a number of advantages. Onplastics, though, UV curable coatingshave been difficult to achieve, mainly be-cause of inferior adhesion to the sub-strate compared to solvent borne sys-tems. Optimisation of existing binder/re-active diluent systems, however, can pro-vide coatings for plastics with very goodperformances.

UV-curing technology for industrial coat-ings has been established for decades.The advantages in terms of processing(high speed, smaller equipment), per-formance (scratch resistance, stain resist-ance, etc) and environmental friendlinessare well known. The use of UV-technologyfor coating plastics has been limited.However, as this article aims to show, thetranslation of the advantages of UV-tech-nology into the field of plastic coatingshas lead to an optimisation of existing UV-resins as well as the development of newUV-binders.

Tailor made solutions required

The increasing use of plastics as construc-tion materials in automotive, electronic andtelecommunication fields implies that theircoatability is becoming very important.However, finding a universal coating forall types of plastics is still a utopia. Theirlow surface energies and sensitivity totemperature and solvent make many plas-tics difficult to coat. The number of differ-ent materials comprising homopolymersor blends of different polymers is enor-mous and most substrates require tailor-made solutions.Moreover, due to different intrinsic prop-erties within a polymer family, differentproperties are targeted. Optimisation andfine-tuning of well-known plastics materi-

UV coatings for plastics

Figure 1: Effect of diluting acrylates on adhesion to several substrates.Blue regions: highly preferred. Green regions: optional in balance with other properties

Figure 2: UV-adhesion promoters for plastics. Blue regions: main component; green regions: optional additive; white region: optional diluting acrylate

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Radiation curing

als by blending or using additives to ob-tain better properties requires simultane-ous adjustment of the corresponding coat-ing formulations. The current trend to-wards cheaper polymers is primarily driv-en by their coatability, which allows to ob-tain the required surface aesthetics.

With UV-technology, one is no longer limit-ed to coat plastics with solvent-basedproducts. Solvent coatings are inhibitedby their high VOC-emissions, the space-consuming, long thermal drying tunnelsrequired and their limited curing speed.

Not only does UV-technology overcome theseproblems, it also offers other advantagessuch as the ability to coat heat-sensitive sub-strates as well as a performance improve-ment e.g. in scratch and stain resistance,which are becoming increasingly important.

UV Curing Technology

UV-resins are well established for surfaceprotection and decoration in many appli-cations such as furniture, wood flooring,construction, graphic arts and electronicshowever, their use on plastic substrates isstill limited. Despite the advantages of theUV-technology, equipment manufacturers,formulators and raw material suppliersface new challenges when attempting tointroduce this technology further into thefield of plastic coatings.

The chemistry used in UV-polymerisationis almost entirely based on acrylate func-tionalised materials. A UV formulation al-ways contains the following ingredients:• Diluting acrylates and acrylated oligomers,

which, as reactive materials, form the back-bone of the coating after drying.

• Photoinitiators, which under exposure toUV-light form radicals that initiate a poly-merisation reaction between dilutingacrylates and acrylated oligomers.

• Fillers and additives, which mostly do notparticipate in the polymerisation reactionand remain embedded in the cured network.

DDiilluuttiinngg AAccrryyllaattee DDeessccrriippttiioonn MMoolleeccuullaarr SShhrriinnkkaaggee SSuurrffaaccee tteennssiioonnwweeiigghhtt ((%%)) aatt 2255°°CC ((mmNN//mm))

IBOA Isobornyl acrylate 208 5.2 32

“Ebecryl 114” Oxyethylated phenol acrylate 236 6.8 39

ODA Octadecyl acrylate 200 8.3 30

“Ebecryl 1039” Urethane monoacrylate 33

TCDA Tricyclodecane diol diacrylate 304 5.9 40

“Ebecryl 145” Propoxylated neopentyl glycol diacrylate 328 9.0 31

DPGDA Dipropylene glycol diacrylate 242 13.0 35

TPGDA Tripropylene glycol diacrylate 300 18.1 34

HDDA Hexane diol diacrylate 226 19.0 36

“Ebecryl 160” Trimethylolpropane ethoxy triacrylate 428 14.1 39

“OTA 480” Propoxylated glycerol triacrylate 480 15.1 36

TMPTA Trimethylolpropane triacrylate 296 25.1 38

“Ebecryl 40” Alkoxylated pentaerythritol tetraacrylate 571 8.7 40

“Ebecryl 140” Ditrimethylol propane tetraacrylate 438 10.0 38

Table 1: Molecular weight, shrinkage, and surface tension of diluting acrylates

Figure 3: A schematic representation of a UV-PUD

Figure 4: Monitoring ofthe colour b val-ues in Xenon-testing. Themarks denotethe appearanceof first cracks


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Radiation curing

Another important parameter to consideris the surface tension of the substrate.Good adhesion is obtained when the sur-face tension of the substrate is higherthan that of the coating, giving an optimalwetting of the substrate. The surface ten-sion of typical diluting acrylates is alsoshown in Table 1. The surface tension ofcommon plastic substrates may vary from20 mN/m ("Teflon") to 44 mN/m (PET). Inaddition, a wide range of surface tensionsmay be encountered for one specific sub-strate material, due to different surfacetreatments. For example, untreatedpolypropylene may have a surface tensionof 28 mN/m whereas corona treated PP cango up to 40 mN/m. A surface treatment withcorona or flame results in the formation ofpolar groups on the substrate.

Different substrates demand different solutions

A formulation leading to a good adhesionon a particular plastic may not delivergood performance on another member ofthe same plastic family. The ability of sev-eral diluting acrylates to serve as an adhe-sion promoter in a coating is shown in Fig-ure 1. In the blue region one can find thepreferred diluting acrylates for given sub-strate materials, whereas the products inthe green region are used in balance withother properties such as diluting powerand reactivity.

Due to its capacity to swell the plastic sur-face, HDDA is an interesting adhesion pro-moter. In the case of PC or impact resistant

Diluting acrylates

UV-coatings are, by their nature, solventfree, so they have to be formulated in away which compensates for the lack of sol-vent in the adhesion on plastics sub-strates. This adhesion is difficult becauseof the inherent polymerisation process ofthe UV-resins.

Shrinkage, solubility and surface tension have to be considered

The speed of the UV-polymerisation processproduces internal stress in the coatingwhich is not found in conventional thermaldrying processes. When exposed to UV-light, the photoinitiator starts a chain reac-tion leading to polymerisation of the liquidcoating accompanied by a decrease in vol-ume and thus shrinkage. The liquid coatingconsists mainly of highly viscous oligomersand low viscosity monomers. The purposeof the diluting acrylates or monomers is thereduction of the viscosity of the total UV for-mulation. These diluting acrylates play therole of "reactive" solvents within conven-tional formulations. But as they react in thefinal polymer, they influence the final prop-erties as well. Low viscosity acrylates withhigh functionality and low molecular weightwill give high reactivity, high cross link den-sity but also high shrinkage. Thus, they willdecrease the adhesion of a UV-coating. Lowviscosity acrylates with low functionality willgive low reactivity, low cross link densityand a high flexibility.

The shrinkage of some diluting acrylates isshown in Table 1. The shrinkage is ex-pressed as the change in density duringcuring of a formulation containing 100%resin and 5% photoinitiator. Althoughthese values give a good indication of theintrinsic properties of a diluting acrylates,the behaviour in a final formulation de-pends on the other components as well.

The solubility parameters of both polymer-ic substrate and formulation indicatewhether adhesion is expected to be diffi-cult or not. Generally, a diluting acrylatewhich can attack or swell a substrate willform an interpenetrating polymer networkbetween substrate and coating, leading toan excellent adhesion. Thus, diluting acry-lates such as oxyethylated phenol acry-late, DPGDA or HDDA are able to attackand swell polycarbonate.

Figure 5: Glossmonitoring inthe Xenon test

RReessiinn DDeessccrriippttiioonn HHoopppplleerr VViissccoossiittyy,, mmPPaa ss ((TT iinn °°CC))

“Ebecryl 740-40TP” Acrylic oligomer in TPGDA 8500 (60)

“Ebecryl 767” Acrylic oligomer in IBOA 8500 (60)

“Ebecryl 745” Acrylic oligomer in blend TPGDA/HDDA 20,000 (25)

“Ebecryl 303” Polymeric resin in HDDA 900 (25)

“Ebecryl 436” Chlorinated polyester resin in TMPTA 1500 (60)

“Ebecryl 438” Chlorinated polyester resin in “OTA 480” 1500 (60)

“Ebecryl 584” Chlorinated polyester resin in HDDA 2000 (60)

“Ebecryl 7100” Amine functional acrylate 1200 (25)

“Ebecryl 168” Methacrylated acidic derivative 1350 (25)

“Ebecryl 170” Acrylated acidic derivative 3000 (25)

Table 2: Viscosity of UV resins for adhesion on plastic substrates


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PS, only a small amount of HDDA is need-ed in a formulation, whereas for harder PShigher quantities are necessary. Becauseof its low surface tension, ODA is a veryuseful diluting acrylate for PP and PE. ODAis mostly used in small amounts becauseof its incompatibility with many acrylatedoligomers at high concentration. "Ebecryl1039" is preferred over "Ebecryl 40" whena higher flexibility is required. TCDA is a di-functional diluting acrylate for rigidSMC/BMC as it has a low shrinkage com-bined with an excellent hardness.

UV adhesion promoters

Diluting acrylates in a UV-formulation re-duce the viscosity and increase the adhe-sion on several plastic substrates. Howev-er, they can reduce the high performanceof an UV coating. Thus, in order to limit thepercentage of diluting acrylates in a UVformulation, one can switch from a onelayer to a multi-layer approach withprimer, basecoat and topcoat. As theprimer is responsible for the adhesion tothe substrate, both basecoat and topcoatmay contain less diluting oligomers.

Almost all automotive paints use theprimer/basecoat/clearcoat build-up in or-der to fulfil the high quality requirements.The basecoat contains the colour pigment,while the topcoat will provide the scratch-resistance etc. This multi-layer approachmakes it easy to switch from one colour toanother without changing the primer andtopcoat. The use of one UV-primer for sev-eral plastic parts could simplify the coat-ing process considerably.

Several UV-resins (Table 2) provide agood adhesion to different plastic sub-strates. In practice, however, as a plasticcan vary from one supplier to another,can be filled or non-filled, can be a blendof two miscible or compatibilised poly-

mers, the value of these resins has to beevaluated with experimental work oneach plastic.

The ability of these resins to serve as an ad-hesion promoter in a primer and/or in acoating is shown in Figure 2. As the adhe-sion of UV-resins towards PC, PVC and PM-MA is mostly obtained with a small amountof diluting acrylate, no adhesion promoter isrequired. Surface treatments, such as coro-na, flame, plasma or IPA cleaning are ad-vised to activate the surface but also to re-move processing additives and contamina-tion. Figure 2 is composed of three areas(blue, green, white), and should serve as aguideline to coat several plastic substrates.The resins in the blue region represent themajor component in a formulation, whichcan be diluted with one of the diluting acry-lates from the white region. Thus, "Ebecryl740-40TP", "767" and "745" are acrylic acry-lates which give good adhesion to manyplastics mainly due to their low functionalityand thus shrinkage. "Ebecryl 303" is a poly-meric resin diluted in HDDA that shows goodadhesion in combination with acrylic acry-lates. "Ebecryl 436", "438" and "584" arechlorinated polyester resins diluted in TMP-TA, “OTA 480” and HDDA, respectively. Theyhave a good reactivity combined with a verylow shrinkage (< 4 %).

The amount of diluting acrylate must bechosen depending on the required viscosi-ty, but also the rigidity of the substrate. Toincrease the reactivity one can take a high-er functional diluting acrylate, or one canadd "Ebecryl 7100", which is a low viscosi-ty amine functional acrylate. "Ebecryl 168"and "170" are methacrylated or acrylatedacidic components which are able to etchthe surface of a filled plastic.

The efficacy of the UV-resins to adhere to aplastic depends on the application processand the contact and drying time. The best wayto evaluate this is through industrial trials.

Low viscosity, high performanceUV-resins

The design of low viscosity, high perform-ance resins for spray application to coatthree-dimensional objects has led to anew generation of binders, radiation cur-able polyurethane dispersions (UV-PUD)(Figure 3). The reaction of diisocyanates(shown in blue in the diagram) with differ-ent polyols (polyester, polyether, polycar-bonate) results in a polyurethane whichforms the basic chemistry of a UV-PUD.Because of their relatively high molecularweight, UV-PUDs show almost no shrink-age after curing, resulting in an excellent

DDiissppeerrssiioonnss PPeerrssoozz SSoollvveenntt rreessiissttaannccee FFlleexxiibbiilliittyy PPVVCC PPCC PPPPhhaarrddnneessss ((ss)) ((AADDRR)),, 1122 µµmm oonn PPCC ((iimmppaacctt,, ccmm)) ((CCoorroonnaa))

DW 7770 311 > 100 > 100 5B 5B 0B

DW 7773 352 > 100 < 20 5B 5B 5B

DW 7825 295 50 > 100 5B 5B 0B

DW 7772 312 > 100 < 20 5B 5B 5B

DW 7849 263 30 > 100 5B 5B 0B

Table 3: Adhesion performance of some “Ucecoat” UV-PUD's. 5B = perfect adhesion

Radiation curing

Table 4: Aliphatic urethane acrylates for exterior application


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Extensive test experience has been gainedwith different types of accelerated testsand cycling conditions. This paper onlyconcentrates on the Xenon test, using aXenon “WeatherOMeter”, according to theASTM G26 method.

Because of the huge variety of plastics, itwas decided to test the UV clearcoats onweather stable white thermoset polymercoated panels at 20 µm coating thickness.A visual evaluation was carried out alongwith colour and gloss monitoring duringthe tests.

Aliphatic urethane acrylates in generaloutperform other oligomer types in bothaccelerated and natural weathering tests.Aromatic polymers, on the other hand, areinherently not stable to light and tend toyellow and degrade rapidly. Also conven-tional polyether and polyester acrylatesare more sensitive to degradation.

UV-resins suitable for formulation of coat-ings for exterior application were identi-fied (Table 4). These include aliphatic ure-thane acrylates based on chemical back-bones that are stable towards degradationunder the influence of light and moisture.These resins have excellent colour andgloss retention properties. Di- and trifunc-tional products result in relatively lowcross link densities and are, in general,less sensitive to crack formation thanhigher functional products which result inmore densely cross linked systems. Never-theless, hexafunctional urethane acrylatesare indispensable in providing superiorscratch resistance.

In a comparative study, all the products,except "Ebecryl 1290" and "Ebecryl 5129",were diluted with 30% HDDA. HDDA is oneof the best performing diluting acrylatesfor exterior use. "Ebecryl 1290" and "Ebe-cryl 5129", both hexa-functional urethane

acrylates, were tested in a mixture with"Ebecryl 284" (20% addition of "Ebecryl1290" and "Ebecryl 5129".)

Figure 4 illustrates the monitoring of thecolour b value in the Xenon test for the 6urethane acrylates. The colour retention isexcellent - after a relaxation phase (b de-creasing), b values increase only veryslightly. The visual appearance of the firstcrack formation is also indicated for eachproduct. Pictorial standards are used tonote degradation with cracking (ASTMD660). "Ebecryl 284", "294/25 HD","1290" and "8402", which were also test-ed by natural exposure in Belgium andFlorida, are still free of cracks after threeyears.

Figure 5 illustrates the monitoring of thegloss in the Xenon test. In general thegloss retention is excellent up to 2000 h ofXenon exposure. Upon prolonged expo-sure the gloss decreases.

adhesion to many substrates. The combi-nation of this property together with thevery low viscosity and the advantages ofUV-technology make these structures verysuitable for either primer or topcoat.

This urethane technology provides versa-tile solutions combining different chainstructures. Such polymers combine hardsegments which could be urethane, urea,allophanates and soft segments such aspolyester, polyether, polycarbonate. Whencompared to traditional 100% UV systems,the UV-PUD's morphology could be adjust-ed depending on the degree of cross link-ing needed and the desired stiffness.

The adhesion of different radiation curablePUDs on different substrates has been test-ed. In addition to adhesion (12 µm dry coat-ing), the Persoz hardness (50 µm dry film onglass), flexibility (impact resistance of a 12µm film on aluminium) and solvent resist-ance (Acetone Double Rubs of a 12 µm filmon PC) is shown in Table 3.

UV resins for exterior applications

Damage of common plastics by solar radi-ation depends on the susceptibility of thepolymer type to solar UV-radiation as wellas the complexity of the weathering envi-ronment to which it is exposed.The magnitude of the damage can be con-trolled using stabilizers in the plastic com-position. Expansion of the use of radia-tion-cured coatings on plastics in exteriorapplications depends on the developmentof binders which combine excellent weath-ering performance with other increasinglymore stringent requirements.Accelerated weathering tests are widelyused to assess the weatherability of poly-mers. The most important parameter gov-erning the reliability of the various meth-ods is the nature of the light source em-ployed. The short wavelength emission ofUV-B lamps can cause unnatural accelera-tion or degradation whereas UV-A lampshave no output below the normal solarcut-off of 295 nm and may allow enhancedcorrelation with actual outdoor weather-ing. With appropriate filters, the Xenon arcspectral light distribution correspondswell to sunlight in both the UV and the vis-ible range. In combination with humiditycontrol features, this makes the Xenontest the most widely accepted standard.

0604

THE AUTHORS➤ Stefan Smeets, PhD, is Manager Technical

Service & Development Industrial Coatings Europe

at Surface Specialties UCB, responsible for

Plastics Coatings.

➤ Ir. Egbert Boerrigter, MBA, is Market Manager

Radcure Europe at Surface Specialties UCB.

➤ Stephan Peeters, PhD, is Manager Global New

Technology Development Radcure at Surface

Specialties UCB.

• With UV-technology, one is no longerlimited to coat plastics with solvent-based products.

• Diluting power, solubility, shrinkageand surface tension of diluting acry-lates have to be considered whenpreparing UV coatings for plastics.

• Several UV-resins provide a good ad-hesion to different plastic substrates.

• Radiation curable polyurethane dis-persions are suitable as primer andtopcoat to coat three-dimensionalplastic objects.

• Several aliphatic urethane acrylatesare suitable to coat exterior plastics.They are stable to degradation bylight or moisture and have also ex-cellent colour and gloss retentionproperties.

Results at a glance

Radiation curing


UV Coatings for Plastics

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