Radiation curing
UV coatings for pl plastics Stefan Smeets Egbert Boerrigt Boerrigter er Stephan Peeters Rapid curing curing,, no or very very low VOC VOC cont content ent , high performan performance ce properties properties - UV-techUV-technology offe offers rs a number number of advan advantage tages. s. On plastics, plast ics, thou though, gh, UV UV cura curable ble coatin coatings gs have been been difficult difficult to achieve, achieve, mainly mainly because of of infe inferior rior adhesi adhesion on to the subsubstrate strat e compared compared to solve solvent nt borne syssystems. Optimisation Optimisation of existing binder/rebinder/reactive diluent systems, however, however, can propro vide coatings for plastics with very good performances.
UV-curin UV-cu ring g tec techno hnology logy for industri industrial al coa coattingss has bee ing been n establ establish ished ed for for decad decades. es. The Th e adv advan anta tage gess in te term rmss of pro proce cess ssin ing g (high speed, smaller equipment), performance (scratch resistance, stain resistance, etc) and envir environment onmental al friendl friendliness iness are well well known. The use of UVUV-techn technology ology forr coat fo coatin ing g plas plasti tics cs ha hass be been en limi limite ted. d. However, Howev er, as this article aims to show, the transla tran slatio tion n of the advan advantag tages es of UV UV-te -techchnolo no logy gy in into to the the field field of pl plas asti ticc co coat atin ings gs has lead to an optimisatio optimisation n of exist existing ing UVresi re sins ns as we well ll as th the e de deve velo lopm pmen entt of ne new w UV-binders.
Figure 1: Effect of dilu Figure diluting ting acrylates acrylates on adhesion adhesion to several several subs substrat trates. es. Blue regions: highly preferr preferred. ed. Green regions: optional optional in balance with other properties
Tailor made solutions requir required ed The incre increasi asing ng use of pl plast astics ics as con constr strucuction materials materials in automotive, automotive, electronic electronic and telecommu telec ommunicat nication ion fiel fields ds impl implies ies that their coat co atab abil ilit ityy is be beco comi ming ng ve very ry im impo port rtan ant. t. However, Howev er, finding finding a universal coatin coating g for alll ty al type pess of pl plas asti tics cs is st stil illl a ut utop opia ia.. Th Thei eir r low surf surface ace ene energie rgiess and sen sensit sitivi ivity ty to temperature temper ature and solvent solvent make many many plastics diffic difficult ult to coat. The number of different mat materi erials als com compri prisin sing g homopoly homopolymer merss or blen blends ds of di diff ffer eren entt po pollym ymer erss is eno norrmouss and most mou most sub substr strate atess req requir uire e tailo tailorrmade solutions. Moreover, Moreov er, due to differe different nt intrin intrinsic sic properties withi within n a polymer family family,, different different properties properti es are targeted. Optimisati Optimisation on and fine-tuning fine-t uning of of well well-known -known plast plastics ics materi materi--
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Figure 2: UV-adhesion promoters promoters for plastics. Blue regions: main component; green regions: regions: optional additive; white white region: optional diluting acrylate
Radiation curing
als by blending or using additives to obtain better properties requires simultaneous adjustment of the corresponding coating formulations. The current trend towards cheaper polymers is primarily driven by their coatability, which allows to obtain the required surface aesthetics.
Figure 3: A schematic representation of a UV-PUD
With UV-technology, one is no longer limited to coat plastics with solvent-based products. Solvent coatings are inhibited by their high VOC-emissions, the spaceconsuming, long thermal drying tunnels required and their limited curing speed. Not only does UV-technology overcome these problems, it also offers other advantages such as the ability to coat heat-sensitive substrates as well as a performance improvement e.g. in scratch and stain resistance, which are becoming increasingly important.
Figure 4: Monitoring of the colour b values in Xenontesting. The marks denote the appearance of first cracks
UV Curing Technology UV-resins are well established for surface protection and decoration in many applications such as furniture, wood flooring, construction, graphic arts and electronics however, their use on plastic substrates is still limited. Despite the advantages of the UV-technology, equipment manufacturers, formulators and raw material suppliers face new challenges when attempting to introduce this technology further into the field of plastic coatings.
The chemistry used in UV-polymerisation is almost entirely based on acrylate functionalised materials. A UV formulation always contains the following ingredients: • Diluting acrylates and acrylated oligomers, which, as reactive materials, form the backbone of the coating after drying.
•Photoinitiators, which under exposure to UV-light form radicals that initiate a polymerisation reaction between diluting acrylates and acrylated oligomers. • Fillers and additives, which mostly do not participate in the polymerisation reaction and remain embedded in the cured network.
Table 1: Molecular weight, shrinkage, and surface tension of diluting acrylates
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Diluting Acrylate
Description
Molecular weight
Shrinkage (%)
Surface tension at 25°C (mN/m)
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
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
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Radiation curing
Diluting acrylates UV-coatings are, by their nature, solvent free, so they have to be formulated in a way which compensates for the lack of solvent in the adhesion on plastics substrates. This adhesion is difficult because of the inherent polymerisation process of the UV-resins.
Shrinkage, solubility and surface tension have to be considered The speed of the UV-polymerisation process produces internal stress in the coating which is not found in conventional thermal drying processes. When exposed to UVlight, the photoinitiator starts a chain reaction leading to polymerisation of the liquid coating accompanied by a decrease in volume and thus shrinkage. The liquid coating consists mainly of highly viscous oligomers and low viscosity monomers. The purpose of the diluting acrylates or monomers is the reduction of the viscosity of the total UV formulation. These diluting acrylates play the role of "reactive" solvents within conventional formulations. But as they react in the final polymer, they influence the final properties as well. Low viscosity acrylates with high functionality and low molecular weight will give high reactivity, high cross link density but also high shrinkage. Thus, they will decrease the adhesion of a UV-coating. Low viscosity acrylates with low functionality will give low reactivity, low cross link density and a high flexibility. The shrinkage of some diluting acrylates is shown in Table 1. The shrinkage is expressed as the change in density during curing of a formulation containing 100% resin and 5% photoinitiator. Although these values give a good indication of the intrinsic properties of a diluting acrylates, the behaviour in a final formulation depends on the other components as well. The solubility parameters of both polymeric substrate and formulation indicate whether adhesion is expected to be difficult or not. Generally, a diluting acrylate which can attack or swell a substrate will form an interpenetrating polymer network between substrate and coating, leading to an excellent adhesion. Thus, diluting acrylates such as oxyethylated phenol acrylate, DPGDA or HDDA are able to attack and swell polycarbonate.
Figure 5: Gloss monitoring in the Xenon test
Table 2: Viscosity of UV resins for adhesion on plastic substrates Resin
Description
“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
“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)
Another important parameter to consider is the surface tension of the substrate. Good adhesion is obtained when the surface tension of the substrate is higher than that of the coating, giving an optimal wetting of the substrate. The surface tension of typical diluting acrylates is also shown in Table 1. The surface tension of common plastic substrates may vary from 20 mN/m ("Teflon") to 44 mN/m (PET). In addition, a wide range of surface tensions may be encountered for one specific substrate material, due to different surface treatments. For example, untreated polypropylene may have a surface tension of 28 mN/m whereas corona treated PP can go up to 40 mN/m. A surface treatment with corona or flame results in the formation of polar groups on the substrate.
Hoppler Viscosity, mPa s ( T in °C)
20,000 (25)
Different substrates demand different solutions A formulation leading to a good adhesion on a particular plastic may not deliver good performance on another member of the same plastic family. The ability of several diluting acrylates to serve as an adhesion promoter in a coating is shown in Fig- ure 1. In the blue region one can find the preferred diluting acrylates for given substrate materials, whereas the products in the green region are used in balance with other properties such as diluting power and reactivity. Due to its capacity to swell the plastic surface, HDDA is an interesting adhesion promoter. In the case of PC or impact resistant
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Radiation curing
Table 3: Adhesion performance of some “Ucecoat” UV-PUD's. 5B = perfect adhesion Dispersions
Persoz har dness (s)
Solvent resistance (ADR), 12 µm on PC
Flexibility (impact, cm)
PVC
PC
PP (Corona)
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
PS, only a small amount of HDDA is needed in a formulation, whereas for harder PS higher quantities are necessary. Because of its low surface tension, ODA is a very useful diluting acrylate for PP and PE. ODA is mostly used in small amounts because of its incompatibility with many acrylated oligomers at high concentration. "Ebecryl 1039" is preferred over "Ebecryl 40" when a higher flexibility is required. TCDA is a difunctional diluting acrylate for rigid SMC/BMC as it has a low shrinkage combined with an excellent hardness.
UV adhesion promoters Diluting acrylates in a UV-formulation reduce the viscosity and increase the adhesion on several plastic substrates. However, they can reduce the high performance of an UV coating. Thus, in order to limit the percentage of diluting acrylates in a UV formulation, one can switch from a one layer to a multi-layer approach with primer, basecoat and topcoat. As the primer is responsible for the adhesion to the substrate, both basecoat and topcoat may contain less diluting oligomers. Almost all automotive paints use the primer/basecoat/clearcoat build-up in order to fulfil the high quality requirements. The basecoat contains the colour pigment, while the topcoat will provide the scratchresistance etc. This multi-layer approach makes it easy to switch from one colour to another without changing the primer and topcoat. The use of one UV-primer for several plastic parts could simplify the coating process considerably. Several UV-resins ( Table 2 ) provide a good adhesion to different plastic substrates. In practice, however, as a plastic can vary from one supplier to another, can be filled or non-filled, can be a blend of two miscible or compatibilised poly-
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Table 4: Aliphatic urethane acrylates for exterior application
mers, the value of these resins has to be evaluated with experimental work on each plastic. The ability of these resins to serve as an adhesion promoter in a primer and/or in a coating is shown in Figure 2. As the adhesion of UV-resins towards PC, PVC and PMMA is mostly obtained with a small amount of diluting acrylate, no adhesion promoter is required. Surface treatments, such as corona, flame, plasma or IPA cleaning are advised to activate the surface but also to remove processing additives and contamination. Figure 2 is composed of three areas (blue, green, white), and should serve as a guideline to coat several plastic substrates. The resins in the blue region represent the major component in a formulation, which can be diluted with one of the diluting acrylates from the white region. Thus, "Ebecryl 740-40TP", "767" and "745" are acrylic acrylates which give good adhesion to many plastics mainly due to their low functionality and thus shrinkage. "Ebecryl 303" is a polymeric resin diluted in HDDA that shows good adhesion in combination with acrylic acrylates. "Ebecryl 436", "438" and "584" are chlorinated polyester resins diluted in TMPTA, “OTA 480” and HDDA, respectively. They have a good reactivity combined with a very low shrinkage (< 4 %).
The amount of diluting acrylate must be chosen depending on the required viscosity, but also the rigidity of the substrate. To increase the reactivity one can take a higher functional diluting acrylate, or one can add "Ebecryl 7100", which is a low viscosity amine functional acrylate. "Ebecryl 168" and "170" are methacrylated or acrylated acidic components which are able to etch the surface of a filled plastic. The efficacy of the UV-resins to adhere to a plastic depends on the application process and the contact and drying time. The bestway to evaluate this is through industrial trials.
Low viscosity, high performance UV-resins The design of low viscosity, high performance resins for spray application to coat three-dimensional objects has led to a new generation of binders, radiation curable polyurethane dispersions (UV-PUD) ( Figure 3 ). The reaction of diisocyanates (shown in blue in the diagram) with different polyols (polyester, polyether, polycarbonate) results in a polyurethane which forms the basic chemistry of a UV-PUD. Because of their relatively high molecular weight, UV-PUDs show almost no shrinkage after curing, resulting in an excellent
Radiation curing
adhesion to many substrates. The combination of this property together with the very low viscosity and the advantages of UV-technology make these structures very suitable for either primer or topcoat.
THE AUTHORS
This urethane technology provides versatile solutions combining different chain structures. Such polymers combine hard segments which could be urethane, urea, allophanates and soft segments such as polyester, polyether, polycarbonate. When compared to traditional 100% UV systems, the UV-PUD's morphology could be adjusted depending on the degree of cross linking needed and the desired stiffness.
➤ Ir. Egbert Boerrigter, MBA, is Market Manager
The adhesion of different radiation curable PUDs on different substrates has been tested. In addition to adhesion (12 µm dry coating), the Persoz hardness (50 µm dry film on glass), flexibility (impact resistance of a 12 µm film on aluminium) and solvent resistance (Acetone Double Rubs of a 12 µm film on PC) is shown in Table 3 .
UV resins for exterior applications Damage of common plastics by solar radiation depends on the susceptibility of the polymer type to solar UV-radiation as well as the complexity of the weathering environment to which it is exposed. The magnitude of the damage can be controlled using stabilizers in the plastic composition. Expansion of the use of radiation-cured coatings on plastics in exterior applications depends on the development of binders which combine excellent weathering performance with other increasingly more stringent requirements. Accelerated weathering tests are widely used to assess the weatherability of polymers. The most important parameter governing the reliability of the various methods is the nature of the light source employed. The short wavelength emission of UV-B lamps can cause unnatural acceleration or degradation whereas UV-A lamps have no output below the normal solar cut-off of 295 nm and may allow enhanced correlation with actual outdoor weathering. With appropriate filters, the Xenon arc spectral light distribution corresponds well to sunlight in both the UV and the visible range. In combination with humidity control features, this makes the Xenon test the most widely accepted standard.
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➤ Stefan Smeets, PhD, is Manager Technical Service & DevelopmentIndustrial Coatings Europe at Surface Specialties UCB, responsible for Plastics Coatings.
Radcure Europe at Surface Specialties UCB.
➤ Stephan Peeters, PhD, is Manager Global New Technology Development Radcure at Surface Specialties UCB.
Extensive test experience has been gained with different types of accelerated tests and cycling conditions. This paper only concentrates on the Xenon test, using a Xenon “WeatherOMeter”, according to the ASTM G26 method. Because of the huge variety of plastics, it was decided to test the UV clearcoats on weather stable white thermoset polymer coated panels at 20 µm coating thickness. A visual evaluation was carried out along with colour and gloss monitoring during the tests. Aliphatic urethane acrylates in general outperform other oligomer types in both accelerated and natural weathering tests. Aromatic polymers, on the other hand, are inherently not stable to light and tend to yellow and degrade rapidly. Also conventional polyether and polyester acrylates are more sensitive to degradation. UV-resins suitable for formulation of coatings for exterior application were identified ( Table 4 ). These include aliphatic urethane acrylates based on chemical backbones that are stable towards degradation under the influence of light and moisture. These resins have excellent colour and gloss retention properties. Di- and trifunctional products result in relatively low cross link densities and are, in general, less sensitive to crack formation than higher functional products which result in more densely cross linked systems. Nevertheless, hexafunctional urethane acrylates are indispensable in providing superior scratch resistance. In a comparative study, all the products, except "Ebecryl 1290" and "Ebecryl 5129", were diluted with 30% HDDA. HDDA is one of the best performing diluting acrylates for exterior use. "Ebecryl 1290" and "Ebecryl 5129", both hexa-functional urethane
acrylates, were tested in a mixture with "Ebecryl 284" (20% addition of "Ebecryl 1290" and "Ebecryl 5129".) Figure 4 illustrates the monitoring of the colour b value in the Xenon test for the 6 urethane acrylates. The colour retention is excellent - after a relaxation phase (b decreasing), b values increase only very slightly. The visual appearance of the first crack formation is also indicated for each product. Pictorial standards are used to note degradation with cracking (ASTM D660). "Ebecryl 284", "294/25 HD", "1290" and "8402", which were also tested by natural exposure in Belgium and Florida, are still free of cracks after three years. Figure 5 illustrates the monitoring of the gloss in the Xenon test. In general the gloss retention is excellent up to 2000 h of Xenon exposure. Upon prolonged exposure the gloss decreases.
Results at a glance • With UV-technology, one is no longer limited to coat plastics with solventbased products. • Diluting power, solubility, shrinkage and surface tension of diluting acrylates have to be considered when preparing UV coatings for plastics. • Several UV-resins provide a good adhesion to different plastic substrates. • Radiation curable polyurethane dispersions are suitable as primer and topcoat to coat three-dimensional plastic objects. • Several aliphatic urethane acrylates are suitable to coat exterior plastics. They are stable to degradation by light or moisture and have also excellent colour and gloss retention properties.
