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Formulations
C HAPTER 5
Formulations Formulations used for radiation curable systems depend on the specific performance requirements of the coatings and have to be adjusted to the application techniques (e.g., viscosity needs). Typically, formulations for UV curable coatings contain 25–90% oligomeric resins, 15–60% reactive diluents, 0.5–8% photoinitiators, 1–5% additives, like levelling agents, defoamers, and optionally pigments, fillers and matting agents. The application fields are very wide and therefore the requirements on the coating properties differ very much. A coarse overview of main applications is given in Figure 5.1. From this broad span of applications it is obvious that formulations also differ very much in their chemistry and composition. Classical applications in wood and paper coating often require highly crosslinked coatings for abrasion, scratch and chemical resistant surfaces. Whereas for future applications, such as exterior wood coating, like window frames, or coil coating
F IG . 5.1. Application overview of uses of UV curable formulations. 140
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FORMULATIONS
TABLE 5.1. General composition of wood coatings and function of the components Component
(%)
Function
Oligomer resin
25–95
Film formation Basic performance properties
Reactive diluent (monomers)
0–60
Film formation Viscosity adjustment
Photoinitiator
1–5
Initiation of curing; speed
Fillers/pigments
0–50
Increased abrasion resistance Increased scratch resistance Cost reduction (e.g., talc) Coloration
Matting agents Additives
Gloss reduction 0–3
Defoaming Wetting, levelling, slip, . . .
much more flexible coating properties are needed. A general discussion about all types of coatings for metal and non-metal substrates including major performance requirements is given in reference1 (Chapters XXXIII and XXXIV). Formulations available to the paint users are developed and tailored by the paint manufacturers (e.g., BASF Coatings, DuPont Performance Coatings, Kansai Paint, PPG for automotive; AKZO, Beckers, DuPont, Valspar, Sherwin Williams for industrial and wood coatings; Dainippon Inks, Sun Chemicals and Flint for graphic arts), who usually do not publish the exact recipes. However, for starting new developments guiding formulations are often made available by raw material producers (BASF AG, Cognis, Cytec Surface Specialities, Eternal and Sartomer) and Paint Research Institutes. Despite published already more than 20 years ago, a basic understanding of UV curable formulations for coatings, inks and paints has been compiled by Holman.2 The purpose of this chapter is to give a brief insight into the formulations for the classical applications and discuss the development of formulations for future applications, such as in food packaging, as well as in exterior uses in automotive, wood or coil coating.
5.1
FORMULATIONS FOR WOOD COATINGS
Examples given below are formulations of high gloss topcoats on wood surfaces. In the past styrene based unsaturated polyesters were used which are still common in some countries, but the more versatile and faster curable acrylics are becoming dominant. Polyester or epoxy acrylates either alone or in combination in amounts up to 60%, diluted with 35% of monomers are mainly used for wood coatings. For parquet and flooring applications the tougher and more abrasion resistant urethane acrylates along with monomers are also employed. A general formulation for wood coatings is shown in Table 5.1, further explaining the typical functions of the ingredients.
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TABLE 5.2. Layer composition of parquet flooring coating Layer
Approx. amount (g/m2 )
Curing
1. Waterbased adhesive primer 2. Undercoat (ctg., corundum) 3. Undercoat (ctg., corundum) 4. Undercoat 5. Top coat
10 20 20 20 5–10
Gellation (incomplete cure) Gellation Gellation Fully cured + grinding Fully cured
TABLE 5.3. Composition of a waterbased adhesion primer for wood Component
(%)
Type
Oligomer resin
50
Emulsion or dispersion Photoinitiator Additives
50 1–3 1–3
Watersoluble 100% UV resin; aliphatic epoxy acrylate, polar polyester acrylate or urethane acrylate Polyester acrylate emulsion (50% solids) All Defoaming, wetting
5.1.1
Industrial Parquet Coating
In order to be able to meet the various requirements of parquet flooring, the construction of prefabricated (multilayer) parquet needs a combination of different layers. The composition of these layers depends on the production line available as well as on the substrate quality. Regardless of the solution chosen, it has to guarantee good adhesion to the substrate, good interlayer adhesion, low abrasion as well as high scratch resistance and high resistance against chemicals, like cleaning agents, red wine, coffee and other typical household chemicals. A typical coating layer construction is shown in Table 5.2.
5.1.2
Adhesion Primer (to the Wooden Substrate)
In order to enable a good adhesion to the wooden substrate, mainly waterbased UV curable primers are used. The main function of this waterbased primer is swelling of the wood, which results in the erection of wood fibres, further increasing the available surface and favouring an entanglement of the wood fibres by the coating to improve the adhesion. A typical formulation used for waterbased primers is given in Table 5.3. In order to improve the interlayer adhesion two strategies are pursued, the incomplete curing of the primer, which results in a gellation of the layer, but only to such an extent, that there are sufficient double bonds remaining for the reaction with the next UV curable layer, and second the grinding of the primer layer in order to increase the roughness and therefore the surface area of the layer.
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FORMULATIONS
TABLE 5.4. Topcoat formulations Component
Oligomer resin
Monomer containing
Monomer free
(%)
Type
(%)
Type
55
Epoxy acrylate, Laromer® LR 8986
43
Epoxy acrylate, Laromer® LR 8986 Polyetheracrylate, Laromer® LR 8967
43 Diluent
26.5
Tripropylenegylcoldiacrylate (TPGDA)
Matting agents
10
7 Talcum 10MOOS 3 Syloid® 162C
Wax
3
Ceraflour® 950
Photoinitiator
3
Irgacure® 500
Synergist
2
Amine-synergist, Laromer® LR 8956
Additives
0.5
Defoamer, Tego® Airex 920
– 10
Syloid ED 80
3.5
Iragcure® 184
0.5
Levelling agent, Byk 361
In the production of top-quality parquet up to three undercoat layers are used. The first two can be formulated with very hard abrasive fillers (e.g., corundum) and are only cured to a low conversion, whereas the third one does not contain the filler. After full curing of the three layers, a consecutive grinding step to prepare the surface for topcoat application is added, where fillers in the third layer would destroy the grinding belt. Finally, the topcoat finishes the construction of the parquet coating. Topcoats for parquet can be based on the combination of trifunctional polyether and polyester acrylate resins,3 as well as on epoxyacrylates, which provide high chemical stability. Besides the high gloss formulations also matt top coats are used. Although matt formulations are difficult to obtain with UV curable 100% liquid formulations, formulations have been developed which yield topcoats with matt appearance (Table 5.4). Such formulations developed for roller coater applications contain matting agent (silicagel and talc), small amounts of solvent can also be used. Such formulations for example are applied in a thickness of about 8–12 µm (8–12 g/m2 ), and exposed to a 120 W/cm lamp at a conveyor belt speed of 5 m/min. The resulting abrasion resistant coating exhibit gloss values of about 30% (Gardner, 60◦ ). Further starting formulations for obtaining high scratch or abrasion resistance have been published.4,5 Special formulations for obtaining high scratch resistance (on beech wood, for example) use large amounts of highly functionalized resins6 or are formulated with nanoparticles (Table 5.5).
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
TABLE 5.5. Highly scratch resistant top coat formulations Component
(%)
Type
Formulation based on high functional acrylates Dipentaerythrolpenta-acrylate Propoxylated neopentylglycol-diacrylate Urethane acrylate Photoinitiator Photoinitiator Oligomer resin
Sartomer® SR 399 Sartomer® SR 9003 Sartomer® CN 965 Benzophenone Irgacure® 184
57 28.5 9.5 3 2
Formulation based on nanoparticles 30
Diluent
22
Nanoparticles (dispersed in resin) Matting agent Photoinitiator Additives
35 9 3.5 0.4 0.4
Epoxy acrylate, Laromer® LR 8986 Polyetheracrylate, Laromer® LR 8967 Laromer® PO 9026V Syloid® ED 80 Irgacure® 184 Levelling agent, Byk 361 Defoamer, Tego® Airex 920
TABLE 5.6. Composition of a typical printing ink formulation Component
(%)
Function
Oligomer resin
40–60
Film formation Basic performance properties
Reactive diluent
10–20
Film formation Viscosity adjustment
Photoinitiator
3–8
Initiation of curing; speed
Pigments or dyes
15–25
Colour image formation
Additives
0–3
Surfactants Waxes
5.2 FORMULATIONS FOR GRAPHIC INKS UV curable systems used in graphic arts applications are divided into the categories of printing inks, containing pigments or dyes, and clear coat overprint varnishes (OPV). Ink formulations have to deal with two main requirements, the performance requirement of the printing process, like offset, letterpress or screen printing, which for instance require different pigmentation and viscosities and the performance requirement of the final printed image.
145
FORMULATIONS
TABLE 5.7. Beige pigmented UV ink Component
(%)
Function
Oligomer resin
75
Polyesteracrylate (Laromer® LR 8800)
Reactive diluent
8.82 4.4
Hexanediol diacrylate Trimethylolpropane triacrylate
Photoinitiator
0.75
Bisacylphosphineoxide (Irgacure® 819, Ciba)
Photoinitiator
1.23
Irgacure® 184 (Ciba)
Pigment
8.82
Titanium dioxide, Rutil
Pigment
0.98
Isoindoline (PY 110; Irgazingelb RTL, Ciba)
TABLE 5.8. UV offset ink UV offset ink components
Yellow (wt%)
Magenta (wt%)
Cyan (wt%)
Black (wt%)
Pigment Permanent yellow GR 01 Litho Rubin D 4574 DD Heliogen® Blue D 7092 Special black 250
15.8 – – –
– 17 – –
– – 17 –
– – 2 15
Resin Laromer® LR 9013 Laromer® LR 9004 Laromer® LR 8986
18.5 40 15.7
20 20 33
20 43 10
20 43 10
4 4 2 100
4 4 2 100
4 4 2 100
4 4 2 100
44 180 40
39 170 60
45 175 55
40 100 50
Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)
5.2.1 Printing Inks (UV Offset, UV Flexo and UV Screen Inks) The film thicknesses of the printing inks are in the range of up to 3 µm for offset and flexo inks and up to 15 µm for screen inks. The printing processes are discussed in Chapter 8.1.2. A general formula for printing inks contains about 40 to 60% resins, and between 15 and 25% of colorant (pigment or dye) as shown in Table 5.6. A specific example of beige pigmented UV ink is given in Table 5.7 (ref. 3). The pigmented UV coating relies on a mixture of photoinitiators, where the short wavelength absorbing Irgacure® 184 is responsible for surface cure and the longer wavelength
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
TABLE 5.9. UV flexo ink UV flexo ink components
Yellow (wt%)
Magenta (wt%)
Cyan (wt%)
Black (wt%)
Pigment Permanent yellow GR 01 Litho Rubin D 4574 DD Heliogen® Blue D 7092 Special black 250
15.5 – – –
– 15 – –
– – 15 –
– – 2 13
Resin/diluent Laromer® LR 9013 Laromer® PO 94F Dipropyleneglycol-diacrylate
15.5 45.6 13
15 51.6 8
15 51.6 8
15 51.6 8
0.2 0.2
0.2 0.2
0.2 0.2
0.2 0.2
Additives Levelling, Byk 307 Defoamer, Foamex N Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)
4 4 2 100
4 4 2 100
4 4 2 100
4 4 2 100
2.9 41 >65
2.0 4 >65
3.2 36 >65
2.4 20 >65
absorbing acylphosphine oxides (either Irgacure® 819 or Lucirin® TPO can be used) enables the throughcure. The coating is exposed to two ferrum added lamps (80 W/cm) at curing speed up to 10 m/min. Starting formulations for further optimization according to the specific needs are given for UV offset, UV flexo and UV screen inks, recommended for paper, board and film applications (e.g., BASF brochure: Make up your print products7 ) in Tables 5.8–5.10. The formulations contain either yellow, red, blue and/or black pigments, usually a grinding resin, like Laromer® LR 9013, polyester or epoxy acrylate binder resins, a photoinitiator combination containing standard photoinitiators facilitating surface cure and an acylphosphine oxide type photoinitiator (e.g., Lucirin® TPO-L), responsible for through cure, and optionally levelling agents and defoamers.
5.2.2
Formulation for Overprint Varnishes (OPV)
Overprint varnishes are applied to impart high gloss and protection to a printed surface, like post cards, catalogue covers, cosmetic cartons, etc.
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FORMULATIONS
TABLE 5.10. UV screen ink UV screen ink components Pigment Heliogen® Blue D 7092 Special black 250 TiO2 Kronos® 2063S Resin Laromer® LR 9013 Laromer® LR 9019 Laromer® UA19T Laromer® LR 8986 Dipropyleneglycol diacrylate
Cyan (wt%)
Black (wt%)
White (wt%)
5 – –
1 4 –
– – 34
5 – 54 7 15
5 – 54 7 15
– 37 – 12
Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Darocure® 1173
1.5 1 0.5 2
1.5 1 0.5 2
4 – – 3
Co-initiator Laromer® LR 8956
6
6
–
2 1 – 100
2 1 – 100
3 6 1 100
Additives Aerosil® 200 CAB 551-001 (20% in DPGDA) Byk® 164 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)
4 – –
3.7 – –
3.5 15 60
The basic performance requirements on overprint varnishes are characterized by: • • • •
High reactivity; High gloss; Excellent wetting of the ink; Low price.
These overprint varnishes are applied with a thickness of about 6–10 µm (g/m2 ) typically in a coating unit as part of a printing press or in a separate step on a coating machine. A typical formulation is based on a high content of multifunctional crosslinkers (for high reactivity), oligomeric resins (epoxy-, polyester- or amine-modified polyether acrylates) and high photoinitiator amounts (Table 5.11). Thus, a general formula for overprint varnishes contains about 30–70% of highly functional reactive diluent, 10–20% resins and photoinitiator amounts up to 8% and often also amine-synergist, in order to obtain high cure speeds.
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
TABLE 5.11. General composition of an overprint varnish formulation Component
(%)
Function
Oligomer acrylate resin
10–20
Film formation Basic performance properties
Reactive diluent
20–30
Film formation Viscosity adjustment
Trifunctional X-linkers
30–70
Coating properties
Photoinitiator
3–8
Initiation of curing; speed
Photosynergists
2–5
Cure speed increase
Additives
0–3
Surfactants Wetting, levelling agent, slip, . . .
TABLE 5.12. Comparison of standard OPV formulation and OPV for inert exposure Component
OPV (%)
Type
Standard
Inert
Oligomer resin
52
52
Epoxy acrylate (high reactivity and chemical resistance), Laromer® LR 8986
Reactive diluent
35
35
Viscosity adjustment, tripropyleneglycol diacrylate (TPGDA)
Photoinitiator
8 –
– 1
Benzophenone Lucirin® TPO
Additives
0.4
0.4
Surfactant and slip additives
In order to reduce the photoinitiator content, for example for formulations, which are designed for packages with indirect food contact (where only very small amounts of migratables are tolerated), curing can be done under inert conditions. The same cure speed can be achieved with formulations containing only 1% photoinitiator compared to 8% when curing under air (Table 5.12). Furthermore, due to the reduced photoinitiator contents, migratables and odour are reduced with such formulations cured under inert conditions.
5.3 SPECIALTY FORMULATIONS (FOR GLASS, POLYCARBONATE, METAL) In the following three examples of specialty formulations applicable as starting formulations for coating glass, plastics or metal surfaces are given.
149
FORMULATIONS
TABLE 5.13. Scratch resistant topcoat for glass Component
(%)
Type
Dipentaerythrolpenta-acrylate Propoxylated neopentylglycol-diacrylate Urethane acrylate Photoinitiator Photoinitiator
64.2 16.4 10 7 2
Sartomer® SR 399 Sartomer® SR 9003 Sartomer CN 965 Benzophenone Darocure® 1173
TABLE 5.14. Flexible starting formulation for UV coating of polycarbonate Component
(%)
Type
Aliphatic epoxyacrylate Trimethylolpropane formal-monoacrylate
60 10
Sartomer® CN 132 Sartomer® SR 531 or Laromer® LR 8887
1,6-Hexanediol diacrylate (HDDA) Photoinitiator Photoinitiator
25 2 3
Irgacure® 184 Darocure® 1173
TABLE 5.15. Formulation for cationic metal coatings Component
(%)
Function
Epoxy resin Inert flexibilizer Photoinitiator
60–70 20–30 1–3
Dow UVR-6160 Polycaprolactone triol Sulfonium salt (e.g., Dow UVI-6990) or Iodonium salt (e.g., Irgacure® 250)
Pigments or dyes Additives
1–2 0–3
Surfactants
Coatings suggested for high scratch resistance on glass (ref. 6) are based on high functional pentaerythrol pentaacrylate (Table 5.13). The reported thickness was about 100 µm and no basecoat was used. The starting formulation proposed for coating plastic polycarbonate (ref. 6) is much more flexible than the formulations described before. It is pointed out, that the use of a flexible basecoat improves abrasion resistance, as well as adhesion and flexibility. Since cationic coating systems based on epoxy resins exhibit little shrinkage, the adhesion of such coatings on metal surfaces is generally better than that of acrylate based formulations. Most of the cationic coatings rely on epoxy resins, flexibilizers and sulfonium or iodonium salts as photoinitiators (Table 5.15). 5.4 FORMULATION SCREENING FOR NEW APPLICATIONS Frequently asked questions are about how to start the formulation screening for a new application. For the traditional applications, like wood, plastic coatings and graphic arts
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
the formulation examples discussed before can be used as a starting point. Some general considerations for the design and selection of raw materials for formulations are given in Figure 5.2. For interior applications, the selection of the raw materials is governed by the demands of the application segment, the required properties and the application method. There are no overall general restrictions. On the other hand, exterior applications and UV curing of pigmented systems require a careful selection of raw materials as well as of the exposure equipment. Since pigments absorb more or less intensely in the UV, the photoinitiators and the lamp have to be tuned to allow enough through-cure. Thus, usually a combination of photoinitiators, a standard hydroxyalkyl ketone (HAK) for surface cure and an Acylphosphine oxide (APO) type for through-cure, is selected. The exposure lamp also should exhibit considerable output at longer wavelength, as do gallium or ferrum added mercury lamps (see Figure 2.21). For exterior applications the selection of all components is critical. The resins have to be “weather-resistant”, particularly oxidative and hydrolytically stable. Hence, the only appropriate resin types are aliphatic urethane acrylates (UA) or acrylated polyacrylates (APA), to be used in combination with hydrocarbon type acrylate diluents (e.g., hexanediol diacrylate, decanediol diacrylate). In order to stabilize the coating and the substrate against degradation and discolouration, the coatings have to contain UV absorbers and radical scavengers, usually of the hindered amine light stabilizer (HALS) type. Since these compounds, similar to pigments, also absorb in the UV, the photoinitiator package is also composed of a (non-yellowing) surface cure type (e.g., Irgacure® 184) and an acylphosphine oxide type photoinitiator. The exposure lamp is preferably of the ferrum added mercury type. The main performance requirements in wood coatings are good adhesion (flexible waterbased primers or reactive acrylates, like isocyanato acrylates) and especially for parquet coatings high abrasion resistance (corundum fillers). The graphic applications often ask for high reactivity (high functional oligomers or amine modified resins) and high gloss. Can and coil coating as well as plastics demand excellent adhesion (low shrinkage, reactive primers) and good flexibility (low functional higher molecular weight resins and/or monofunctional reactive diluents). High scratch resistance can be obtained with high crosslink densities and/or added nanoparticles. High hardness can be achieved with high Tg coatings (high crosslink density or rigid components) and high flexibility by low crosslink density (high molecular weight between crosslinks) and flexible chains (polyether, polysiloxanes). Last but not least the desired application method determines the choice of the raw materials. Powder applications require a glass transition temperature of the powder above room temperature, preferably above 40 ◦ C, and melting temperatures below 120 ◦ C. Thus, high Tg resins and/or crystalline resins/diluents have to be used in combination with low volatile, solid photoinitiators. In liquid systems, the application viscosity decreases from roller over curtain and vacuum coating to spray coatings. The viscosity adjustment can be done preferably with reactive diluents, however, where low molecular weight monomers are undesirable or prohibited (porous substrates, spraying), solvents or waterbased emulsions or dispersions may be used.
FORMULATIONS
F IG . 5.2. Considerations for selection of starting formulations.
151
152 UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.3. Schematic structures of flexible and highly crosslinked urethane acrylate resins and different functional diluents for formulation screening with high-throughput methods.
153
FORMULATIONS
TABLE 5.16. HTS set-up for screening urethane acrylates Varied components
Components kept fixed in all formulations
Binder
Reactive thinner
Reactive thinner (wt%)
Photoinitiator
Additives
UV protection
UA 1 ↓ UA 26
TMPMFA HDDA TMPTA
0, 10, 30, 50 0, 10, 30, 50 0, 10, 30, 50
3.5% Irgacure® 184 0.5% Lucirin® TPO
1% defoamer 0.3% levelling
1.3% UV absorber 0.7% HALS
5.5
INCREASING EFFICIENCY IN FORMULATION SCREENING: HIGH-THROUGHPUT SCREENING (HTS)
In order to illustrate the method of high-throughput screening (HTS), the course of the development of weathering and scratch resistant aliphatic urethane acrylate resins is given in the following example.8 The screening was based on the formulation of aliphatic urethane acrylate resins together with reactive diluents of varying functionality. The aliphatic urethane acrylate resins used were synthesized by the reaction of a low viscous isocyanato-acrylate (ICA: Laromer® 9000 (BASF AG)) with commercially available diols and hydroxyalkyl acrylates. Molecular weight and acrylate functionality of the urethane acrylates was varied systematically. Twenty-six different aliphatic urethane acrylates were synthesized and formulated with the 3 reactive thinners as shown in Figure 5.3. The combination of 26 UA resins with the 3 diluents in 4 different concentrations resulted in 260 formulations. As fixed components 4 wt% photoinitiator (mixture of Irgacure® 184, Lucirin® TPO), 1 wt% defoamer, 0.3% levelling additive and 1.3 wt% UV absorber and 0.7 wt% HALS (UV protection) were added to the formulations (Table 5.16). The workflow of the high-throughput formulation testing is elucidated in Figure 5.4. This tool setup can be used for formulation screening with already existing raw materials, however, a parallel synthesis robot can also be applied to synthesize tailor-made new components (resins) in a very efficient way before the formulation step. A modular coatings robot (Figure 5.6) was used to dispense the raw materials and stir the formulations, then to draw down the coating films and finally to cure them using UV light. Various semiautomated application tests yielded materials properties like hardness, elasticity, scratch and chemical resistance as well as UV conversion. Sophisticated data management supported the design of experiments, configures the robot, captures the recipes, process parameters and measurement results in a database and offers data visualization. This allows selection of promising formulations and to harvesting of structure property correlations of coating properties, process parameters and chemistries applied. In subsequent steps more focused libraries can be generated. The resins can be synthesized in a classical manner or with a HTS reactor with automated dosing and parallel setup as shown in Figure 5.5. The formulations were coated with the HTS equipment shown in Figure 5.6 onto glass substrates by doctor blading (slit width 200 µm) with a wet paint thickness of 150 µm. After 15 min flash-off at room temperature the coatings were subjected to a 20 min treatment at 100 ◦ C. Subsequently, UV curing was performed at 60–80 ◦ C with 5 J/cm2 under
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.4. Scheme of high throughput formulation testing.
F IG . 5.5. Parallel reactors for synthesis.
inert conditions in a CO2 bath in order to eliminate oxygen inhibition. All the properties were measured with the HTS equipment (Figure 5.7). The mechanical properties of the cured films, like hardness and elasticity, were determined by Fischerscope® measurements
FORMULATIONS
F IG . 5.6. HTS parallel coating equipment.
F IG . 5.7. HTS combinatorial application testing.
155
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.8. Determination of hardness and elasticity (fischerscope® ).
F IG . 5.9. Fischerscope measurement (hardness versus elastic work fraction) of highly X-linked UA formulations from HTS.
FORMULATIONS
157
F IG . 5.10. Fischerscope measurement (hardness versus elastic work fraction) of UA formulations from HTS of all 260 formulations.
(Figure 5.8). Scratch resistance was evaluated by a sand scrub test. UV conversion was determined by confocal Raman microscopy at the surface (0 µm) and 15 µm layer depth. Chemical resistance was evaluated for H2 SO4 , NaOH, Pancreatin, tree resin and H2 O at two different temperatures each. At first glance, one might be overwhelmed by the flood of data generated from the screening. With visualization tools, however, interesting correlations can be revealed. A plot of hardness vs. elasticity, as shown in Figure 5.9, for a subset of highly crosslinked urethane acrylate resins (see Figure 5.3, top right) as a function of the used hydroxyacrylate building block on one hand and the type (i.e., functionality) and concentration of the reactive thinner on the other hand. The picture becomes even more complex (Figure 5.10), if additionally urethane acrylate resins are incorporated which contain flexibilizing diols in the center of the resin molecule (see Figure 5.3, top left). From the data in Figure 5.10 molecular resin structures can be identified which result in coatings of high elastic work (>70% of total) at low hardness (highly flexible systems) or at high hardness (highly crosslinked coatings). The evaluation of the scratch testing showed high scratch resistance for the coatings which exhibited high elastic work, either the flexible or the highly crosslinked. The chemical resistance data (Figure 5.11) were plotted in the same type of graph (hardness versus elastic work) and the differentiation of good and bad chemical resistance is shown by the shape (filled circle, empty circle and plus (+), respectively). The results demonstrate that the best chemical resistance is in the upper right corner of every plot, thus produced by the coatings with high crosslink density. The highly crosslinked urethane acrylate coatings identified by the HTS screening have then been subjected to intensive classical scratch resistant tests. According to a model published by Jones et al.9 , there are three different ways
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.11. Chemical resistance results of UA formulations from HTS (the formulations are plotted the same as in diagram in Figure 5.10).
TABLE 5.17. “Three response, two mechanism model” for evaluation of scratch resistance Responses to marring stress
Mechanisms of marring
Mechanisms of healing
Fracture Plastic deformation Elastic deformation
Fracture Plastic deformation
Viscoelastic creep
coating surface can respond to marring stress, addressing two different mechanisms of marring, and one mechanism of healing. In this model, fracture and plastic deformation lead to observable marring, while elastic deformation does not. Instead, elastically deformed mars recover their original dimensions almost instantaneously. Thus, the three responses in this model lead to only two marring mechanisms (Table 5.17). According to the model described above and the results of the high-throughput screening, the urethane acrylates which combine a high ratio of elastic work (We /Wtot ) and high universal hardness (Hu ) in Fischerscope testing should exhibit a very interesting combination of hardness and elasticity.10 After further optimization of the respective formulations these products have been tested as automotive clear coats, especially on plastic parts against the conventional two pack (2C PU) formulations. The results are shown in Table 5.18. Such highly crosslinked coatings based on aliphatic urethane acrylates, identified by the high-throughput screening, showed excellent chemical resistance and extreme scratch resistance.
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FORMULATIONS
TABLE 5.18. Applications test for automotive clear coats High functional aliphatic urethane acrylate (UV) based on Laromer® 9000 3.5% Irgacure® 184/0, 5% Lucirin® TPO, UV curing Larolux® Scratch resistance Initial gloss at 20◦ AMTEC-Kistler 10 cycles at 20◦ (%) DIN 55668 10 cycles after reflow 2 h 80 ◦ C at 20◦ (%) 40 cycles at 20◦ (%) Chemical resistance Gradient oven Base coat brilliant silver
H2 SO4 (◦ C) NaOH (◦ C) Pancreatin (◦ C) Tree resin (◦ C) H2 O (◦ C)
UV
89 85 87 83
2C PU scratch resistant (bench mark) 85 74 87 48
75 57 69 75 75
44 69 71 69 75
Such HTS methods are now often used for the screening of formulations. Further examples describing the development of UV refinish primers and clear coats have been published recently.11
REFERENCES 1. Wicks Jr., Z.W., Jones, F.N. and Pappas, S.P. (Eds.), “Organic Coatings, Science and Technology, Vol. 2, Applications, Properties, and Performance”. John Wiley, 1994, Chapters XXXIII and XXXIV, pp. 273–311. 2. Holman, R. (Ed.), “UV&EB Curing Formulations for Printing Inks, Coatings and Paints”. SITA Technology, London, 1984. 3. Müller, B. and Poth, U., “Lackformulierung und Lackrezeptur: Das Lehrbuch für Ausbildung und Praxis”. Vincentz Verlag, 2003, Chapter 2. 4. Bankowsky, H.H., Paint Coat. Ind. June, 44–48 (1997). 5. Bankowsky, H.H., Enenkel, P., Beck, E., Lokai, M. and Sass, K., The formulation and testing of radiation curing binders for abrasion-resistant parquet varnishes, RadTech, Baltimore, MD, End User Workbook. 2000, pp. 210–222. 6. Silberzan, I., Magny, B., Elie, K., Roche, S. and Loubert, J.L., An insight in scratch resistance of UV cured coatings, RadTech Europe, Conference Proceedings, Vol. I. 2003, pp. 171–177. 7. www.basf.com/pigment and www.basf.com/resin. 8. Heischkel, Y., Schwalm, R., Kutschera, M., Schrof, W., Koltzenburg, S., Beck, E., Larbig, H., Menzel, K. and Gruber, N., RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 297–303. 9. Jones, N., Shen, W., Smith, S.M., Huang, Z. and Ryntz, R.A., Progr. Org. Coat. 34, 119 (1998). 10. Schwalm, R., Polym. Paint Colour J. 189 (4421), 18 (1999). 11. Bach, H., Gambino, C., Galeza, L., Dvorchak, M.J., Fäcke, T., Detrembleur, C., Ehlers, M., Mundstock, H., Schmitz, J. and Weikard, J., UV refinish primer and clearcoat, RadTech Europe 2003, Conference Proceedings. 2003, pp. 731–738.
Formulations Formulations used for radiation curable systems depend on the specific performance requirements of the coatings and have to be adjusted to the application techniques (e.g., viscosity needs). Typically, formulations for UV curable coatings contain 25–90% oligomeric resins, 15–60% reactive diluents, 0.5–8% photoinitiators, 1–5% additives, like levelling agents, defoamers, and optionally pigments, fillers and matting agents. The application fields are very wide and therefore the requirements on the coating properties differ very much. A coarse overview of main applications is given in Figure 5.1. From this broad span of applications it is obvious that formulations also differ very much in their chemistry and composition. Classical applications in wood and paper coating often require highly crosslinked coatings for abrasion, scratch and chemical resistant surfaces. Whereas for future applications, such as exterior wood coating, like window frames, or coil coating
F IG . 5.1. Application overview of uses of UV curable formulations. 140
141
FORMULATIONS
TABLE 5.1. General composition of wood coatings and function of the components Component
(%)
Function
Oligomer resin
25–95
Film formation Basic performance properties
Reactive diluent (monomers)
0–60
Film formation Viscosity adjustment
Photoinitiator
1–5
Initiation of curing; speed
Fillers/pigments
0–50
Increased abrasion resistance Increased scratch resistance Cost reduction (e.g., talc) Coloration
Matting agents Additives
Gloss reduction 0–3
Defoaming Wetting, levelling, slip, . . .
much more flexible coating properties are needed. A general discussion about all types of coatings for metal and non-metal substrates including major performance requirements is given in reference1 (Chapters XXXIII and XXXIV). Formulations available to the paint users are developed and tailored by the paint manufacturers (e.g., BASF Coatings, DuPont Performance Coatings, Kansai Paint, PPG for automotive; AKZO, Beckers, DuPont, Valspar, Sherwin Williams for industrial and wood coatings; Dainippon Inks, Sun Chemicals and Flint for graphic arts), who usually do not publish the exact recipes. However, for starting new developments guiding formulations are often made available by raw material producers (BASF AG, Cognis, Cytec Surface Specialities, Eternal and Sartomer) and Paint Research Institutes. Despite published already more than 20 years ago, a basic understanding of UV curable formulations for coatings, inks and paints has been compiled by Holman.2 The purpose of this chapter is to give a brief insight into the formulations for the classical applications and discuss the development of formulations for future applications, such as in food packaging, as well as in exterior uses in automotive, wood or coil coating.
5.1
FORMULATIONS FOR WOOD COATINGS
Examples given below are formulations of high gloss topcoats on wood surfaces. In the past styrene based unsaturated polyesters were used which are still common in some countries, but the more versatile and faster curable acrylics are becoming dominant. Polyester or epoxy acrylates either alone or in combination in amounts up to 60%, diluted with 35% of monomers are mainly used for wood coatings. For parquet and flooring applications the tougher and more abrasion resistant urethane acrylates along with monomers are also employed. A general formulation for wood coatings is shown in Table 5.1, further explaining the typical functions of the ingredients.
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TABLE 5.2. Layer composition of parquet flooring coating Layer
Approx. amount (g/m2 )
Curing
1. Waterbased adhesive primer 2. Undercoat (ctg., corundum) 3. Undercoat (ctg., corundum) 4. Undercoat 5. Top coat
10 20 20 20 5–10
Gellation (incomplete cure) Gellation Gellation Fully cured + grinding Fully cured
TABLE 5.3. Composition of a waterbased adhesion primer for wood Component
(%)
Type
Oligomer resin
50
Emulsion or dispersion Photoinitiator Additives
50 1–3 1–3
Watersoluble 100% UV resin; aliphatic epoxy acrylate, polar polyester acrylate or urethane acrylate Polyester acrylate emulsion (50% solids) All Defoaming, wetting
5.1.1
Industrial Parquet Coating
In order to be able to meet the various requirements of parquet flooring, the construction of prefabricated (multilayer) parquet needs a combination of different layers. The composition of these layers depends on the production line available as well as on the substrate quality. Regardless of the solution chosen, it has to guarantee good adhesion to the substrate, good interlayer adhesion, low abrasion as well as high scratch resistance and high resistance against chemicals, like cleaning agents, red wine, coffee and other typical household chemicals. A typical coating layer construction is shown in Table 5.2.
5.1.2
Adhesion Primer (to the Wooden Substrate)
In order to enable a good adhesion to the wooden substrate, mainly waterbased UV curable primers are used. The main function of this waterbased primer is swelling of the wood, which results in the erection of wood fibres, further increasing the available surface and favouring an entanglement of the wood fibres by the coating to improve the adhesion. A typical formulation used for waterbased primers is given in Table 5.3. In order to improve the interlayer adhesion two strategies are pursued, the incomplete curing of the primer, which results in a gellation of the layer, but only to such an extent, that there are sufficient double bonds remaining for the reaction with the next UV curable layer, and second the grinding of the primer layer in order to increase the roughness and therefore the surface area of the layer.
143
FORMULATIONS
TABLE 5.4. Topcoat formulations Component
Oligomer resin
Monomer containing
Monomer free
(%)
Type
(%)
Type
55
Epoxy acrylate, Laromer® LR 8986
43
Epoxy acrylate, Laromer® LR 8986 Polyetheracrylate, Laromer® LR 8967
43 Diluent
26.5
Tripropylenegylcoldiacrylate (TPGDA)
Matting agents
10
7 Talcum 10MOOS 3 Syloid® 162C
Wax
3
Ceraflour® 950
Photoinitiator
3
Irgacure® 500
Synergist
2
Amine-synergist, Laromer® LR 8956
Additives
0.5
Defoamer, Tego® Airex 920
– 10
Syloid ED 80
3.5
Iragcure® 184
0.5
Levelling agent, Byk 361
In the production of top-quality parquet up to three undercoat layers are used. The first two can be formulated with very hard abrasive fillers (e.g., corundum) and are only cured to a low conversion, whereas the third one does not contain the filler. After full curing of the three layers, a consecutive grinding step to prepare the surface for topcoat application is added, where fillers in the third layer would destroy the grinding belt. Finally, the topcoat finishes the construction of the parquet coating. Topcoats for parquet can be based on the combination of trifunctional polyether and polyester acrylate resins,3 as well as on epoxyacrylates, which provide high chemical stability. Besides the high gloss formulations also matt top coats are used. Although matt formulations are difficult to obtain with UV curable 100% liquid formulations, formulations have been developed which yield topcoats with matt appearance (Table 5.4). Such formulations developed for roller coater applications contain matting agent (silicagel and talc), small amounts of solvent can also be used. Such formulations for example are applied in a thickness of about 8–12 µm (8–12 g/m2 ), and exposed to a 120 W/cm lamp at a conveyor belt speed of 5 m/min. The resulting abrasion resistant coating exhibit gloss values of about 30% (Gardner, 60◦ ). Further starting formulations for obtaining high scratch or abrasion resistance have been published.4,5 Special formulations for obtaining high scratch resistance (on beech wood, for example) use large amounts of highly functionalized resins6 or are formulated with nanoparticles (Table 5.5).
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
TABLE 5.5. Highly scratch resistant top coat formulations Component
(%)
Type
Formulation based on high functional acrylates Dipentaerythrolpenta-acrylate Propoxylated neopentylglycol-diacrylate Urethane acrylate Photoinitiator Photoinitiator Oligomer resin
Sartomer® SR 399 Sartomer® SR 9003 Sartomer® CN 965 Benzophenone Irgacure® 184
57 28.5 9.5 3 2
Formulation based on nanoparticles 30
Diluent
22
Nanoparticles (dispersed in resin) Matting agent Photoinitiator Additives
35 9 3.5 0.4 0.4
Epoxy acrylate, Laromer® LR 8986 Polyetheracrylate, Laromer® LR 8967 Laromer® PO 9026V Syloid® ED 80 Irgacure® 184 Levelling agent, Byk 361 Defoamer, Tego® Airex 920
TABLE 5.6. Composition of a typical printing ink formulation Component
(%)
Function
Oligomer resin
40–60
Film formation Basic performance properties
Reactive diluent
10–20
Film formation Viscosity adjustment
Photoinitiator
3–8
Initiation of curing; speed
Pigments or dyes
15–25
Colour image formation
Additives
0–3
Surfactants Waxes
5.2 FORMULATIONS FOR GRAPHIC INKS UV curable systems used in graphic arts applications are divided into the categories of printing inks, containing pigments or dyes, and clear coat overprint varnishes (OPV). Ink formulations have to deal with two main requirements, the performance requirement of the printing process, like offset, letterpress or screen printing, which for instance require different pigmentation and viscosities and the performance requirement of the final printed image.
145
FORMULATIONS
TABLE 5.7. Beige pigmented UV ink Component
(%)
Function
Oligomer resin
75
Polyesteracrylate (Laromer® LR 8800)
Reactive diluent
8.82 4.4
Hexanediol diacrylate Trimethylolpropane triacrylate
Photoinitiator
0.75
Bisacylphosphineoxide (Irgacure® 819, Ciba)
Photoinitiator
1.23
Irgacure® 184 (Ciba)
Pigment
8.82
Titanium dioxide, Rutil
Pigment
0.98
Isoindoline (PY 110; Irgazingelb RTL, Ciba)
TABLE 5.8. UV offset ink UV offset ink components
Yellow (wt%)
Magenta (wt%)
Cyan (wt%)
Black (wt%)
Pigment Permanent yellow GR 01 Litho Rubin D 4574 DD Heliogen® Blue D 7092 Special black 250
15.8 – – –
– 17 – –
– – 17 –
– – 2 15
Resin Laromer® LR 9013 Laromer® LR 9004 Laromer® LR 8986
18.5 40 15.7
20 20 33
20 43 10
20 43 10
4 4 2 100
4 4 2 100
4 4 2 100
4 4 2 100
44 180 40
39 170 60
45 175 55
40 100 50
Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)
5.2.1 Printing Inks (UV Offset, UV Flexo and UV Screen Inks) The film thicknesses of the printing inks are in the range of up to 3 µm for offset and flexo inks and up to 15 µm for screen inks. The printing processes are discussed in Chapter 8.1.2. A general formula for printing inks contains about 40 to 60% resins, and between 15 and 25% of colorant (pigment or dye) as shown in Table 5.6. A specific example of beige pigmented UV ink is given in Table 5.7 (ref. 3). The pigmented UV coating relies on a mixture of photoinitiators, where the short wavelength absorbing Irgacure® 184 is responsible for surface cure and the longer wavelength
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
TABLE 5.9. UV flexo ink UV flexo ink components
Yellow (wt%)
Magenta (wt%)
Cyan (wt%)
Black (wt%)
Pigment Permanent yellow GR 01 Litho Rubin D 4574 DD Heliogen® Blue D 7092 Special black 250
15.5 – – –
– 15 – –
– – 15 –
– – 2 13
Resin/diluent Laromer® LR 9013 Laromer® PO 94F Dipropyleneglycol-diacrylate
15.5 45.6 13
15 51.6 8
15 51.6 8
15 51.6 8
0.2 0.2
0.2 0.2
0.2 0.2
0.2 0.2
Additives Levelling, Byk 307 Defoamer, Foamex N Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)
4 4 2 100
4 4 2 100
4 4 2 100
4 4 2 100
2.9 41 >65
2.0 4 >65
3.2 36 >65
2.4 20 >65
absorbing acylphosphine oxides (either Irgacure® 819 or Lucirin® TPO can be used) enables the throughcure. The coating is exposed to two ferrum added lamps (80 W/cm) at curing speed up to 10 m/min. Starting formulations for further optimization according to the specific needs are given for UV offset, UV flexo and UV screen inks, recommended for paper, board and film applications (e.g., BASF brochure: Make up your print products7 ) in Tables 5.8–5.10. The formulations contain either yellow, red, blue and/or black pigments, usually a grinding resin, like Laromer® LR 9013, polyester or epoxy acrylate binder resins, a photoinitiator combination containing standard photoinitiators facilitating surface cure and an acylphosphine oxide type photoinitiator (e.g., Lucirin® TPO-L), responsible for through cure, and optionally levelling agents and defoamers.
5.2.2
Formulation for Overprint Varnishes (OPV)
Overprint varnishes are applied to impart high gloss and protection to a printed surface, like post cards, catalogue covers, cosmetic cartons, etc.
147
FORMULATIONS
TABLE 5.10. UV screen ink UV screen ink components Pigment Heliogen® Blue D 7092 Special black 250 TiO2 Kronos® 2063S Resin Laromer® LR 9013 Laromer® LR 9019 Laromer® UA19T Laromer® LR 8986 Dipropyleneglycol diacrylate
Cyan (wt%)
Black (wt%)
White (wt%)
5 – –
1 4 –
– – 34
5 – 54 7 15
5 – 54 7 15
– 37 – 12
Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Darocure® 1173
1.5 1 0.5 2
1.5 1 0.5 2
4 – – 3
Co-initiator Laromer® LR 8956
6
6
–
2 1 – 100
2 1 – 100
3 6 1 100
Additives Aerosil® 200 CAB 551-001 (20% in DPGDA) Byk® 164 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)
4 – –
3.7 – –
3.5 15 60
The basic performance requirements on overprint varnishes are characterized by: • • • •
High reactivity; High gloss; Excellent wetting of the ink; Low price.
These overprint varnishes are applied with a thickness of about 6–10 µm (g/m2 ) typically in a coating unit as part of a printing press or in a separate step on a coating machine. A typical formulation is based on a high content of multifunctional crosslinkers (for high reactivity), oligomeric resins (epoxy-, polyester- or amine-modified polyether acrylates) and high photoinitiator amounts (Table 5.11). Thus, a general formula for overprint varnishes contains about 30–70% of highly functional reactive diluent, 10–20% resins and photoinitiator amounts up to 8% and often also amine-synergist, in order to obtain high cure speeds.
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
TABLE 5.11. General composition of an overprint varnish formulation Component
(%)
Function
Oligomer acrylate resin
10–20
Film formation Basic performance properties
Reactive diluent
20–30
Film formation Viscosity adjustment
Trifunctional X-linkers
30–70
Coating properties
Photoinitiator
3–8
Initiation of curing; speed
Photosynergists
2–5
Cure speed increase
Additives
0–3
Surfactants Wetting, levelling agent, slip, . . .
TABLE 5.12. Comparison of standard OPV formulation and OPV for inert exposure Component
OPV (%)
Type
Standard
Inert
Oligomer resin
52
52
Epoxy acrylate (high reactivity and chemical resistance), Laromer® LR 8986
Reactive diluent
35
35
Viscosity adjustment, tripropyleneglycol diacrylate (TPGDA)
Photoinitiator
8 –
– 1
Benzophenone Lucirin® TPO
Additives
0.4
0.4
Surfactant and slip additives
In order to reduce the photoinitiator content, for example for formulations, which are designed for packages with indirect food contact (where only very small amounts of migratables are tolerated), curing can be done under inert conditions. The same cure speed can be achieved with formulations containing only 1% photoinitiator compared to 8% when curing under air (Table 5.12). Furthermore, due to the reduced photoinitiator contents, migratables and odour are reduced with such formulations cured under inert conditions.
5.3 SPECIALTY FORMULATIONS (FOR GLASS, POLYCARBONATE, METAL) In the following three examples of specialty formulations applicable as starting formulations for coating glass, plastics or metal surfaces are given.
149
FORMULATIONS
TABLE 5.13. Scratch resistant topcoat for glass Component
(%)
Type
Dipentaerythrolpenta-acrylate Propoxylated neopentylglycol-diacrylate Urethane acrylate Photoinitiator Photoinitiator
64.2 16.4 10 7 2
Sartomer® SR 399 Sartomer® SR 9003 Sartomer CN 965 Benzophenone Darocure® 1173
TABLE 5.14. Flexible starting formulation for UV coating of polycarbonate Component
(%)
Type
Aliphatic epoxyacrylate Trimethylolpropane formal-monoacrylate
60 10
Sartomer® CN 132 Sartomer® SR 531 or Laromer® LR 8887
1,6-Hexanediol diacrylate (HDDA) Photoinitiator Photoinitiator
25 2 3
Irgacure® 184 Darocure® 1173
TABLE 5.15. Formulation for cationic metal coatings Component
(%)
Function
Epoxy resin Inert flexibilizer Photoinitiator
60–70 20–30 1–3
Dow UVR-6160 Polycaprolactone triol Sulfonium salt (e.g., Dow UVI-6990) or Iodonium salt (e.g., Irgacure® 250)
Pigments or dyes Additives
1–2 0–3
Surfactants
Coatings suggested for high scratch resistance on glass (ref. 6) are based on high functional pentaerythrol pentaacrylate (Table 5.13). The reported thickness was about 100 µm and no basecoat was used. The starting formulation proposed for coating plastic polycarbonate (ref. 6) is much more flexible than the formulations described before. It is pointed out, that the use of a flexible basecoat improves abrasion resistance, as well as adhesion and flexibility. Since cationic coating systems based on epoxy resins exhibit little shrinkage, the adhesion of such coatings on metal surfaces is generally better than that of acrylate based formulations. Most of the cationic coatings rely on epoxy resins, flexibilizers and sulfonium or iodonium salts as photoinitiators (Table 5.15). 5.4 FORMULATION SCREENING FOR NEW APPLICATIONS Frequently asked questions are about how to start the formulation screening for a new application. For the traditional applications, like wood, plastic coatings and graphic arts
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
the formulation examples discussed before can be used as a starting point. Some general considerations for the design and selection of raw materials for formulations are given in Figure 5.2. For interior applications, the selection of the raw materials is governed by the demands of the application segment, the required properties and the application method. There are no overall general restrictions. On the other hand, exterior applications and UV curing of pigmented systems require a careful selection of raw materials as well as of the exposure equipment. Since pigments absorb more or less intensely in the UV, the photoinitiators and the lamp have to be tuned to allow enough through-cure. Thus, usually a combination of photoinitiators, a standard hydroxyalkyl ketone (HAK) for surface cure and an Acylphosphine oxide (APO) type for through-cure, is selected. The exposure lamp also should exhibit considerable output at longer wavelength, as do gallium or ferrum added mercury lamps (see Figure 2.21). For exterior applications the selection of all components is critical. The resins have to be “weather-resistant”, particularly oxidative and hydrolytically stable. Hence, the only appropriate resin types are aliphatic urethane acrylates (UA) or acrylated polyacrylates (APA), to be used in combination with hydrocarbon type acrylate diluents (e.g., hexanediol diacrylate, decanediol diacrylate). In order to stabilize the coating and the substrate against degradation and discolouration, the coatings have to contain UV absorbers and radical scavengers, usually of the hindered amine light stabilizer (HALS) type. Since these compounds, similar to pigments, also absorb in the UV, the photoinitiator package is also composed of a (non-yellowing) surface cure type (e.g., Irgacure® 184) and an acylphosphine oxide type photoinitiator. The exposure lamp is preferably of the ferrum added mercury type. The main performance requirements in wood coatings are good adhesion (flexible waterbased primers or reactive acrylates, like isocyanato acrylates) and especially for parquet coatings high abrasion resistance (corundum fillers). The graphic applications often ask for high reactivity (high functional oligomers or amine modified resins) and high gloss. Can and coil coating as well as plastics demand excellent adhesion (low shrinkage, reactive primers) and good flexibility (low functional higher molecular weight resins and/or monofunctional reactive diluents). High scratch resistance can be obtained with high crosslink densities and/or added nanoparticles. High hardness can be achieved with high Tg coatings (high crosslink density or rigid components) and high flexibility by low crosslink density (high molecular weight between crosslinks) and flexible chains (polyether, polysiloxanes). Last but not least the desired application method determines the choice of the raw materials. Powder applications require a glass transition temperature of the powder above room temperature, preferably above 40 ◦ C, and melting temperatures below 120 ◦ C. Thus, high Tg resins and/or crystalline resins/diluents have to be used in combination with low volatile, solid photoinitiators. In liquid systems, the application viscosity decreases from roller over curtain and vacuum coating to spray coatings. The viscosity adjustment can be done preferably with reactive diluents, however, where low molecular weight monomers are undesirable or prohibited (porous substrates, spraying), solvents or waterbased emulsions or dispersions may be used.
FORMULATIONS
F IG . 5.2. Considerations for selection of starting formulations.
151
152 UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.3. Schematic structures of flexible and highly crosslinked urethane acrylate resins and different functional diluents for formulation screening with high-throughput methods.
153
FORMULATIONS
TABLE 5.16. HTS set-up for screening urethane acrylates Varied components
Components kept fixed in all formulations
Binder
Reactive thinner
Reactive thinner (wt%)
Photoinitiator
Additives
UV protection
UA 1 ↓ UA 26
TMPMFA HDDA TMPTA
0, 10, 30, 50 0, 10, 30, 50 0, 10, 30, 50
3.5% Irgacure® 184 0.5% Lucirin® TPO
1% defoamer 0.3% levelling
1.3% UV absorber 0.7% HALS
5.5
INCREASING EFFICIENCY IN FORMULATION SCREENING: HIGH-THROUGHPUT SCREENING (HTS)
In order to illustrate the method of high-throughput screening (HTS), the course of the development of weathering and scratch resistant aliphatic urethane acrylate resins is given in the following example.8 The screening was based on the formulation of aliphatic urethane acrylate resins together with reactive diluents of varying functionality. The aliphatic urethane acrylate resins used were synthesized by the reaction of a low viscous isocyanato-acrylate (ICA: Laromer® 9000 (BASF AG)) with commercially available diols and hydroxyalkyl acrylates. Molecular weight and acrylate functionality of the urethane acrylates was varied systematically. Twenty-six different aliphatic urethane acrylates were synthesized and formulated with the 3 reactive thinners as shown in Figure 5.3. The combination of 26 UA resins with the 3 diluents in 4 different concentrations resulted in 260 formulations. As fixed components 4 wt% photoinitiator (mixture of Irgacure® 184, Lucirin® TPO), 1 wt% defoamer, 0.3% levelling additive and 1.3 wt% UV absorber and 0.7 wt% HALS (UV protection) were added to the formulations (Table 5.16). The workflow of the high-throughput formulation testing is elucidated in Figure 5.4. This tool setup can be used for formulation screening with already existing raw materials, however, a parallel synthesis robot can also be applied to synthesize tailor-made new components (resins) in a very efficient way before the formulation step. A modular coatings robot (Figure 5.6) was used to dispense the raw materials and stir the formulations, then to draw down the coating films and finally to cure them using UV light. Various semiautomated application tests yielded materials properties like hardness, elasticity, scratch and chemical resistance as well as UV conversion. Sophisticated data management supported the design of experiments, configures the robot, captures the recipes, process parameters and measurement results in a database and offers data visualization. This allows selection of promising formulations and to harvesting of structure property correlations of coating properties, process parameters and chemistries applied. In subsequent steps more focused libraries can be generated. The resins can be synthesized in a classical manner or with a HTS reactor with automated dosing and parallel setup as shown in Figure 5.5. The formulations were coated with the HTS equipment shown in Figure 5.6 onto glass substrates by doctor blading (slit width 200 µm) with a wet paint thickness of 150 µm. After 15 min flash-off at room temperature the coatings were subjected to a 20 min treatment at 100 ◦ C. Subsequently, UV curing was performed at 60–80 ◦ C with 5 J/cm2 under
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.4. Scheme of high throughput formulation testing.
F IG . 5.5. Parallel reactors for synthesis.
inert conditions in a CO2 bath in order to eliminate oxygen inhibition. All the properties were measured with the HTS equipment (Figure 5.7). The mechanical properties of the cured films, like hardness and elasticity, were determined by Fischerscope® measurements
FORMULATIONS
F IG . 5.6. HTS parallel coating equipment.
F IG . 5.7. HTS combinatorial application testing.
155
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.8. Determination of hardness and elasticity (fischerscope® ).
F IG . 5.9. Fischerscope measurement (hardness versus elastic work fraction) of highly X-linked UA formulations from HTS.
FORMULATIONS
157
F IG . 5.10. Fischerscope measurement (hardness versus elastic work fraction) of UA formulations from HTS of all 260 formulations.
(Figure 5.8). Scratch resistance was evaluated by a sand scrub test. UV conversion was determined by confocal Raman microscopy at the surface (0 µm) and 15 µm layer depth. Chemical resistance was evaluated for H2 SO4 , NaOH, Pancreatin, tree resin and H2 O at two different temperatures each. At first glance, one might be overwhelmed by the flood of data generated from the screening. With visualization tools, however, interesting correlations can be revealed. A plot of hardness vs. elasticity, as shown in Figure 5.9, for a subset of highly crosslinked urethane acrylate resins (see Figure 5.3, top right) as a function of the used hydroxyacrylate building block on one hand and the type (i.e., functionality) and concentration of the reactive thinner on the other hand. The picture becomes even more complex (Figure 5.10), if additionally urethane acrylate resins are incorporated which contain flexibilizing diols in the center of the resin molecule (see Figure 5.3, top left). From the data in Figure 5.10 molecular resin structures can be identified which result in coatings of high elastic work (>70% of total) at low hardness (highly flexible systems) or at high hardness (highly crosslinked coatings). The evaluation of the scratch testing showed high scratch resistance for the coatings which exhibited high elastic work, either the flexible or the highly crosslinked. The chemical resistance data (Figure 5.11) were plotted in the same type of graph (hardness versus elastic work) and the differentiation of good and bad chemical resistance is shown by the shape (filled circle, empty circle and plus (+), respectively). The results demonstrate that the best chemical resistance is in the upper right corner of every plot, thus produced by the coatings with high crosslink density. The highly crosslinked urethane acrylate coatings identified by the HTS screening have then been subjected to intensive classical scratch resistant tests. According to a model published by Jones et al.9 , there are three different ways
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UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS
F IG . 5.11. Chemical resistance results of UA formulations from HTS (the formulations are plotted the same as in diagram in Figure 5.10).
TABLE 5.17. “Three response, two mechanism model” for evaluation of scratch resistance Responses to marring stress
Mechanisms of marring
Mechanisms of healing
Fracture Plastic deformation Elastic deformation
Fracture Plastic deformation
Viscoelastic creep
coating surface can respond to marring stress, addressing two different mechanisms of marring, and one mechanism of healing. In this model, fracture and plastic deformation lead to observable marring, while elastic deformation does not. Instead, elastically deformed mars recover their original dimensions almost instantaneously. Thus, the three responses in this model lead to only two marring mechanisms (Table 5.17). According to the model described above and the results of the high-throughput screening, the urethane acrylates which combine a high ratio of elastic work (We /Wtot ) and high universal hardness (Hu ) in Fischerscope testing should exhibit a very interesting combination of hardness and elasticity.10 After further optimization of the respective formulations these products have been tested as automotive clear coats, especially on plastic parts against the conventional two pack (2C PU) formulations. The results are shown in Table 5.18. Such highly crosslinked coatings based on aliphatic urethane acrylates, identified by the high-throughput screening, showed excellent chemical resistance and extreme scratch resistance.
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FORMULATIONS
TABLE 5.18. Applications test for automotive clear coats High functional aliphatic urethane acrylate (UV) based on Laromer® 9000 3.5% Irgacure® 184/0, 5% Lucirin® TPO, UV curing Larolux® Scratch resistance Initial gloss at 20◦ AMTEC-Kistler 10 cycles at 20◦ (%) DIN 55668 10 cycles after reflow 2 h 80 ◦ C at 20◦ (%) 40 cycles at 20◦ (%) Chemical resistance Gradient oven Base coat brilliant silver
H2 SO4 (◦ C) NaOH (◦ C) Pancreatin (◦ C) Tree resin (◦ C) H2 O (◦ C)
UV
89 85 87 83
2C PU scratch resistant (bench mark) 85 74 87 48
75 57 69 75 75
44 69 71 69 75
Such HTS methods are now often used for the screening of formulations. Further examples describing the development of UV refinish primers and clear coats have been published recently.11
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