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Deliverable [11.8] from Project: ML² – Multi Layer Micro Lab

Title: Demonstration of a Coating System UV-Curing and Nano-Imprint Lithography

Content:

1. Executive Summary .................................................................................................................................. 2

2. Introduction ............................................................................................................................................... 3

3. Principle of UV-Curing.............................................................................................................................. 5

3.1. Basics in recipe development ................................................................................................................. 5

3.2. The design of suitable coating systems for UV lacquering ................................................................. 5

4. UV-LED vs UV-Lamp ................................................................................................................................. 6

5. UV-Curing Process ................................................................................................................................... 8

5.1. Polymerization Mechanism ...................................................................................................................... 8

6. Nano-Imprint Lithography (NIL) ............................................................................................................ 11

6.1. Embossing Roller configuration ........................................................................................................... 11

6.2. Inverted quartz roller configuration ...................................................................................................... 12

7. UV-Curing-Process Development: An Example .................................................................................. 12

7.1. Optimization of the ink formulations or of the coating equipment ................................................... 12

7.2. UV-Lacquer Optimization ....................................................................................................................... 13

7.2.1. Chemically optimized formulations: ..................................................................................................... 13

7.2.2. Shrinkage of UV-Lacquer during UV-Curing ........................................................................................ 13

7.3. Manifold Effects influence the film quality: Substrate, Primer, Coating ink, Coating process and Drying Process ........................................................................................................................................ 14

7.3.1. Effect of the Substrate: .......................................................................................................................... 14

7.4. „Drip-Off-Process“ .................................................................................................................................. 14

7.4.1. Typical values of Brilliance .................................................................................................................... 16

7.5. Coating Technologies ............................................................................................................................ 17

7.6. Challenges in transferring UV-Coating Processes from one coating system to another ............... 17

7.7. Suppliers for UV-Curing lacquers ......................................................................................................... 18

8. Optimization of the Coating Equipment: Determination and Comparison of the Process-Regimes for an Standard UV-Lacquer System .................................................................................................... 19

8.1. Results of Printing Trials at Coatema ................................................................................................... 19

9. UV-Modules integrated at Coatema ...................................................................................................... 20

9.1. Equipment Solution for RD: The Smartcoater ..................................................................................... 20

9.2. Equipment Solution for RD: The Click&Coat Approach ..................................................................... 21

9.3. Equipment Solution for Pilot lines and Production: UV-LED-Solution ............................................. 22

10. References ............................................................................................................................................... 25

Authors: Dr. N. Meyer, Dipl.-Ing. A. Weber and T. Kolbusch, all Coatema Coating Machinery GmbH

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1. Executive Summary

UV-Curing and Nano-Imprint Lithography are both process technologies which are manifoldly applied in industry

and in R&D to optimize devices on flexible substrates as well as for precise structuring and patterning of well-

defined structures. As shown a typical topic in R&D is how UV-Curing affects the morphology and performance

of membranes.

In the beginning we describe the principle of UV-Curing and the composition of suitable inks and lacquers. A

significant optimization was introduced by the development of UV-LEDs in comparison to conventional UV-

Lamps. Here the monodisperse wavelength of the UV-LEDs offers precise initiation condition to start the UV-

Polymerization in comparison to the broadband emission of conventional UV-Lamps. More features and

differences of UV-LEDs vs. conventional UV-Lamps are discussed and offer alternatives for industry as well as

for R&D.

Basics of chemistry are describes as well. A successful UV-polymerization consists of an Initiation, a

polymerization and a termination step. If all 3 steps are well-controlled polymers with more than 100.000

consecutive polymerization insertions can be obtained. It is pointed out that the UV-Polymerization has to take

place under an inert atmosphere. Oxygen contamination will lead to unwanted side reactions resulting in early

and uncontrolled termination, also with critical incorporation of oxygen species in the polymer layer effecting its

stability, lifetime and device performance.

In general the strategy for UV-curing development can be achieved by modification of the UV-lacquer chemistry

and modifying the lacquer composition by adding additives, diluters or solid compounds. As an alternative

strategy different coating technologies can be applied or modified to coat a given lacquer composition under

industrial specifications. As an Engineering company Coatema has developed more than 30 different application

systems which offer manifold different process regimes and address different lacquer properties. Last but not

least it is obvious that a successful qualification sometimes follows both strategies.

The Nano-Imprint-Lithography (NIL) Technology is also presented. Here the standard embossing roller

configuration is discussed as well as the inverted quartz roller configuration. Whereas with the embossing roller

only UV-transparent substrates can be applied the inverted quartz roller allows the Curing and NIL on in-

transparent substrates such as metal foils.

As an example the process optimization of a top-coating is experimentally performed at Coatema and its effect

for the brilliance of the top-coating is discussed. Different process parameters and some slight modifications of

the lacquer as well as the use of different coating systems offer a huge range to realize different top coatings

with significant effects.

The next paragraph gives a short overview about local and worldwide suppliers. Here pure chemistry

compounds are available for coating experts, who work with their own lacquer compositions. Alternative ready

to use lacquers are offered for less experienced coating operators.

Finally Coatema gives an overview about different UV-modules which are used for Curing and Nano-Imprint-

Lithography and which have been integrated in different equipment designs by Coatema in R&D-Projects as

well as on request by industrial customers.

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2. Introduction

There are a number of different approaches in micro or nano structuring of surfaces. In the case of the ML²-

Project we are speaking about polymer substrates which are handled, structured, coated and laminated roll to

roll. Compared to silicon wafer technologies the advantages are a much higher yield and a significant cost

reduction of the processes at around 1/3 compared with the discrete silicon wafer processes.

Compared to silicon wafer materials the handling of polymer films and the application of UV crosslinking

chemistry is causing technical challenges and red brick walls in the continuous roll to roll production, which have

to be solved for scaling up this novel technology.

Basically we have two different technology approaches in microfluidics which are UV nanoimprint lithography

(NIL) and hot embossing or thermal nanoimprint. The first system is using a UV-crosslinkable polymer layer in

which the structure is imprinted by an imprint roller and at the same time UV-crosslinked so that the 3D-

structure in the novel layer is stabilized before leaving the imprint tool. In comparison the hot embossing is using

a heated imprint roller which is directly embossing the nano or micro structure into the polymer film. Also with

this last method the so called freezing down effect below the glass transition temperature of the substrate

polymer is important to reach a high aspect ratio and very well defined and reproducible structures in the

polymer substrate.

Fig. 1 displays the economic benefit of UV-Curing[1]

compared to conventional solvent- or water based coating

processes. Due to the liquid nature of most monomers applied in UV-Curing the materials are processed in high

concentrations or even without any solvents offering a 2 fold advantage: First, without or only minimum use of

solvents the requirements for expensive drying is significantly reduced. Second, the UV-Curing process enables

a high material flux to generate thick films if desired. Another advantage is, that the viscosity of the UV-curable

ink can be adjusted in a wide range end enables the combination of precise ink-jetting with subsequent UV-

Curing.

Figure 1: right: The economic benefit of UV-Curing: In comparison to solvent or aqueous based coatings neat UV-

inks can be processed and UV-crosslinked, which needs no additional drying step and results directly in thick

films.[1]

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Table 1: Advantages for UV-Curing[5]

No. Effect in UV-Curing Benefit for the Manufacturer

1 Total Curing within seconds Fast processing, enables fast quality control

2 Solvent-free process No Drying, no solvent emission,

3 1 component System No pot-life = no waste of inks

4 Highly crosslinked systems Robust layer, high chemical and physical stability

In this deliverable 11.8 we describe the principle for UV-Curing and it’s optimization in a case study as an

example for the future process development for hydrodynamic substrates in the ML²-Project.

Due to its optimized chemistry and quantitative conversion the UV-Curing is manifoldly applied in R&D and

Industry all over the world. The UV-Curing is applied to optimize or stabilize complex device structures in

various thin film applications. As one typical example for current R&D Figure 2 shows a publication where the

morphology and performance of Desalination-Membranes is optimized.[2].

Figure 2: Applied acrylate UV-Curing in the current R&D of membranes.

[2]

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3. Principle of UV-Curing

Commercially available UV-Lacquers have been developed based on anionic, cationic and radical

polymerization processes and some engineered solutions, like the Dual-Cure[3]

systems contain several

monomer species, like acrylics and polyurethanes, which polymerize or crosslink in different mechanisms.

3.1. Basics in recipe development

Based on a basic recipe the UV-Curing process and the chemical composition of the Curing Ink (UV-Lacquer) is

adjusted to the specific customer needs and requirements by adding additives in a modular approach. This

modular approach offers also the various options to adjust the lacquer to the requirements of the existing

coating equipment.

Optimizing the recipe design by additives, the coating manufacturer can adjust some of the further

characteristics to the lacquer layer. E.g. by adding well-defined cross-linkers and some softening agents the UV-

Curing process can be fitted to the substrates and the applied process regime resulting in high yields, defect-

free homogeneous coating and fast production with low equipment maintenance.

In the recipe the rheology of the ink as well as the process parameters and the reaction time for drying or

crosslinking can be influenced, e.g. by adding a non-reactive diluter like water or solvent. This optimization is

widely used to adjust the coating to the surface tension of the substrate, to influence contact angle and its shear

behavior. Last but not least adding diluters influences the reaction time for the drying/crosslinking step and

influences the shrinkage of the formed polymer chain or polymer network.

A main problem for UV-Lacquer applications is the volume shrinking by 2-7% during the polymerization step. All

monomers react to build up a denser polymer chain or a denser polymer network resulting in a film with higher

macroscopic mass density. This stress in the chemical reaction is the origin for the manifoldly observed

shrinkage. The shrinkage effect generates additional tension in the substrate which may result in uneven and

rough substrate foils leading to non-perfect coatings with cracks, defects and non-intended thickness variations.

To overcome this shrinkage-effect experts have added a few % of solvents of softening agents to the lacquer to

increase the flexibility during film formation and reduce stretching forces. Another concept increases the

stiffness of the substrate by adding nano particles.

A today´s still not solved problem for lacquers is their non-compatibility to normal gear pumps. Around 70% of

standard recipes cannot be handled with standard gear pumps, due to gear pump induced polymerization. The

acrylics are activated by the gear pump shear forces and start to polymerize prior to the application onto the

substrate. This results in unintended high viscous lacquers, less controllability and finally in a non-reproducibility

of the UV-Curing step.

3.2. The design of suitable coating systems for UV lacquering

The development of coating recipes is based on a design which fits to the production process and in detail

which fits to the coating system, coating thickness, coating accuracy and operation speed. The target is to

develop a recipe, which fulfills all economical and functional requirements in the coating and UV-Curing step.

This has been demonstrated for many coating systems and in this context they are also the basis for the UV

reactive systems which are already used in a lot of different coating applications or UV-crosslinking processes.

As an example we will discuss UV-Curing coatings being used as lacquering (or also called top layer) on top of

underlying coatings.

In this report we want to show some of the relevant methods which are typically used by manufacturers

according to available chemistry and equipment and desired product and product performance.

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4. UV-LED vs UV-Lamp

Today mainly 2 different options exist to generate the desired UV-Light for the desired UV-Curing process. The

conventional UV-lamp is mostly based on Mercury, but there are also UV-lamps available based on Gallium and

Iron emission spectra. The new innovative technology is based on the novel UV-LEDs, which have been

optimized towards high flux of radiation and defined wavelength emission. Many OEM-Suppliers offer different

solutions for the generation of the UV-light based on performance data, radiation density, component size, price,

etc. Coatema has integrated manifold UV-components from suppliers such as GEW[4]

, Hönle[5]

, IST[6]

,

Phoseon[7]

, 4Pico[8]

and others and offers here various engineering solutions for its partners and customers

depending on the desired specifications and available budget. As an example Figure 7 displays a conventional

UV-Lamp and UV-LED system by Hönle.

Figure 3: UV-Emitters from Hönle: right: Conventional Mercury lamp vs. UV-LED System (right).[5]

Both Technologies are widely used in R&D and industry but show also some significant differences.

Whereas Mercury or Iron radiate a full spectrum of wavelength in the range of 200 to 450 nm the UV-LEDs

radiate discrete and well-defined wavelength depending on the UV-LEDs used. As displayed in Figure 8 various

UV-LEDs are commercially available which radiate precisely at single wavelengths of 365 nm, 385 nm, 395 nm

or 405 nm. It is noteworthy that especially at the low wavelength of 365 and 385 nm the radiation intensity has

not reached the values of the UV-LEDs with 395 and 405 nm. That may result in the limitation that a UV-Curing,

which shall work only with wavelengths of 365 or 385 nm, is significantly reduced in its capacity and allows only

moderate web speeds, whereas with higher wavelengths a higher radiation flux is generated enabling faster

Curing and processing. More details about UV-Lamps and their specifications can be found here.[9]

Figure 4: left: Emission Spectra of conventional Hg-Lamp, right: Emission spectra of various UV-LEDs.[5]

Table 2 gives an overview about the features comparing UV-lamps and UV-LEDs which are important for the

industrial use in UV-Curing. Conventional UV-Lamps are still used as standard solutions due to their broad

acceptance and broad emission spectrum, but UV-LEDs allow to choose the right emission wavelength for a

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desired chemistry composition where the absorption is optimized. Here UV-LEDs offer the option to be more

sensitive while no additional radiation is generated which could be harmful to the formed film or the substrate

being coated. But this benefit in selectivity and differentiation potentially demands also to replace the used UV-

LED unit with another wavelength emitting UV-LED unit, if the UV-Curing composition is changed.

Conventional UV-Lamps generate heat radiation and ozone. Both could be harmful, due to unintended chemical

side reactions and thermal stress which cannot be totally compensated by optimized coating parameters, e.g.

reduced substrate temperature and/or nitrogen purged compartments. As pointed out in Figure 7 the UV-

generated radicals react with oxygen in air resulting in a less controlled polymerization or even a premature

termination of the polymerization or crosslinking. If the UV-Curing is realized with an inert atmosphere, e.g. with

a constant nitrogen flow the polymerization and crosslinking is controlled much better. This may result in better

film homogeneity and finally in improved performance data or longer device lifetimes. Coatema has realized

several UV-Curing modules with housing for operation in nitrogen mode.

Whereas UV-Lamps require a heat-up phase the UV-LEDs deliver directly the desired radiation. This allows

manufacturers to process several products within only one process line and offers to address several niche

markets with their products.

A significant improvement offers the UV-LED concerning its lifetime. UV-LEDs reach more than 20.000 h of

operation whereas UV-Lamps have to be replaced after 1000-5000 h, causing unintended downtimes and

additional spare parts.

The maintenance of UV-Lamps has to be scheduled more frequently but is nowadays less expensive. If UV-

LEDs have to be replaced due to a shift in the radiated wavelength or due to hardware failures the complete

LED-unit has to be exchanged, which results in higher costs.

Table 2: Comparison of features for the conventional UV-lamp, based on mercury and UV-LEDs.[5]

No. Feature Conventional UV-Lamp (Hg-Spectrum)

UV-LEDs (LED-Spectrum)

1 Wavelength 200-450 nm (see Fig.4 left)

Single, defined emission of LED at 365nm, 385 nm. 395 nm or 405 nm

2 Ozone generation yes no

3 Heat radiation towards substrate

yes

no

4 Efficiency ca.30% 15-40%

5 Size ratio high (depends on application)

Low (compact design)

6 Operation mode Heat-up phase required Standby-Mode: 15-40%

Shutter required

No Heat-up phase No Standby-mode or Shutter required

7 Cooling Air and/or water cooling Mostly water, infrequent air

8 Typical lifetime 1.000-5.000 h >20.000 h

9 Maintenance conventional UV analyzer exchange of only radiator

difficult to analyze all LEDs exchange of LED unit

10 Investment low higher

Summing up it can be predicted that UV-LEDs will gain market share especially where the sensitivity in UV-

Curing and the selectivity in radiated wavelength is required. The conventional UV-Lamp technology will lose

market share but will stay in business especially for mass production and low-cost solutions.

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5. UV-Curing Process

As already mentioned the UV-Curing is manifoldly applied in R&D as well as in production. Large production

numbers are related to curing and drying of coated films as well as in pressure sensitive adhesives (PSA) of

flexible substrates. Here manufacturers use ready-to-use chemical compositions provided by the suppliers or

compose their own coating recipe solutions based on own Know-How and experience, which offers higher

margins of the value chain. The industrial market reaches here several billions € per annum.

In research UV-Curing is applied as a proven technology to modify and improve selected layers for novel

applications, e.g. Hydrofluidics, membranes, encapsulation or organic electronics like OLEDs or OPVs. Here the

chemical functional groups for UV-Curing are chemically attached or synthesized into the compounds for the

novel film application and the UV-Curing step is integrated into the process flow for device preparation. In case

of a future pilot production the process flow will be analyzed and scaled towards industrial demands and

specifications. Coatema is involved in both disciplines the Process development in the R&D Departments as

well as in the scale-up activities to realize pilot lines and production lines for the novel products.

5.1. Polymerization Mechanism

The polymerization and cross linking mechanism has been investigated in detail for many years. Nevertheless

some fundamental basics shall be shortly discussed. Most polymerization reactions have to fulfill a 3-step

reaction pathway. First the Initiation, which generates the starting radicals for the polymerization. This is realized

by a thermal or radiation sensitive molecule reacting directly with heat or the UV-radiation (see Fig. 7). Second,

the chain growth, were the monomers are attached stepwise to the radical and generate the polymer chain. And

third the termination itself, were of the polymerization stops. Here the active polymerization species is

deactivated and the polymerization comes to its end. Please note that the termination can be favoured by

impurities in the process or even by oxygen as pointed out in Fig. 7 by Hönle. A polymer chain usually consists

of more than 100.000 Monomers, which requires that the 100.000 steps of monomer addition have to take place

before a termination of the reactions occurs. Monomers with only one functional group attach to the growing

polymer chain and extend the chain. Only, if the Monomer contains two or more functional groups, this

Monomer is enabled to take part in two or more polymerization steps and linking two or three polymer chains

chemically together. Only that case leads to the cross-linking effect and forming a dense network of all polymer

chains. Therefore bi-functional or multifunctional monomers are called cross-linkers.

Figure 5 displays the general Mechanism of Polymerization, its active species and the required chemical

structure of the Alkenes for polymerization, which is present e.g. in the acrylics. All Figures have been taken

from a lecture of Prof. Crouse from Tennessee Technological University.[10]

Figure 5: Introduction to Polymerization: left: General Mechanism; right: Monomers.[10]

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Figure 6: Introduction to polymerization: left: Active Intermediates for radical or ionic polymerization; right: Variety

of Monomers and Substituent Effects promoting polymerization mechanism.[10]

Figure 6 shows that a polymerization can follow several reaction pathways. Here the main routes via a radical or

ionic pathway are described. Additionally the ionic case has to be differentiated in carbocations and carbanions.

It is important to point out that all these polymerization pathways differ in their energetics and all of them are

influenced by impurities. Therefore the purity of the chemical, the purity of the equipment and its handling in

inert atmosphere (absence of oxygen) are essential to enable reproducible polymerization conditions.

Additionally it is shown that the Alkenes structure is the functional group which takes part in the polymerization

and that group is common in many available compounds in chemistry, which strengthen the general utilization of

such polymerization reactions.

Figure 7: Initiating the Polymerization in atmosphere and in nitrogen.[10]

Advantages for inert UV-drying.[5]

As it is shown in Figure 7 the UV-Curing and UV-Drying is established in industry for many years and the trend

is to realize inert UV-Installations to have better control of the UV-Curing chemistry and to minimize potential

impurities or oxygen in the process.

Table 3 shows commercial cross-linkers, which are reactive molecules with more than 1 polymerizable

functional group. These cross-linkers react statistically with different polymer chains and thereby form a covalent

strong network between all polymers chains. This crosslinking results in high chemical and physical stability of

the formed layer.

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Table 3: Cross-linkers based on acrylate chemistry.[2]

Figure 8 displays how the UV-Curing Process is applied in membrane formation. Due to the fact that the UV-

Curing is only applied from the top side this process configuration directly results in a differentiation of the

Membrane top- and back-side. The top-side is functionalized with a crosslinked acrylic layer whereas the

backside is not functionalized.

Figure 8: Modern application of UV-Curing for asymmetric PSU synthesis and functionalization.[2]

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6. Nano-Imprint Lithography (NIL)

The success of UV-Curing is also based on Nanoimprint-Lithography which is widely used for film structuring

and patterning. Fig. 9 shows the Principle of Nanoimprinting. First a coating of the soluble lacquer is applied and

directly structured using an embossing roller. During the embossing step the UV-Lacquer is totally cured

whereby the structure is hardened and well-defined. When the Substrate is released from the embossing roller

the structure is well-defined, rewound or further processed. Here 2 different NIL-configurations have been

developed, which are used in R&D and in industry. Coatema has integrated both engineering solutions for

industrial customers.

Figure 9: Principle of UV-Curing with embossing roller

6.1. Embossing Roller configuration

A standard Roller configuration for NIL consists of a temperature controlled embossing roller. For this process it

is possible to use either embossed roller or sleeve roller technology. Often a sleeve roller is used which allows

to change the NIL design easily and is also cost efficient. Fig. 10 displays the detailed view of such typical

embossing rollers, which have been realized by the Fraunhofer-IPT and the VTT Technical Research Center of

Finland. For a successful NIL-Process the UV-radiation is generated directly above the roller and the Design of

the UV-Lamp is realized to cover about half of the roller dimensions. This means that the UV-radiation always

has to pass through the substrate to start and finalize the UV-polymerization. As a consequence in the standard

NIL roller configuration only UV-transparent substrates can be processed.

Figure 10: The nanoimprint rollers with its structure can be seen, courtesy of Fhg-IPT and VTT

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6.2. Inverted quartz roller configuration

Another approach in equipment Design for UV-Curing and NIL is the inverted quartz roller design as shown in

Fig 11.[11]

Here a roller with or without embossing is realized fully out of quartz, which is transparent for UV-

radiation. Therefore the UV-radiator is integrated in the quartz roller design. This configuration allows to start

and finalize the UV-Polymerization without radiation through the substrate foil. As the result this inverted

configuration allows UV-Curing processes and NIL on non-transparent substrates, were radiation cannot pass

through the backside.

Figure 11: UV-Module in operation. The Nanoimprint roller with its structure can be seen.

7. UV-Curing-Process Development: An Example

To demonstrate the potential of UV-Curing Coatema decided to demonstrate the process development based

on a standard lacquer, which is used to improve and modify the shine and brilliance of a top coat which can be

applied to nearly any surface.

7.1. Optimization of the ink formulations or of the coating equipment

The industrial process development is mostly driven by time, cost and yield. Here we want to discuss the

various options for the UV-Curing process development where in general 2 routes for optimization can be

realized, depending on the product and its later application.

If the product shall be finished with a UV-Curing to improve its appearance there are many standard UV-Curing

recipes and ready to use formulations from the chemical suppliers available. Here you just order several

commercial solutions, which are suitable to your available coating equipment and apply and test these on your

end-product.

If your product device needs a special coating which has to fulfill special duties between several layers or which

will interact with reagents and solvents like it is discussed in the ML²-Project you may have limited freedom to

modify the chemical composition of the lacquer or the acrylic modified chemical material which shall be coated

and UV-cured in the following process sequence. In that case you still have the option to modify the coating

equipment and to apply several coating heads available. Coatema has realized more than 30 different coating

heads which enable the coating process of a given ink at different process regimes (Figure 12).

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Figure 12: Various coating systems are available from Coatema.

7.2. UV-Lacquer Optimization

Commercially available UV-Lacquers have been developed based on anionic, cationic and radical

polymerization processes and some engineered solutions, like the Dual-Cure systems contain several monomer

species, like acrylics and polyurethanes, which polymerize or crosslink in different mechanisms.

7.2.1. Chemically optimized formulations:

Today’s standard formulations consist of modular components which are ready to use and provided by the

chemical suppliers. They address several features of the coating equipment, the substrates and the process

regime which shall be applied.

7.2.2. Shrinkage of UV-Lacquer during UV-Curing

Due to the polymerization all Monomers react to form covalent bonds resulting in a polymer with higher density

compared to the monomer starting material. This increase in density results consequently in a shrinkage of the

polymer volume of about 2-7% and leads finally to tension and stress in the macroscopic film being formed. As

a consequence this effect can result in curl, pinholes, cracks or even cuts and large defects in the film and

cannot be tolerated. This trend is also influenced by the target film thickness.

Here the process development and the optimization of the chemistry recipe could successfully improve the

performance. By adding only a few % of solvent the polymerization speed can be reduced and slowed down

towards the end of the polymerization resulting in less induced tension with much better film homogeneity. The

few % solvents will slowly migrate out of the film and evaporate from the substrate surface within hours or days.

Similar molecules with reduced vapor pressure are known as softening agents. These materials are also used

manifoldly in industry and often are already used as additives for the production of flexible substrates.

As described these softening additives have a positive effect on the film formation but they may have also a

negative effect in the film bonding between the substrate and the film formed by UV-Curing or UV-Drying.

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7.3. Manifold Effects influence the film quality: Substrate, Primer, Coating ink, Coating process and

Drying Process

7.3.1. Effect of the Substrate:

The main effects caused by the substrate are:

Surface energy

Reactivity to form covalent bonds (Grafting) versus van der Waals-bonded layers

Swelling (Activity to incorporate solvents in the substrate polymer)

Penetration (Activity to transport solvent or molecules through the polymer substrate = Membrane)

If for a given substrate these properties have to be adjusted for a following layer experts often apply an

additional primer step. Here this additional chemical primer acts as adhesion-promoting layer. This can be

realized as an additional layer or by an additional physical activation, e.g. by applying a Corona or Plasma-

treatment of the substrate prior to the coating step. Here it is noteworthy to develop always several process

alternatives for a given process. Especially for the future scaling onto pilot lines the Engineering Team needs

alternative approaches to reach the pilot lines targets within the cost budget.

7.4. „Drip-Off-Process“

The Drip-Off-Process has been developed especially for the UV-Curing of top-coatings and final finishing. Good

results have been demonstrated for 3 and 5 roller configurations, for screen printing and for Slot-Die Coating

Processes. For each Coating technology the lacquers have been adjusted by individual additives to enable the

best layer quality during the coating step itself.

The Drip-Off-Process and its lacquers have been developed to generate a brilliant or an opal top coat and both

kinds of finishing can be realized in a single coating step. In a 5-Coating station (like a 5 color offset printing

machine) the 5th coating unit (traditionally used for special color or effect coatings) consists of an oil based

coating. This will be applied in the last finishing step and the UV-Coat will coat huge areas of the substrate. The

last finishing in the printing process is normally done in a flexo-like coating system with a chamber knife or

doctor blade. In this step, a large-area brilliant UV-Curing lacquer is applied. During the interaction between

partial printing of “5th color” and UV-coating, UV coating achieve partially a textured layer around of the brilliant

UV-Curing finishing. The UV coating is repelled and the texture-effect is kept at the well-defined areas. As a

consequence manifold visual effects can be realized in between the textured and brilliant areas.

An important advantage is that this Drip-Off-Process can de directly inspected at the coating line offering the

Coating Engineers options for inline adjustment. This insitu approach represents economic advantages

compared to conventional top coats were the effect cannot be monitored in an insitu-fashion. This Drip-Off-

Process has been established for print-media and graphics with precise differentiation for bright and dark area

effects. This drip-Off-Finish has a significant economic impact. Besides the improvement of the brilliance the

Drip-Off-Coat results with its crosslinking UV-Curing also in an improvement of the coating life time and its

stability against abrasion. Here this UV-Curing reflects its basic properties of an encapsulation layer. Figure 13

displays a typical Drip-Off Effect.[12]

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Figure 13: Examples for the Drip-Off-Effect.[12]

Figure 14 displays the described opal-brilliant effect of the Drip-Off-Process. In the top-left area of the picture

the coat is opal whereas in the bottom-right the brilliance of the coating is kept.[12]

Figure 14: Applied Drip-Off-Process to realize large area opal-brilliant effects.[12]

Principle and Characterization of coating Brilliance

The brilliance of coatings is determined by the degree of reflection as shown in Figure 15. More details

about the quanitative analysis can be given by the suppliers of such lacquers and Drip-Off-Coating

systems.[13]

Here the reference value of 100 represents a total reflection without any light scattering. To

enable the comparison between different layers the angle of incidence has always to be given.

Figure 15: Principle of light reflection and typical angles of incidences. [13]

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7.4.1. Typical values of Brilliance

For comparison typical values of reflection have been defined for the range of highest brilliance to textured and

decorative coatings, displayed in Table 4.

Table 4: Comparison of Features for the conventional UV-lamp, based on Mercury and UV-LEDs.[5

]

No. Feature Angle of

incidence

Amount of

Reflection

1 Highest Gloss 60° 98

2 Gloss 60° 86

3 Antiglare 60° 50

4 Semi-matt 60° 35

5 Matt 60° 15

6 Textured 60° 15-86

To achieve the light-scattering effect 2 alternatives have been developed. Flattering agents (pyrogenic silicate)

Nano particles (nano-dispersed silicates) can be added or the UV-Crosslinking is driven intentionally in an

inhomogeneous process regime. As a result various skinning effects, e.g. orange peel effect, standard skinning,

ice-structure or a true skin effect can be realized just by adjusting the coating process parameters, which have

been realized in Coating trials at Coatema.14]

Figure 16 and 17 display some of these visual effects.

Figure 16: left: Melinex 725 Substrate coated with UV-Lacquer by 3 different coating heads, right: 2 rollers in

reversed mode (both rollers in stainless steel).[14]

Figure 17: left: 2 rollers in reversed mode (one roller in stainless steel- one roller in rubber with imprint structure),

right: UV-opaque coating by Mayer-Bar / Doctor Knife, drying, bossing with structured roller and finally

UV-Curing.[14]

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7.5. Coating Technologies

To apply the best Coating Technology 2 alternative approaches have been established.

1. Optimization of the Coating Equipment & Process with fine-tuning of the UV-Curing lacquer

2. Optimization of the UV-Curing lacquer and fine-tuning of the coating equipment / Process.

Roll-to-Roll manufacturing for UV-Lacquers is usually realized with coating technologies based on:

Mayerbar

Doctor-blade

Slot-Die

Roller Systems with several Rollers

Flexo-printing

Gravure printing

Role-to-Roll Silk Screen Printing

All Techniques differ quite a lot in transferring the UV-Lacquer to the substrate, resulting in different thickness

regimes and Lacquer specifications.

The Roller coating systems in Flexo- and Gravure Printing for example transfer the Lacquer in a direct mode

and split it into 2 fractions. One fraction is transferred to the substrate and another fraction remains with the

roller. This effect is essential to achieve a smooth and homogeneous coating.

7.6. Challenges in transferring UV-Coating Processes from one coating system to another

Figure 18 and Figure 19 display UV-Coating trials at Coatema, were the Meniscus can be seen during a slot-Die

coating experiment. Here different parameters have been tested and optimized to achieve homogenous coating

results. The forming of a stable meniscus is essential for a successful homogenous coating and depends on the

gap between substrate and Slot-Die, on the substrate speed and coating thickness.

Figure 18: Slot-Die-Coating Experiments of UV-Lacquers at Coatema.[14]

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Figure 19: UV-Curing Experiments at Coatema. [14]

As discussed before the volume shrinkage during the UV-Curing may occur and cause some problems

especially for very thin substrate foils. Here a precise and careful process optimization of the UV-Coating and

Curing parameters may help to overcome these problems.

7.7. Suppliers for UV-Curing lacquers

Coatema is engineering Coating Equipment for customers worldwide since 40 years and has qualified manifold

suppliers of chemistry. Those have been global players as well as local suppliers, which have been nominated

by the customers. Therefore we cannot name a complete list but only typical suppliers we have worked with,

Monomers, e.g. Acrylics:

BASF Coatings GmbH, Germany, http://www.basf-coatings.com Manifold acrylics, (mono-functional and

polyvalent for crosslinking), solvents and Initiators,

Sun Chemical Ltd., UK: www.sunchemical.com, Manifold acrylics (mono-functional and polyvalent for

crosslinking, Solvents, and initiators.

Additives: (Softening additives, stabilizer, solvents)

Byk-Chemie GmbH, Germany: www.byk.com, Softening additives, stabilizers, solvents

Tego (Evonik Ressource Efficiency GmbH): www.tego.de, Softening additives, stabilizers, solvents

UV-Lacquers for Flexo- and Offset-Printing:

BASF Coatings GmbH, Germany, www.basf-coatings.com

Schmidt-Flint GmbH; Germany, www.flintgrp.com

Schmid-Rhyner AG, Switzerland, www.schmid-rhyner.ch

Sun Chemical Ltd., UK, www.sunchemical.com/

UV-Automotive:

Bostik GmbH, Germany, www.bostik.de,

Becker Chemie GmbH, Germany, www.becker-chemie.de

UV-Screenprint / Lacquers for rotary screen printing / Digital Printing

Marabu GmbH & Co. KG, www.marabu.de

Sun Chemical Ltd., UK, www.sunchemical.com

Sericol (FUJIFILM Europe GmbH), www.sericol.com

Dekalin (Diffutherm B.V.), The Netherlands, www.dekalin.com

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8. Optimization of the Coating Equipment: Determination and Comparison of the Process-Regimes for

an Standard UV-Lacquer System

To demonstrate the Optimization of the coating equipment Coatema has chosen a well-known UV-Lacquer

which is typically used for label printing and flexographic printing and performed the tests with support from

FhG-IPT. Usually this UV-Lacquer is processed with a Flexo-Printing system chambered doctor blade and

anilox rollers.

Experimental:

Commercial UV-lacquer “Brilliant 3501” with high viscosity, high reactivity and high brilliance (Schmid-Rhyner,

Switzerland) was coated using several Coatema Coating Heads. As substrate we used PET-foil with thickness

of 125 µm (Melinex O, untreated Du Pont). For UV-Curing we used UV-Curing Module with an Hg-lamp (180-

450 nm) from IST (Model M300 U1) at atmosphere.

8.1. Results of Printing Trials at Coatema

During the trials at Coatema we have tested 7 different coating systems in different configurations and

investigated the quality of the UV-cured lacquer as thick films (>10 µm) and thin films (<10 µm). We detected

the amount of reflectance with a standard setup as described earlier (Table 5).

Table 5: Comparison of different coating equipment technologies for top-coatings

Coating

Head

Film Thickness

Reflectance at (60°)

[%] Film flatness Edges Wetting Comment

Screen Printing

PET-Screen

< 10µm 78 disturbed squeezed i.O. PET-Screen structure

> 10µm 82 disturbed squeezed i.O. Shrinkage

Gravure printing

grid with120°

t < 10µm 65 hazy

sharp i.O. i.O.

> 10µm 88 o.k., brilliant squeezed i.O. i.O.

Standard Flexo printing,

< 10µm 77 o.k., brilliant sharp i.O. i.O.

> 10µm 86 o.k., brilliant squeezed i.O. i.O.

Flexo Printing, anilox roller /

chambered doctor blade

< 10µm 84 o.k., brilliant sharp i.O. i.O.

> 10µm 92 o.k., brilliant sharp i.O. i.O.

Flexo Printing, anilox roller /

chambered doctor blade

60°C

< 10µm 90 o.k., brilliant sharp i.O. i.O.

> 10µm 94 o.k., brilliant sharp i.O. i.O.

Slot-Die,

gap = 50µm

< 10µm not detected Roll-off

not detected

Strong repellent

no meniscus

> 10µm 94 flow marks sharp repellent no process regime

detected

Slot-Die,

Gap = 50µm,

60°C

< 10µm 92 disturbed sharp repellent only small process regime detected

> 10µm 94 disturbed sharp repellent only small process regime detected

As expected the best results with the highest reflectance (92% & 94%) was achieved for the Flexo-Printing with

anilox roller and chambered doctor blade. High reflectance (94%) was also achieved for the Slot-Die System

operated with and without a 60° tilt.

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9. UV-Modules integrated at Coatema

9.1. Equipment Solution for RD: The Smartcoater

UV-Curing is widely established in R&D. In Fig. 20 Coatema has realized a double sided UV-Curing in a

Smartcoater configuration. Due to its compact design the Smartcoater is often chosen for inert UV-Curing

processes (Figure 21). See also some specifications about the Smartcoater and the UV-Module integrated.

Figure 20: Design Concept for 2-fold UV-Curing and detailed Realization in a Smartcoater.

Figure 21: Smartcoater with 2 integrated UV-Curing Modules.

Performance characteristics of the engineering solution:

Smartcoater: web speed = 10 m/min, web width = 250 mm; inert atmosphere possible UV-Modules: Hönle, Hg-Lamp, 230 W/cm, inert configuration possible

21

9.2. Equipment Solution for RD: The Click&Coat Approach

With the FhG-IPT we have integrated a temperature controlled Nano-Imprint Lithography NIL unit, which is used

for process optimization and pilot line production in the ML²-Poject. Fig. 22 and Fig. 23 show the pilot line

configuration. Due to the Click&Coat design all modules can be rearranged and reconfigured according to

different product specifications. Using only one Click&Coat-Process line this flexible design allows to address

manifold product process-sequences and thereby manifold niche markets.

Figure 22: Click&Coat configuration of R2R-Pilot System at IPT. The UV-Module is integrated into the Embossing

unit

Figure 23: The R2R-Pilot System at IPT with its Slot-Die unit and the Nano-Imprint-Lithography with its UV-Lamp.

Performance Characteristics of the Click&Coat Pilotline with NIL:

Click&Coat Pilot Line: Flexible modular Design, web speed = 0,2-10 m/min, web width = 500 mm NIL Module: GEW, Hg-Lamp, 140W/cm, inert configuration

22

Together with the Joanneum in Austria we have realized an R2R Pilot line with an UV-NIL-Module.

Figure 24: R2R-UV-Nanoimprinting System at Joanneum for R&D in Organic Electronics.

Figure 25: System configuration of the R2R-UV-Nanoimprinting system.

Figure 26: System configuration of the R2R-UV-Nanoimprinting system.

Performance Characteristics of the Base Coater with NIL:

Base Coater: Web speed = 1-50 m/min, web width = 250 mm UV-Module: IST-Metz, Hg-Lamp, 160 W/cm, inert configuration

9.3. Equipment Solution for Pilot lines and Production: UV-LED-Solution

An example for a production system with the innovative UV-LED solution is shown next.

23

Figure 27: Single LED Spot 100 and alignment of 9 LED Spots to realize a working width of 1600 mm.

Figure 28: Production line with integrated UV-LED Solution.

Performance Characteristics of the Production Line with UV-Curing:

Pilot Line: Web speed = 0,2-2,0 m/min,Web width = 1600 mm UV-Curing Module: Hönle, 9 x LED Spot 100 (365 nm) with up to 1,5 W/cm2, air-cooled and

with different wave lengths available

24

A conventional Engineering solution with Mercury lamps has also been realized for a production line.

Figure 29: Design concept for integration.

Figure 30: UV-LED Curing system during Factory Acceptance test at Coatema. UV-Module unfolded and UV-Module

folded for Process.

Figure 31: UV-Module in operation. The Nanoimprint roller with its structure can be seen.

Performance Characteristics of the Production line with UV-Curing:

Production Line: web speed = 1-30 m/min, web width = 1000 mm UV-Curing Module: GEW, Hg-Lamp, 150 W/cm, inert configuration

25

10. References

[1] D. H. Taylor, President, Specialty Papers & Films, Inc., Inkjet-printing processes for packaging and labeling: an Introduction, www.convertingquarterly.com, 2011 Quarter 4, 46.

[2] I. StruzYnska-piron, M. R. Bilad, J. Loccufier, L. Vanmaele, I. F. J. Vankelecom, Influence of UV curing on Morphology and performance of polysulfone membranes containing acrylates, J. of Membrane Science, 2014, 462, 17-27.

[3] Dual Cure is a brand of BASF SE: (see http://Brand+Darocur-Brochure- high+lights +Radiation +cur-ing with+resins+and+photoinitiators+for+industrial+coatings+and+graphic+arts+Laromer+Irgacure +Lucirin+Darocur-English.pdf

[4] GEW Ltd., Crompton Way, Crawley West Sussex RH10 9QR , UK, www.gewuv.com

[5] Dr. Hönle AG UV Technology, Gräfelfing, Germany, www.hoenle.de [6] IST Metz GmbH, Nürtingen, Germany, www.ist-uv.de/wir-ueber-uns/presse/led-teil-2/ [7] Phoseon Technology, 7425 NW Evergreen Parkway, Hillsboro OR 97124 USA, www.phoseon.de [8] 4pico B.V., Jan Tinbergenstraat 4B, 5491 DC Sint Oedenrode, The Netherlands, www.4pico.nl [9] www.msscientific.de/lamp_overview.html [10] Prof. Course, Tennessee Technological University, https://iweb.tntech.edu/dcrouse/421ppt/4210CH6.pdf [11] by coutesy of Dr.W. Schippers, Managing Director of Nanotexx GmbH, www.nanoptics.de [12] Hochschule für Technik, Wirtschaft und Kultur, Leipzig (HTWK), Veredelungslexikon [13] Byk http://www.byk.com/de/support/instrumente/digitaler-katalog.html, [14] Experimental results have been achieved by Coatema during trials for the EU-Project Multilayer Micro

Lab “ML²” and in additional trials for industrial customers.