Presentation on Flexographic Printing Techniques. At:by:Leslie D. Embury January 2010

The Gwent Group of Companies Suppliers to Major Multinational Companies in :The Automotive Industry. The Fuel Cell Industry. The Electro-ceramics Industry. The Pharmaceutical Industry. The Bio Science Industry. The Electronic Display Industry. and General Electronics.

Large Area Printing • The Application of Flexography for the Volume Printing of Electronics.

Synopsis Flexography is an additive printing process that has the potential to achieve the necessary resolution and precision for volume printing of plastic electronics and OLED lighting. The Technology Strategy board funded the “Accuflex” project, this project is developing a flexographic printing system for volume production of high resolution plastic electronics, achieving fine line registration within large areas on continuous webs. This will require improved control of the printing process, particularly the plate (image carrier) technology; web handling and drying to create accurate fine line and precise features with the requisite inter layer registration that is essential for the use of these processes for the manufacture of sophisticated flexible displays, RFID, intelligent packaging, bio sensors, disposable electronics and intelligent textiles. The initial target products for Accuflex are large area lighting and conducting tracks for packaging, e.g. on tablet or blister packs.

Accuflex • • • • • • • •

A UK government funded project. The Gwent Group.: Formulation of improved conducting inks. Timsons Ltd.: Press development include precision digital drive system on their T-Flex press. Tectonic Ltd.:– On line inter unit registration measurement and full web inspection. Millennium Lasers Ltd.: on-line trimming. Asahi plc.: Flexographic plate technology. Innovia Ltd.: Substrate technology, including biodegradable films. WCPC, Swansea University: Fundamental research in flexography, particularly on contoured and textured plates to address the demands of feature resolution and quality.

Areas of Technology to be addressed. • • • • • • •

Machine registration. Ink formulation. Printing geometry. Substrate stability & surface parameters. Transfer plate development. Optical registration. Laser trim.

Flexographic Printing

Flexographic Printing

Flexoprinting 10-60 meters per minute

Flexographic Printing • The TFlex 508 flexographic printing press is being used as a basic platform for Accuflex project due to its robust construction, high specification drive elements and adaptability

Flexographic Printing

Flexographic Printing

Flexographic Printing

Current Printing Techniques. •

• • • • • • •

The process of choice currently for deposition of conductive inks is screen printing, which are typically demands high viscosity materials producing a thick film which have prolonged drying and curing times. A variation being Stencil printing. There are significant challenges in formulating materials for flexographic printing. Additionally, the system must compete with alterative possibilities. Direct Gravure. Ink Jet Direct Write. Subtractive Technologies.

Screen Printing Fig 1 The basic screen print process

Squeegee

Screen mesh Paste

Screen frame Substrate Nest

Emulsion

Screen Printing

Printing speed: 60 -1500 sheets per hour

High Definition Screens Screen with 500 mesh calendared fabric wire diameter 8 µm line/space 20 µm/20 µm

Material agglomeration Three particle agglomerate limits maximum particle size of powder to 12-15μm (assuming aperture of 45 μm)

Print Limitations 300SDS 40µm

C2090903R3 76% Pt

400UT 40µm

V330 40µm

Stencil Printing

Electroformed apertures

Positive advantage since no threads to reduce open area. Negative one direction only, without a dotting process.

Stencils • •

20 micron lines and spaces Speed of printing 60-1500 sheets per hour

Ink Jet

Piezo-Electric Deposition Up to 24 meters/min for digital printing

Ink Jet

• Nano silvers printed 50 micron lines • Using Current technology 30–50 µm lines

Ink Jet

SEM of Ink Jetted GOX layer

Glucose Biosensor response

Ink Dispersion Equipment for flexo Ink

Ink Requirements for Flexographic Printing • • •

• • • • •

The process demands low viscosity inks, the films deposited are thinner The solvents in the ink must be compatible with the plates. Increased flexibility is required and the drying times are significantly shorter. The initial demands are for conductive inks, this process requirement mean a fundamental rethink in formulation in terms of:Particle size. Particle shape Solvent and Resin systems that can be used. Their development had led to a conductive ink which is capable of being printed by flexography

Flexographic Printing Inks Raw Materials Selection • • •

Solvents and the roles they play in the formulation. Polymer types and considerations. Functional Phases, Conductive powders.

Flexographic Printing Inks Function and Selection of solvents Dissolve the selected resin system. Maintain printability. Control drying rate. Resin must remain fully dissolved during printing and drying process. • Generally several solvents are used with different boiling points and evaporation rates. • Direct effect on the drying of the printed films. • • • •

Flexographic Printing Inks Solvent Properties to Consider • Solvents and diluent can be used. • Solvents dissolve the resin, diluents do not. • Solvents must remain in the ink formulation during drying process to maintain resin solubility. • Combining solvents to give a blend gives the best drying properties to a printed ink. • Drying is a “wicking” process that draws solvent from the printed ink film.

Flexographic Printing Inks Function of the Resin • Binder system to hold the particulates together. • Gives the final dried film strength and adhesion to a given substrate. • Transfer process from photopolymer to the substrate.

Flexographic Printing Inks Formulation Considerations for Resins. • • • • • • •

Many types of resins can be used. Resin type depends on the substrate and adhesion required. Resin must prefer to adhere to the substrate rather than the photopolymer, otherwise no transfer will take place. Resin type used will also depend on the types of solvents that can be used to dissolve it. Dielectric effect of the resin used will affect the electrical conductivity of the dried film. Shrinkage of the resin on drying will have an effect on the electrical conductivity.

.

Solvent release of the resin on drying

Flexographic Printing Inks Functional Components • • • • • •

Silver Powders Silver Flakes Carbon Powders Graphite Powders Silver Complexes Silver Metallo-Organics

Flexographic Printing Inks Function of Conductive Powders • Give final dried print electrical conductivity. • Particle size will also determine the rheology of the final ink.

Flexographic Printing Inks Formulation Considerations for Conductive Powders • • • •

Particle size. Particle size distribution. Tap density. Surface area.

Flexographic Printing Inks Manufacturing of Silver Powders • • • •

Chemical Precipitation. Coating of Powders. Processing of Powders. Manufacture of Flakes.

Flexographic Printing Inks Effect of Silver Powder Coating • Coatings are used to prevent agglomeration of the silver powder. • Coating isolate each silver particle. • Poor removal of the coating reduces conductivity. • Solvents must be capable of removing the silver coating.

Flexographic Printing Inks Selecting Silver Powder or Flake • Powders • Particle Size Distribution

Flexographic Printing Inks Selecting Powder or Flake • Typical Flake

• Particle Size Distribution

Flexographic Printing Inks Nano-Silver Powder • • • • • • • • •

30nm mono dispersed powder (25 -45nm) Organic content minimised. No surfactants or dispersants added. Powder supplied “wet” in a solvent. Easier to disperse. PSD (SEM) D10 10nm D50 30nm D90 50nm

Flexographic Printing Inks Selection of Silver Powder or Flake • Powders are spherical. • Have less particle to particle contact. • Generally give low conductivity in polymer systems. • Can be added to improve conductivity of flake formulations

• Flakes are flat platelets. • Have greater overlap of particles when printed. • Generally give higher conductivity in polymer formulations. • Wide range of flake sizes.

Functional Conductive Flexographic Inks Property

Silver Ink C2080815D1

Carbon Ink C2090424D2

Ink Type

Solvent Based

Solvent Based

Drying Requirement

Forced air 60 – 70°c

Forced air 60 – 70°c

Viscosity @ 50s-1(typically)

0.8 Pa s

0.5 Pa s

Volume Resistance Ohms/cm*

5.2475x10-5

2.07x10-2

Maximum Print Speed**

30m/min

30m/min

Flexographic Printing Inks Rheology of Conductive Flexographic Inks. Carbon Flexo Ink C2080529D7 #2080618.25 0.3 0.25

Viscosity Pa s

Silver Flexo Ink C2080815D1#2090115.08 0.7000 0.6000

Viscoaity Pa s

0.5000

0.2 0.15 0.1 0.05

0.4000

0 0

0.3000

50

100

150

200 Shear Rate s-1

0.2000 0.1000 0.0000 0

50

100

150

200 Shear Rate s-1

250

300

350

400

250

300

350

400

Trial Formulations • All Silver powders and flakes have differing Tap Densities, and Surface areas. • Silver Flakes range from 1.5 – 6.0 g/cm3 • Silver Powders range from 0.8 – 4.9 g /cm3 • Surface Area range from 0.2 – 3.6 m2/g • Require a Model formulation to be able to assess different Silver Powders and Flakes. • Allows comparisons to be made between the performance of flakes and powders within the formulations.

Volumetric Ratio’s • Allows the same volume of Silver to be added to the same volume of Resin in the formulation. • Eliminates differences between the Tap Densities of different powders and flakes.

Trial Formulations Product Code

C2080605R4

C2080605R3

C2080605R2

C2080609R2

C2080609R1

C2080603R10

Volumetric Ratio

12.5:1

10:1

7.5:1

7:1

6:1

5:1

Weight % Silver

97.26

96.67

95.60

95.30

94.57

93.54

Weight % Resin

2.74

3.33

4.40

4.70

5.43

6.46

Conductivity (Ohms)

2.66

2.43

2.54

1.44

1.268

2.34

In Dried Film

Volumetric Ratio Trial Results

7

3

6

2.5

5

2

4

1.5

3

1

2 1

0.5

0

0 12.5:1

10:1

7.5:1

7:1

Volumetric Ratio

6:1

5:1

% Resin Conductivity

Conductivity (Ohms) of Bar Coated Films

Resin Content in Dried Film %

Relationship between Resin Content and Conductivity for Conductive Flexographic Inks

Mixing Silver Powder and Flake

3.5 3 2.5 2 1.5 1 0.5 0

6 5 4 3 2 1 0 10.56:1

10.56:1 Volumetric Ratio

% Resin Conductivity

Conductivity (Ohms) of Bar Coated Films

Resin Content in Dried Film %

Effect of Combining Silver Flake and Silver Powder on Conductivity (2:1)

Modifying Resin Type Effect of Changing Resin Type on Conductivity

Resin Content in Dried Film %

5

1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

4 3 2 1 0 7:1

7:1 Volumetric Ratio

Conductivity (Ohms) of Bar Coated Films

% Resin Conductivity

Trials • Resin and Solvent type have significant effect on the print quality. • Optimisation of Organic components in the ink to match surface energies of the film substrate being used. • C2080609D1 (7:1)

Effect of Print Orientation to Print Quality • 30 micron lines 45° orientation

• 30 micron lines (vertical)

• 30 micron lines (horizontal)

Effect of Print Orientation to Print Quality • 100 micron lines 45° orientation

• 100 micron lines (vertical)

• 100 micron lines (horizontal)

Ink Rheology Problems

Ink Rheology Problems.

Carbon Only Inks • Removal of Graphite to reduce PSD • Highest Conductivity Carbon Used

Resistance Results • Carbon Only formulations have resistances that are x10 larger Product Code

C2090204R9

C2090204R10

C2090204R12

C2080529D7

Volume Resistance Ohm/cm

0.00564

0.00322

0.00520

0.0207

Carbon/Graphite C2090424D2 • Re-formulated to remove slow solvents. • New formulation printed on Nilpeter press. • Drying system. • Print Speeds. • Achieved 35m/min. • No set off. • 40m/min some set off.

9 nozzles per dryer Nozzle air velocity >3000 FPM (600 m/min) Air volume 340 CFM ( 9.6 m/min) per mpdule (2 stations) Heater temperature 170 F. (77 C.)

Glucose Demostrator • Trails using various carbon flexo inks were carried out on two substrates supplied by Innovia. • Natureflex 45NVL and Natureflex 45NK. • Natureflex 45NK was found to give the lowest Coefficient of Variance (CV%) .

Glucose Demonstrator • Three inks on total required for the glucose demonstrator to be printed. • Electrode material, Carbon Flexo ink, used for the working and pseudo reference electrode. • Insulation, used to isolate the tracks and define working and pseudo electrode areas. • Hydrophobic Dielectric, used to channel sample to electrode area.

Inks Used • Carbon Graphite formulation was modified to give the lowest CV% when printed on Naturflex 45NK, C2091026D4 # 2091027.16. • Insulation in Flint UV curable formulation,UVF00345-405#2090722.12 Flexo Overprint varnish. • Hydrophobic Dielectric, solvent based flexo ink which was specially formulated to give good printing characteristics, D2091001D8 #2091031.02.

Bio-sensor Production

Possible Products

Possible Products

Possible Products

Conclusions • High speed printing by Flexographic techniques is possible. • Ink formulations can be produced to suit the majority of inks types that are needed. • High cost machinery is offset by lower production costs. • High volume production is obtainable. • Good electrical /electrochemical properties can be obtained

The Gwent Group Gwent Electronic Materials Ltd. Applied Enzyme Technology Ltd. Gwent Biotechnology Ltd. Gwent Sensors Ltd. LRH Ltd.

Les Embury Managing Director The Gwent Group, Monmouth House, Mamhilad Park, Pontypool NP4 0HZ. United Kingdom. [email protected] http://www.gwent.org Tel: +44 (0)1495 750505 Fax: +44 (0) 1495 752121