Case Examples of Applying the Oleophobic (Oil Repellent) Technology

Oleophobic technology that supports the future

The oleophobic technique has become essential in not only fiber (textile) and exterior wall applications but also semiconductors and displays in the electronics field and life sciences in the biotechnology field, and is expected to be used in space satellites and telecommunications in the future.

The oleophobic technology is widely used in various fields, particularly advanced fields such as leading-edge electronic devices. The possibilities of the technology are expanding as technological innovation continues in diverse areas.

This site examines the possibilities of the technology mainly in the fields of electronics and life sciences through case examples.

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Case Examples of the Oleophobic Technology

The oleophobic technology is widely used in fields such as leading-edge electronic devices. Fine processing on backplanes has improved in line with advances in processing capacity as well as miniaturization of devices. High durability is required in addition to improved performance. By adding the ability to repel water or oil (oil solutions), or hydrophobic and oleophobic functions, to these devices, fine processing and high durability can be achieved. This section introduces examples of using the technology in leading-edge electronic devices and life sciences.

CASE 01Oleophobic Photoresist
CASE 02Oleophobic coatings
CASE 03Oleophobicity in microfluidics

Usage examples of Oleophobic Photoresists

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What is the oleophobic technology?

The mechanism of repelling liquids

Perhaps the best known example of the oleophobic technology is frying pans that have undergone fluorine processing.
A drop of oil (liquid) dripped onto the surface of such a frying pan forms a ball shape and rolls because of the balance between the surface tension of the frying pan (solid) and the surface tension of the oil (liquid) and the interfacial tension between the oil and the frying pan.

The angle θ between the solid surface and the tangent from the point of contact of the liquid in this state is called wettability (contact angle). This relationship is expressed by Young’s formula (Fig. 1). The wettability (contact angle) changes when the surface (interfacial) tension in each of the terms in the formula changes. For example, the solid becomes less wettable if its surface tension (γS) is reduced, and repels liquids better. That is, oleophobicity involves reducing the surface tension of the solid.

To improve the oleophobicity, materials are often coated with an oleophobic agent, or a base material with a small surface tension is used for the solid itself.

AGC Seimi Chemical Co., Ltd.:
http://www.seimichemical.co.jp/eng/product/fluoro/

Technologies and Materials of Hydrophobicity and Oleophobicity, CMC Publishing Co., Ltd.:
https://www.cmcbooks.co.jp/user_data/overseas.php

Young’s formula (expresses the balance of forces on the solid surface)

Fig. 1: Wettability of a liquid on a solid surface

The figure shows the wettability of a liquid on a solid surface. The angle θ between the solid surface and the tangent from the point of contact of the liquid when the surface tensions of the liquid and solid and the interfacial tension are in balance is called wettability (contact angle). This relationship is expressed by Young’s formula, γS = γSL + γL cosθ, where γL is the surface tension of the liquid, γS is the surface tension of the solid, and γSL is the interfacial tension between the solid and liquid. The behavior of the droplet when water or oil is coated on the substrate is considered the wettability, and is used to examine the degree of repellency from waterproofing and oil repellency addition.

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CASE 01Oleophobic Photoresists

ATX®Series

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The oleophobic technology has been used in the photolithography process for manufacturing OLED display backplanes in recent years, as there were underlying issues in mass producing larger displays. The conventional function of the photoresists used in the photolithography process is to serve as a photosensitive processing material that forms the desired shape. By adding the oleophobic technology, it became possible to mass produce larger OLED displays. This section introduces a case example of using the oleophobic technology in the photolithography process.

CASE 01-1Oleophobicity for manufacturing OLED displays

Why oleophobicity became necessary for manufacturing OLED display panels

The conventional methods of manufacturing OLED display panels are the white vapor deposition + color filter method and the RGB vapor deposition method. These manufacturing processes require large vacuum devices and fine masks. However, using these methods for larger displays involves extreme technical difficulties, as well as increased manufacturing cost due to low production efficiency, increased material cost due to low material utilization ratio, and environmental impact.
Meanwhile, the RGB printing method, which has been subject to research and development, reduces the processes of large vacuum devices and requires no fine mask. This enables the mass production of medium-sized monitor displays, higher production efficiency, and lower production cost. JOLED Inc. launched a mass production line using the RGB printing method to produce a 32-inch display (OLEDIO™) in 2021.

JOLED Inc.:
https://www.j-oled.com/eng/technology/

JOLED Inc.:
https://www.j-oled.com/eng/press/20210329/

The oleophobic technology is essential in the RGB printing method.

Issues with larger OLED displays solved by the oleophobic technology

OLED uses organic compounds (inks) as the luminescence materials. There are also RGB pixels of width 10 to 30 μm aligned inside the OLED display panel, as shown in Fig. 2.

In the RGB printing method, which is an efficient way of manufacturing large OLED displays, luminescence materials are applied onto the RGB pixels inside the OLED display panel by inkjet. To prevent color mixture between adjacent pixels when ink is applied to the pixels as the luminescence material, the wall (bank) that separates the pixels must be oleophobic. In the photolithography process, a hydrophobic and oleophobic layer is formed only at the top part of the wall (bank) that separates the RGB pixels (see Fig. 2) to prevent color mixture.

This figure shows RGB pixels on a inkjet OLED display substrate. It is structured so that hydrophobicity (water repellent) and oleophobicity (oil barrier) is expressed only at the top of the bank, to ensure that the ink slides down and settles in the correct position even when the ink is applied to the top of the bank. The surface transfer property and orientation characteristic of fluorine are used to form the oil barrier.

Fig. 2: RGB pixels on an inkjet OLED display substrate

The oleophobic photoresist controls the movement of luminescence material during the application of ink and ensures that the luminescence material settles in the precise location within the RGB pixels (see Fig. 3).

A cross-sectional drawing of the function of the hydrophobic and oleophobic layer of the oleophobic photoresist. An oleophobic (oil barrier) layer achieved by the fluorine component is formed at the top of the oleophobic photoresist, which is also called the bank material, formed on the substrate so that the ink slides down and settles inside the pixel even when it is dripped onto the hydrophobic(water repellent) and oleophobic(oil repellent) layer. The surface transfer property and orientation characteristic of fluorine are used to form the hydrophobic and oleophobic layer.

Fig. 3: Function of the hydrophobic and oleophobic layer in an oleophobic photoresist

While there is also a different application of photoresists for processing, where they are removed after the photolithography process, the oleophobic photoresist used in manufacturing OLED displays is not removed but remains as a permanent part of the product.
In addition, a fluorine compound is used in the oleophobic photoresist. Oleophobicity is expressed only at the top of the bank by using surface transfer toward the interface with air and the orientation characteristic unique to fluorine.

Requirements of photoresists for inkjet OLED display panels

Contact AngleWater: 100°, xylene: 46°
Thickness0.5〜2μm
Resolution10μm
Exposure wavelengthsi-line, ih-line, ghi-line

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CASE 02Oleophobic coatings

SURECO®

CYTOP®

Case examples of using the oleophobic technology in coatings include anti-fingerprint coatings, antifouling coatings, and industrial inkjet nozzles. Depending on the application, such features as hydrophobicity, waterproofing, moisture proofing, acid resistance, and insulation may also be required in addition to oleophobicity.

CASE 02-1Anti-fingerprint coatings

SURECO®

Case examples of using oleophobicity in anti-fingerprint coatings include lenses for eyeglasses,　automobile displays, smartphones, PCs, and protective films. A surface treatment agent containing fluorinated polyether compound is used, and a diluent solvent suitable for the application is also used.
Wet coating such as spraying, dipping and spinning or dry coating such as vacuum deposition is used to apply and form a thin-film coating on the base material. The coating layer repels sebum, reduces adhesion of fingerprints and improves ease of cleaning fingerprints. Suitable base materials include glass, metals and resins.

Oleophobicity required in anti-fingerprint coatings

Anti-fingerprint coating agents for various products require such properties as hydrophobicity (water repellency) and oleophobicity, water-slippability, antifouling, fingerprint antifouling (easy to wipe off stains), thin film (approximately 10 nm as a dry film), low coefficient of friction, and high abrasion resistance. The functions required in anti-fingerprint coatings for different products are listed below.

Performance required for different products

Lenses for eyeglasses

Abrasion resistance
Hydrophobicity (thin film of nm order, 100° or larger)
Displays for automobiles

Abrasion resistance
UV resistance
Hydrophobicity(thin film of nm order, 100° or larger)
Touch panels (smartphones, PCs)

Abrasion resistance
Hydrophobicity(thin film of nm order, 100° or larger)

CASE 02-2Oleophobicity in antifouling coatings

SURECO®

Case examples of using oleophobicity in antifouling coatings include glass parts of household appliances, PC cases, and screen masks. A surface treatment agent containing fluorinated polyether compounds are used, and a diluent solvent suitable for the method of application is also used.
Wet coating such as spraying, dipping and spinning or dry coating such as vacuum deposition is used to apply and form a thin-film coating on the base material. The coating repels oil (oil solutions) and reduces the staining on the base material. Suitable base materials include glass, metals and resins.

Oleophobicity required in antifouling coatings

Antifouling coatings for various products require such properties as hydrophobicity and oleophobicity, water-slippability, antifouling, fingerprint antifouling (easy to wipe off stains), thin film (approximately 10 nm as a dry film), low friction coefficient, and high abrasion resistance. The functions required in antifouling coatings for different products are listed below.

Performance required for different products

Household appliances (glass parts)

Hydrophobicity(thin film of nm order, 100° or more)Abrasion resistance
PC cases

Liquid repellent coating
Screen masks

Hydrophobicity(thin film of nm order, 100° or more)Abrasion resistance

CASE 02-3Industrial inkjet nozzles

CYTOP®

Textile printing is an example where industrial inkjet nozzles require oleophobicity. It is possible to print complex designs on fibers using the high-definition inkjet technology. The oleophobic technology is used in the industrial printers because they require durability of high-speed, continuous ink (paint) discharge. Application of an oleophobic coating solvent (surface modifier) on the tips of the inkjet nozzles improves the liquid shedding performance and durability of the nozzles.

Oleophobicity required in industrial inkjet nozzles

Industrial inkjet nozzles used for textile printing require properties such as hydrophobicity, alkali resistance, solvent (ink, paint) resistance, and abrasion resistance. The oleophobicity requirements for inkjet nozzles are listed below.

Examples of coating agents used for industrial inkjet nozzles (for textile printing)

Coating thickness

0.1～1um
Required characteristics

Hydrophobicity
Alkali resistance
Solvent (ink, paint) resistance
Abrasion resistance, etc.

CASE 03Oleophobicity in microfluidics

CYTOP®

Microfluidics is a fluid manipulation technology used for fine processing on channels with flat surfaces, that utilize the base materials such as resin, glass and silicon. The channel surfaces are of nanometer to millimeter order dimensions. Some of the critical application fields of microfluidics include life sciences, chemistry and analysis.
For example, microfluidics is used in biochip devices, for detection of cells, DNA or proteins. Rapid PCR testing is another example of a critical microfluidic application, which have become widely popular during the COVID-19 pandemic. Biochips use the oleophobic technology to dynamically control the droplets of the test sample, etc.

CASE 03-1Oleophobicity in biochips

Case examples of oleophobicity in biochips include surface modification of the base materials. A biochip is an artificial substrate which can be used to measure the concentration of certain molecules and compounds. DNA, protein, sugar chains, etc. are fixed onto the finely patterned substrate. Then, the biological molecules and compounds are introduced onto the substrate to bring them in contact. Finally, their interactions are measured to quantify the fixed molecules and compounds. A thin-film coating of fluororesin is applied to modify the surface of the biochip.

Oleophobicity required in biochips

Oleophobicity is widely used in biochips to perform fluorescence observations of the sample. By providing hydrophobicity and oleophobicity as well as a hydrophilic structure in the fine patterning on the biochip, it is possible to move the sample into the hydrophilic part and perform fluorescence observation.
Dynamic control by applying a voltage to the sample droplet on the oleophobic coating is also possible. When a voltage is applied to the droplet, the electrowetting phenomenon occurs leading to the wettability (contact angle) changes allowing the droplet to move, separate or mix.

The hydrophobic and hydrophilic patterning is created with a hydrophobic and oleophobic coating. Oleophobicity (oil barrier) enables static control (sliding, moving, separating and mixing) of the liquid by using the wettability property. Hydrophobicity and oleophobicity suppress the adhesion of biological molecules and compounds, facilitating fluorescence observation and measurement of the reactions of the sample inside the hydrophilic part.

Fig. 5: Conceptual drawing of the fine hydrophobic and hydrophilic patterning on a biochip

The contact angle, or “wettability,” is changed by assigning the negative electrode on the substrate side of the biochip made of inorganic glass substrate and the positive electrode on the liquid side, and applying a voltage to change the surface tension of the liquid, surface tension of the solid, and the interfacial tension between the solid and the liquid. This controls the movement of the liquid, enabling dynamic control.

Fig. 6: Dynamic electrical control of a minute droplet

Examples of fluororesin coating materials used in biochips

This section introduces various fluororesin coatings used on biochips, the performance of the resins, etc.

Comparison of surface contact angle depending on the presence of coating

Coating	Base material	Water	n-hexadecane
None	Glass substrate	44°	21°
Present //CYTOP®	Glass substrate	112°	53°

* Biochips require high hydrophobicity and oleophobicity.

Comparison of critical surface tension of resins

CYTOP®	PTFE	PMMA
19mN/m	18mN/m	39mN/m

* Biochips require a low critical surface tension.

Comparison of absorption coefficient of resins

CYTOP®	High-density polyethylene	Polyimide
0.01％ or lower	0.01％ or lower	0.5％

* Biochips require an extremely low water absorption coefficient.

Reasons for Choosing AGC

AGC has used the oleophobic technology in two main areas.

Introduction of a lineup of standard products equipped with oleophobicity.
Proposal of customized products for particular purposes in electronic device manufacturing processes, etc.

Customized Products of AGC

We will make proposals according to the purpose of oleophobicity, such as electronic device manufacturing processes and coating on base materials. We have a solid track record in the fields of photoresists, various types of coating, microfluidics, and biochips.

Standard Products of AGC

Our lineup of products with oleophobicity includes:

ATX®Series

SURECO®
CYTOP®

Customized Products of AGC

AGC manufactures fluororesins from raw materials with excellent repellency.
We discuss with our customers, propose solutions to meet their performance requirements, and connect them to practical applications. We receive inquiries for applications in a wide range of fields, particularly the electronics (including nano-sized applications), life sciences, and automotive(mobility) industries.

Inquiries

Understanding our customers

Sample work pieces

Evaluation and feedback

Adoption

We suggest the suitable composition of oleophobic photoresist
along with your process.

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