UV-Durable, Water Repellent Coatings for Automotive Applications

Abstract

Silane-modified perfluoro polyether (PFPE) nanocoatings are widely used on glass touchscreen surfaces, such as smartphones and tablets, for fingerprint mitigation and easy cleaning. Although current PFPE coatings have excellent water/oil repellency and abrasion resistance, they typically lack long-term out-door durability. Perfluoro carbon (PFC) based materials are well-known as water repellent coatings for automotive glass due to their comparatively higher UV resistance, but they have poor abrasion durability. Recently, the growing use of electronics and sensors in the automotive industry has created a demand for UV-durable anti-fouling coatings. To meet this demand, Daikin has created a new fluorocoating, Optool UD120, based on our proprietary PFPE technology with significantly increased UV resistance while maintaining water/oil repellency and abrasion resistance properties. This advanced technology is well suited for applications such as camera lenses, LIDAR modules, touch displays and windows.

Key words: Coatings; Nano coating; UV resistance; water repellent coating; Fluoropolymer

1. Introduction

Over the last decade, fluoropolymers have become ubiquitous as coatings for cover glass surfaces such as smartphones, tablets, PCs, and other touch screen devices. Their anti-smudge, easy-to-clean, and smooth haptic properties make them ideal for this growing market application. Today, silane-modified perfluoro polyether (PFPE-Si) is widely used for these coatings. The PFPE molecule exhibits high water/oil repellency by creating a coating with low surface free energy [1]. Ether groups contained in the structure make it flexible and oil-like, which generates a pleasing surface feel and contributes to abrasion resistance [2]. Additionally, the silane-groups contained at the end of the PFPE chain allow for chemical bonds with glass surface Si-OH groups, further improving abrasion durability. For its ability to impart these unique properties, PFPE-Si is widely used on the previously mentioned substrates; however, poor UV-durability has limited the applications to mostly indoor settings.

For UV-exposed applications, such as automotive glass, the traditional fluoro-coating of choice has been silane-modified perfluorocarbon (PFC-Si). PFC-Si’s demonstrate good water/oil repellency and UV resistance. However, the PFC’s crystallinity is so high that it develops a rigid structure that causes the PFC-Si coatings to leave the surface feeling rough and unable to survive very much mechanical abrasion. Thus, the market requires a new water/oil repellent coating that has both abrasion durability and UV resistance properties. With this objective in mind, Daikin has developed an innovative polymer that successfully combines these three properties.

To begin with, Daikin considered the chemical structure of PFPE-Si. Its ability to create robust abrasion resistance and smooth surface haptics made it an ideal foundation from which to start the develop-ment of a new and improved material. Focus next turned to the chemical bonding energy in each component of that PFPE-Si molecule. Any material’s chemical bonds will decompose when exposed to energy greater than its chemical bonding energy. In an outdoor exposure situation, the energy generated by the UV spectrum of light is often greater than that of the bonding energy between different components of a molecule. Three important components are bound together within PFPE-Si polymers, each with different molecular bonding energy holding them in place: a PFPE chain, a silane component, and a linker component that links the PFPE and silane moieties (Image 1).

Image 1: PFPE-Si chemical structure and reaction

The PFPE chain contains C-F bonds, which have a high bonding energy. The silane component is covalently bound into the polymer with a siloxane bond (-Si-O-Si-), which also has a high bonding energy. Lastly, the linker component’s bonding energy is comparatively weak. Considering this thought process, the team surmised the most likely candidate for UV degradation was the linker component in PFPE-Si. Thus began work to improve the linker’s chemical structure. After evaluating various linker structures, Daikin finally succeeded in synthesizing a new PFPE-Si with significantly improved UV-resistance. This polymer also continues to impart the haptics and abrasion durability typical of PFPE-Si’s. In the following report, we will detail the properties of this new polymer, Optool UD120, from Daikin.

2. Preparation and evaluation

In this study three fluoro-coating materials were evaluated:

1. Optool UD120 – With improved linker structure

2. Optool DSX – Daikin’s flagship PFPE-Si

3. PFC-Si material – Trimethoxy(1H,1H,2H,2Hperfluoro-n-octyl) silane [C6F13CH2CH2Si-(OCH3)

Each material was further diluted in Novec HFE7200 (a 3M product) to achieve an active ingredient concentration to 0.1 mass %. Prior to the application of the coating, the chemically strengthened glass substrates were cleaned via plasma treatment. Coatings were then applied to the substrates by double fluid nozzle spray equipment with an adjusted spray amount of 50g/m2. Next, coated substrates were baked in an oven at 100ºC for 1 hour in order to drive the chemical reaction of the fluoro-coating material with the chemically strengthened glass substrate via covalent bonding. Finally, after a further 24 hours at 25ºC atmosphere, the coatings were ready for the following evaluations:

UV resistance test (QUV test):

For the UV resistance evaluation, sample water contact angle was measured after QUV exposure with the following settings:

– UVB: 310nm

– Irradiance: 0.63W/m2

– Water shower time: 3min/hour

– Dark board temperature (where the sample substrate was sitting): 63ºC

Steel wool abrasion resistance test:

Steel wool (#0000) was used as the abrasive material. The 1 x 1cm steel wool was adhered to a 1 kg load and then used to abrade the coated substrate at 60 cycles per minute with a 6cm stroke.

Kinetic coefficient friction test:

A LabThink CoF system, model FPT-F1 was used to determine the kinetic coefficient of friction (KCOF). A 2 x 2cm portion of copy paper was used as friction material. The copy paper friction material was adhered to a 0.2 kg sled which was tethered to a load cell, and then pulled across the coated substrate at a constant rate of 200 mm/min. Friction between the copy paper and the coating substrate was measured in order to calculate a kinetic coefficient of friction value, which is the ratio of the pull force (kinetic friction) measured by the load cell to normal force (the weight of the friction sled).

3. Results and Discussion

UV resistance test (QUV test):

Figure 1 shows the water contact angle of each coating after QUV exposure. The material with the improved linker structure, Optool UD120, shows significantly improved UV resistance. After 450 hours of QUV exposure, its water contact angle remained greater than 100 degrees. This was much longer than Optool DSX and PFC-Si, which had water contact angles drop below 100 degrees after ~100 and ~130 hours, respectively. From this data, it is apparent that the linker component’s chemical structure has an important impact on the UV resistance of a fluoro-coating.

Figure 1. Water contact angle after QUV exposure; (a) is Optool UD120 coated glass, (b) is Optool DSX coated glass and (c) is PFC-Si coated glass.

Steel wool abrasion resistance test:

Figure 2 shows the water contact angle of each coating during the steel wool abrasion test. Steel wool tests are done on pristine coated samples; i.e. before being subjected to steel wool abrasion, the coated substrates were not exposed to UV, salt spray, or any other environmental degradants. PFC-Si shows poor abrasion resistance with the water contact angle dropping to around 80 degrees after only 500 cycles of steel wool abrasion. On the other hand, both PFPE-Si-based materials, Optool DSX and the Optool UD120, retained water contact angle values of greater than 100 degrees after 3,000 and 9,000 cycles of steel wool abrasion, respectively. Although still only a hypothesis, Daikin believes the molecular conformation caused by the linker moiety is the reason that the Optool UD120 shows enhanced abrasion resistance as compared to the Optool DSX.

Figure 2. Water contact angle after steel wool abrasion test; (a) is Optool UD120 coated glass, (b) is Optool DSX coated glass, and (c) is PFC-Si coated glass.

Kinetic coefficient friction test:

Figure 3 shows the kinetic coefficient of friction (KCOF), used as a proxy for the haptic feel of “slipperiness” on the coating surface. Typically, a fluoro-coating’s primary fluorine-chain structure has an effect on this KCOF value. The flexible molecular structure of both the Optool DSX and the Optool UD120 creates surfaces with lower KCOF as compared with the PFC-Si material. The Optool UD120 has a roughly equivalent KCOF value as compared to Optool DSX.

Figure 3. Kinetic coefficient friction test result; (a) is Optool UD120 coated glass, (b) is Op-toolDSX coated glass and (c) is PFC-Si coated glass.

4. Conclusion

To improve the UV-durability of anti-fouling coatings, Daikin has synthesized a new fluoro-coating material based on proprietary PFPE technology. The chemical structure of the linker-component of this new PFPE-Si material has been exchanged for a linker with higher bond energy and thus greater UV-durability. Daikin has evaluated this new material, Optool UD120, coating on glass and compared it with general PFPE based coatings (Optool DSX) and PFC based coatings. The Optool UD120 generates coatings with advanced UV resistance and good performance in other critical properties such as steel wool durability, and coefficient of friction. This advanced material is well suited for outdoor use on camera lenses, LIDAR modules, touch displays and automotive window glass.

5. References

[1] S.S. Chhatre, J.O. Guardado, B.M. Moore, T.S. Haddad, J.M. Mabry, G.H. McKinley, R. E. Cohen, “Fluoroalkylated Silicon-Containing Surfaces-Estimation of Solid-Surface Energy,” Applied Materials and Interfaces 2, 3544-3554, (2010). [2] J.Scheirs, Ed., “Modern Fluoropolymers,” John Wiley & Sons, Ltd, 460, (1997).