Wettability & Non-stick Properties

Wettability of Fluoropolymers

Wettability refers to the ability of a liquid to spread on a solid surface. In the case of fluoropolymers, they exhibit low surface energy due to the presence of fluorine atoms. The low surface energy makes them hydrophobic and oleophobic, meaning they repel water and oil, respectively. When water or oil comes into contact with a fluoropolymer surface, it forms droplets and beads up instead of spreading out, which allows for easy removal or rolling off. This property is known as the "lotus effect" or self-cleaning behavior, as dirt and contaminants have difficulty adhering to the surface.

The water and oil repellency of fluoropolymers is minimally affected by temperature changes, chemicals or UV exposure, making them suitable for a variety of applications requiring resistance to liquids, stains and corrosion. These include non-stick coatings for cookware, oil and grease resistant materials in the food industry, waterproof clothing, stain-resistant fabrics and more.

Non-stick Properties of Fluoropolymers

On the one hand, the strong carbon-fluorine bonds in fluoropolymers form highly stable inert materials, i.e., chemically inert. On the other hand, weak intermolecular forces between fluorine atoms and other atoms or molecules in fluoropolymers result in low surface energy. The combination of low surface energy and chemical inertness makes fluoropolymers highly non-stick.

This non-stick property enables fluoropolymers to be used in a variety of industries, including food processing, packaging, medical, chemical, and aerospace industries. For example, in the aerospace industry, fluoropolymer coatings are used on aircraft components to prevent icing, reduce friction and improve performance.

Effect of Structure on Wettability of Fluoropolymers

The wettability of fluoropolymers is influenced by polymer composition and structure. Perfluorinated polymers have lower wettability than partially fluorinated polymers due to:

• Partially fluorinated resins are more polar than fully fluorinated polymers because the electronegativity of the substituting element (hydrogen or chlorine) is less than that of fluorine.
• The length of the C-X bond (X, H or Cl) along the polymer backbone is different from the length of the C-F bond. The electronegative and electropositive centers are in balance, but at a distance within the chain, resulting in a differential separation of charge, allowing electrostatic interactions between adjacent chains.
• The presence of polarity in partially fluorinated polymers strongly affects interfacial properties, resulting in enhanced wettability to contacting liquids as a function of the degree of polarity.

Wettability of fluoropolymer surface [1]

Comparison of Wettability of Fluoropolymers

The wettability of different liquids on polymer surfaces varies, as summarized in the table below. The smaller the contact angle, the stronger the wettability.

Liquid Types	Liquids	Order of Contact Angles
Nonpolar aprotic liquids	Heptane Octane Decane Tridecane Hexadecane Squalene	FEP > PFA > PTFE > ETFE > PVDF > ECTFE > PVF
Polar aprotic (bifunctional) liquids	DMF Nitrobenzene DMSO Acetonitrile Pyrrole	PTFE, FEP, PFA > ETFE > ECTFE > PVDF > PVF
Monofunctional polar aprotic liquids	Methylene Bromoform α-Bromonaphthalene Pyridine	PTFE, FEP, PFA > ETFE > ECTFE ≥ PVDF > PVF

Reference

1. Sangwha Lee, Langmuir 2008, 24, 9, 4817–4826.