Hydrophobic coatings and the drive to replace plastic packaging
The first in a series of articles for Plastic Free July

Hydrophobic coatings represent an exciting direction of research in the fields of chemistry and materials science. In particular, so-called “superhydrophobic” coatings can provide materials with extremely high water repellency and self-cleaning properties. It is hoped that such coatings can create novel materials to be used as part of the growing drive to replace plastic packaging. For instance, biodegradable paper with a superhydrophobic coating is envisaged as a replacement for the single-use plastics which currently predominate food packaging. Moreover, it is hoped that new, environmentally-friendly coating materials can be developed to replace current coating materials, many of which are based on non-biodegradable fluorinated polymers.

Analysis of patent publications indicates that innovators are filing an increasing number of patent applications related to “superhydrophobic” technologies. The graph below shows the number of international (PCT) patent applications published from 1995 to 2018, where the claims contain the term “superhydrophobic” or equivalent. There has been a significant increase from little or no publications in the late 90s, to fifty or more each year for the last five years.

Clearly, there is an increasing drive to gain patent protection in the field of superhydrophobic coatings. However, there are some considerations specific to this field that should be taken into account when drafting new patent applications.

A patent claim must be deemed to be clear before a patent may be granted, so that a third party can reasonably determine whether or not they may be infringing the patent.  Accordingly, patent applications should be carefully drafted to minimise the risks of receiving clarity objections. This requires particular consideration for patent applications directed towards superhydrophobic coatings, due to the possible ambiguity in the term “superhydrophobic”.

A “superhydrophobic” surface is commonly defined as one which has a contact angle to water of at least 150°. However, there is not yet an internationally agreed definition of the term. For instance, there is no entry for “superhydrophobic” in the IUPAC Compendium of Chemical Terminology (“the Gold Book”), which gives many agreed definitions of technical terms. A patent claim defining a coating as “superhydrophobic” without further definition could thus be deemed to be unclear. Moreover, the ambiguity in the term could result in a patent examiner adopting a different interpretation of “superhydrophobic” than intended by the patent applicant. Accordingly, when drafting a patent application in this field, it is advisable to include a definition of the term “superhydrophobic”.

An informal review shows that different definitions are used for “superhydrophobic” in patent applications, and that sometimes no definition is included at all. This is perhaps not surprising, given the lack of an agreed definition of the term. The most common definition is that mentioned above, of a contact angle to water of at least 150°. However, other definitions are used, such as a contact angle of significantly greater than 120°, at least 125°, and at least 140°. Moreover, in some cases an additional requirement to the definition is that the superhydrophobic coating must have a “sliding angle” to water of less than 10°. The different definitions used in current patent applications illustrates the need to include a clear definition when drafting a new patent application in this field.

A further issue with the clarity of “superhydrophobic” lies in the use of the contact angle in most definitions, because the term “contact angle” itself may be deemed unclear. For instance, there are different types of contact angle, the static contact angle, the advancing contact angle, and the receding contact angle, and these may have different values. Moreover, these different contact angles may be measured with different techniques with the possibility for different results, casting further doubt on the clarity of a definition based on the “contact angle”. Accordingly, to ensure the clarity of the term “superhydrophobic”, it is advisable to specify the type of contact angle used to define the term, and to specify one method for measuring this contact angle.

A further consideration is that the term “ultrahydrophobic” can be used synonymously with “superhydrophobic”. It is advisable for a patent application to pick one of the terms, provide a clear definition, and use the term consistently.

In summary, hydrophobic coatings represent an exciting opportunity for innovators looking to develop alternatives to single-use plastics and the lack of an agreed definition of “superhydrophobic” should not be a barrier to gaining patent protection in this field. Instead, the risks of the ambiguity in the term can be avoided if a clear definition is included when drafting new patent applications.

View PDF