An overview of today’s functional coatings

Traditionally, the primary functions of coatings are to protect and decorate substrates. More recently, growth has occurred in the research and development, as well as commercialisation, of coatings which have novel functions in addition to having traditional protection and decoration properties. These coatings are often referred to as functional coatings. Materials such as thermochromic, energy efficient coatings for buildings and antireflective coatings for eye glasses have been commercially available for nearly two decades. These classes of coatings generally provide significant added value. In general, they provide their function in three distinct zones: at the interface of the coating and air, in the bulk of the coating, and at the interface of coating and substrate. While the list of functional coatings is long, examples of some of the commercial and most recognisable functional coatings are: antifouling, antimicrobial, colour shifting, conductive, easy-clean, photo- and thermochromic, self-healing and superhydrophobic coatings.

Science and technology of functional coatings

All coatings have surface functionalities. The standard properties of adhesion, corrosion and scratch resistance, gloss, hydrophobicity, antimicrobial, etc., are properties of the surface rather than of the bulk. Bulk properties are those such as cohesive strength, low gas and moisture permeability, and general durability.

Broadly speaking, all conventional coatings are also functional except that their function is either limited or fixed to a particular property. For this reason, functional coatings or multi-functional coatings share many of the compositional concepts and formulation guidelines of ordinary coatings. Therefore, functional coatings must possess similar basic properties to conventional coatings and use similar ingredients, in addition to other, specific materials.

In general, the unique features of functional coatings, whether surface functionality, or bulk, are brought about by the following three classes of materials: specifically designed responsive polymers; responsive additives, diluents, or solvents; and responsive pigments. Material selection is often one of the most important parts of the coating formulation. Simply incorporating a responsive ingredient in a conventional coating formulation will not result in a functional coating. Functional coatings as a group can be divided into two classes with different, distinct functions. First, those coatings that are intrinsically functional, such as superhydrophobic, antimicrobial, antifouling, antireflective radar absorbing, self-stratifying, or conductive coatings. Second, the class that is extrinsic or stimuli responsive and function by specific external or internal triggering mechanisms. This class includes coatings such as thermochromic, colour shifting, touch sensitive, corrosion- and explosive-resistant, self-healing and shape-memory materials.

Antimicrobial coatings

Coatings can be designed to kill or inhibit the growth of bacteria through three mechanisms: (1) coatings that can resist the attachment of bacteria, (2) coatings that release biocides that will kill the bacteria, and (3) coatings that can kill bacteria on contact. Coatings can also combine two or more of these mechanisms.

To inhibit bacterial attachment, the surface of a coating must be hydrophobic. To accomplish this, coatings contain fluoropolymers and organosilicone compounds that have very low surface energies. Therefore, they do not allow the collection of liquid water and hence are not favourable surfaces for bacterial settlement. There are also many suitable bactericides and fungicides such as small molecule antibiotics, quaternary salts (“quats”), chloramines, or triazines that can be included in a coatings formulation. However, compounds that are not bound to the backbone of the principle coating resin can leach out over time, greatly reducing their useful life.

Biocidal polymers contain active functional groups such as quaternary amines, various quinolone carboxylic acid derivatives such as Norfloxacin, or various N-halamines. These polymers may be classified as “biocide release,” or “non-contact kill”. Biocide release systems are available commercially Meanwhile, inorganic compounds such as zinc oxide, titanium dioxide and silver compounds may be dispersed in ordinary polymers to afford a “contact kill” class of antimicrobial coatings.

“Coatings go to work” also details the underlying physical and chemical processes for super hydrophobic coatings, self-stratifying coatings, photochromic coatings, self-healing and self-repairing coatings and thermochromic coatings. You can read the complete article in the January issue of the European Coatings Journal.