Biocides: protection in packaging

Learn about the different types of biocides, their action and functionalities, depending on the application.

By Julián A. Restrepo*

Biocides (also erroneously called microbicides or bactericides), in their most general definition, are those substances used in the chemical industry (in sectors such as paints, inks, pigment dispersions, food, among others), as agents intended to destroy, counteract, neutralize and prevent action (or exercise other control), of any micro-organism considered harmful to man.

Contextualizing its use in the paint industry, it refers to those additives used, either for the protection of the paint on the packaging (called "biocides for protection in can" or simply "biocides in-can") or for the protection of the dry film of the coating, to prevent or prevent the growth of microorganisms in the paint.

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There are also biocides for the disinfection of production facilities, where their use is not intended for a particular product (which will be consumed by the end user), but for the disinfection of paint production plants. Naturally, this type of biocides are more active and aggressive than the previous ones, since in this case we are talking about "aggressively attacking" these microorganisms, so that there is no contamination of the manufactured products.

It must be said that the term microbicide is mainly used in the pharmaceutical industry to designate those substances or compounds intended to prevent infections or diseases in man. While the term bactericide is partially inaccurate to designate a biocide, since it refers to that substance only with the ability to fight bacteria, but as will be seen later, biocides not only fight bacteria, but a wide spectrum of microorganisms.

The basic objective of this paper is to discuss the most relevant aspects about the chemical biocides used for the protection of water-based paints in the container and to give some guidelines for the choice of the correct biocide, based on the biocides currently used in the Latin American paint market; ignoring much of the issue concerning recent legislation in European countries, for example.

Mechanism of action: Biocides usually act on the cell membrane of the microorganism, penetrating it and destroying the systems that allow the microorganism to live: The biocide causes the lysis of the protein or lipo-protein wall of the organism and penetrates inside interrupting the biochemical reactions that sustain life in the organism [1].

Types of biocidal products: Biocides can be synthetic chemicals, of biological origin or come from physical sources. Here's how you have:

a. Physical agents: They are those that do not correspond to chemical substances, but rather to physical methods that allow to destroy microorganisms, such as heat or sources of high-energy radiation (ultraviolet light, infrared radiation), which oxidize the protein wall and practically burn the microorganism.

b. Biological: They are those substances of biological origin, created naturally by living beings for self-defense; they are usually of the protein type, among which are some enzymes, for example, lysozyme.

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c. Chemicals: They are those substances or compounds derived from chemical synthesis, and can be organic or inorganic in nature. Some examples are: chlorine dioxide (ClO2), isothiazolins, benzoisothiazolins chloramines, alkyl bromides, alkyl chlorides or aryl, among others.

Contaminants in aqueous paints
It should be said that it is not common to talk about biocides for solvent-based paints, since in itself the organic solvent content prevents the growth of microorganisms; although we also talk about the fact that solvent-based paints do not contain water1 and do not use cellulosic thickeners2.

When analyzing the sources of contaminants in water-based paints, they can be mentioned, mainly:

Water: Since depending on its origin, it can present bacterial contamination. Taking into account that the water in aqueous paints can reach a content greater than 50% by weight, it becomes a raw material that can provide an appreciable amount of microorganisms.
Loads and extensions: Such as calcium carbonates, kaolins and talcs (the common loads used in paints), because they can contain organic matter. Especially, when it comes to loads supplied in sludge ("sludge loads"), since in many cases the material is not subjected to a drying process, but at all times can be found in the aqueous phase in the beneficiation process, so it can be presented (in some cases), risk of the presence of microbiological contamination, which is why sludge manufacturers include in their formulation a certain biocide content.
Containers: Since they may contain traces of dust or other contaminants.
Equipment, plants and operators: Either by the presence of biolayers or enzymatic contamination.

It should be noted that, no raw materials are mentioned such as:
a) Resins in aqueous dispersion, since generally, the manufacturer of these formulates his product with a quantity of biocidal product such that it allows to keep it free of microbiological contamination; in addition, in some cases, its 'residual monomer' content is sufficient to have a biocidal effect.
b) Pigments, since these are synthetic and come almost exclusively from chemical processes, where the product at the end of the process is washed and dried, and bearing in mind that it is powdered material, it is not expected to find this type of contamination in them.
c) Additives, many of those used in aqueous paints are supplied in liquid phase (dissolved or in aqueous dispersion), and many of them have a biocidal character; in other cases, due to their manufacturing process, this type of contamination is not expected to be found in them either.

Main types of contaminants in paints
Bacteria: They grow in neutral or slightly alkaline solutions, at a pH between 4-9.
Algae: They require water and sunlight.
Yeasts: They feed on sugars or starches.
Fungi: They grow in acidic solutions, at a pH between 4-6.
Molds: They require decaying organic matter.
Virus: They require living things or cells. It is worth mentioning that, in reality, virus contamination is very rare in paints, and occurs only in environments of high contamination.