The preservation of paint during the storage and sale stage, prior to application, requires the use of biocides that prevent the degradation of compounds.
By: Julián A. Restrepo R.*
Some time ago, specifically in the March and April 2009 issue (vol. 14, No. 2, pp. 20-22), the importance of biocides as protective elements for packaging had been addressed. In this edition we will return and expand the concepts, in order to get a global idea on the subject.
As a first step, we will deal with the characteristics of a good biocide for preservation in the packaging. An effective biocide must meet the following qualities:
* Effective at low concentration: The lower the required dose, the more economical the use of the biocide; it is desirable that it be effective at low doses of use. Therefore, it is required to present a "good cost/benefit ratio".
* Broad spectrum of activity: That is, it must cover a wide range of microorganisms (bacteria, fungi, algae and yeasts).
* Effective in a wide temperature and pH range: It must have resistance to the temperature and pH of manufacture, and conditions of use.
* Compatible with other components of the formulation: There is no 'antagonism' between the biocide and other materials.
* Wide range of solubility in water: The ideal is to have biocides completely soluble in water, to improve their effectiveness.
* High persistence: Being effective over time and avoiding bio-resistance, preventing microorganisms from generating resistance to their toxicity.
* Easy to neutralize: It must have deactivating mechanisms for subsequent neutralization, or in case of spills.
* Low human toxicity: It must not be detrimental to safe handling by the operator. In fact, in broader terms, it is desirable that the biocidal product has low toxicity and ecotoxicity (refers to its effect on the environment).
Terminology of interest
At this point it is important to clarify a number of terms related to the use of biocides:
* The phenolic coefficient [2], is an experimental value that is measured to those substances that have biocidal properties, taking as a reference the biocidal capacity of phenol. This coefficient makes it possible to determine the relative efficiency of a biocidal product, or rather, its effectiveness as a biocidal additive.
* The minimum inhibitory concentration (MIC), expresses the minimum amount of biocide necessary to prevent microbial growth, in a laboratory test (dissolution technique) and is determined visually.
Sanitization, is the cleaning operation carried out in the production facilities for the control and elimination of pathogenic microorganisms from the environment, either by physical or chemical methods. This process uses special biocides (chlorine dioxide, dry chlorine compounds, glutaraldehyde, iodophor compounds, among others), and allows the elimination of biolayers.
* Biolayers (or biofilms), are layers or thin films formed in production facilities by microorganisms, as a result of their metabolic activity [3].
* Microbial resistance or bio-resistance, refers to the mechanisms that microorganisms develop to survive over time even with the presence of a dose of biocide that was initially lethal, so that more and more biocide is needed to obtain the desired effect.
* Enzymatic contamination: Enzymes are organic catalysts produced by microbes (e.g., pseudomonas). They are not commonly inactivated by the action of biocides. The easiest way to prevent it is through disinfection of possible affected sites (sanitization).
Effectiveness of a biocidal product
It is measured through microbiological tests, among which are commonly used:
* Colony-forming unit (cFU) counts, the results of which are reported in cfu/mL units.
*Challenge tests, where a sample is inoculated with bacteria every week for a month.
* Tests at temperatures of 35ºC for several weeks.
It should be said that water-based paints require biocides for their protection in the packaging and for the protection of the paint film. While solvent-based paints in liquid form do not require the use of biocides, but do require protection of the paint film.
It is not to be forgotten that the vast majority of aqueous paints (mainly architectural paints), employ cellulosics as thickening agents (which are nothing more than carbohydrates), and although there are some commercial varieties improved enzymatically, these additives are still very susceptible to microbial attack. Thus, in a conventional architectural painting you will find water and carbohydrates, which are the best conditions for microbial growth (if the pH is adequate).
It is important to note that there are currently uretic thickeners, which are of different chemical nature, so they are an alternative to cellulosic thickeners (in terms of their tendency to biodegradation).
Characterisation of water-based biocidal products
The biocides used for protection in the packaging in aqueous paints are basically aqueous solutions, usually of one or more active components and their concentration varies from 1.5 - 28% by weight. The most commonly used active compounds are:
* Formaldehyde releasers (LFs): Such as Ethylene glycol-hemiformal and benzyl-hemiformal (phenyl-methoxy-methanol).
Isothiazolins (ITs): Generally a mixture between CMIT (5 chloro,2-methyl,4 isothiazolin-3-one) and MIT (2 methyl, 4 isothiazolin-3-one).
* Benzoisotazoline (BIT).
* Bronopol (2-bromine,2 nitro-1,3-propanediol) or BOP.
In some cases, the solutions are not only composed of water, but are accompanied by glycols (propylene glycol) and glycerin.
Selection criteria
Based on these active compounds, the market includes:
* Isothiazolins + formalin releaser: fight bacteria, fungi and yeasts; pH of use: 2.5 – 9.0. They act in the "head space".
* Isothiazolins (without formalin releasers): Fight bacteria, fungi, yeasts and algae; pH of use: 2.5 – 9.0.
* Formalin releaser: Fight aerobic and anaerobic bacteria; pH of use: 3.0 – 12.0. They act in the "head space".
* Benzoisothiazolinone: Fight bacteria, fungi and yeasts; pH of use: 4.0 – 12.0.
* Bronopol + Isothiazolins: Fight aerobic and anaerobic bacteria, fungi and yeasts; pH of use: 4.0 – 9.0.
As a guide, the following summary is presented that allows to establish the required biocide under different conditions:
* pH>9, temperature >45 ºC, without limitations for formaldehyde: It is suggested to use a biocide based on formalin releaser.
* pH>9, temperature >45 ºC, formaldehyde free: It is suggested to use a biocide based on Benzoisothiazolinone.
* pH<9, temperatura <45 ºC, sin limitaciones para el formaldehído: Se sugiere emplear un biocida basado en una mezcla de Isotiazolinona y liberador de formol.
* pH<9, temperatura <45 ºC, libre de formaldehído: Se sugiere emplear un biocida basado en una mezcla de Bronopol e Isotiazolinona.
Final comments
Compounds such as Bronopol and Isothiazolins degrade under the same conditions: They decompose at pHs greater than 9; except for benzoisothiazolin, which can withstand pHs close to 12, and formalin releasers act above and below 45 °C.
Periodic rotation of the biocide (i.e. the constituent active ingredient) is recommended; for example, to change at least between 9 to 12 months the biocidal product based on mixture of formaldehyde + Isothiazolins for the biocide based on mixture of Bronopol + Isothiazolins. But remember to change the biocide based on the active ingredient and its concentration.
Some manufacturers dispute the existence of bio-resistant microorganisms (resistant strains), while others recommend the rotation of the biocide to avoid immuno-resistance of microorganisms.
It should be said that, chemically the reducing media are conducive to the growth of microorganisms: thus, technically in rigor, for the choice of the correct biocide it is very important to know the oxido-reducing conditions of the system.
To establish the proper dosage of biocide in a particular formulation, it is very important to know what is the minimum inhibitory concentration of microorganisms from which the paint is required to be protected.
References
[1] http://es.wikipedia.org/wiki/Biocida
[2] http://www.virtual.unal.edu.co/cursos
[3] http://www.agualatinoamerica.com/docs/PDF/070804%20Nivel%201.pdf
[4] Bayer Chemicals (Lanxess Group) website: http://www.lanxess.com/en/hubpage.html
http://www.protectedbypreventol.com/mpp/en/global/
* M.Sc. PhD. Innovation Coordinator. Pinturas Sapolín, Invesa S.A.[email protected]
Girardota, Colombia.
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