Introduction:
Bio-degradation is major problem in industries like textiles, automotive, medical, home appliances, paint and coatings, construction, packaging etc. Bio-cides have been in use to prevent microbial attack and hence to prevent damage of products. Though usage of bio-cides is essential in many industries, it is well known that they have negative impact on environment as well as on human health especially pregnant women, unborn life, small children, or people with serious chronic illness.
According to Central Insecticides Board & Registration Committee (CIBRC), common preservatives which are used in waterborne coating (example – diuron, carbendazim etc.) are causing harm to living beings and the environment.
Thus now a days, scientists are facing a new challenge to balance environmental obligations and product’s performance.
In this review, three approaches are discussed which have been explored in textile, paper, food, biomedical, cosmetics etc. fields and can be alternates for sustainable antimicrobial products.
Exploring metallic nano-particles / nano-composites, antimicrobial polymers, incorporation of material based on natural resources are the approaches focused in this article.
Nano-particles/ nano-composite polymers:
From ancient time, silver (Ag) and copper (Cu) are renowned for their anti-microbial efficiency. Ag and Cu nano-particles have attracted attentions for their applications in numerous fields. In past few years, copper being heavy metal, having known its side effects on human being and other living organisms, copper’s direct use as preservative has been controlled in many industries.
However, silver nano-particles have unparalleled advantages due to its broad antimicrobial spectrum, high stability, low human toxicity.
Mechanism of anti-microbial effect of silver is not completely understood. Three mechanisms are proposed for silver nano-particles are, (i) gradual release of silver ions, followed by disruption of ATP production and DNA replication, (ii) direct damage to cell membrane by silver nano-particles and (iii) silver nano-particles and silver ion generation of reactive oxygen species.
Figure 1 explains possible mechanisms of silver to kill microbes.
These mechanism involves interaction of silver with thiols or disulfide groups of enzymes (4,6,9,13). Various routes are studied by scientists to use Ag NPs in textile, biomedical, dentistry, food packaging industries but rarely work has been done in paint industry. Though AG NPs are popular due to their unique properties, some detrimental effects of them are release of free silver ions, easy agglomeration, difficulty to be immobilized on surfaces, lack of controllability in synthesis etc. It leads to reduction in their efficiency (4,13,14,15).
The release of free silver ions is strictly controlled by many national law and control agencies worldwide (5,9). Polymerization is one of the best routes to control many of the above factors and enhance.
Antimicrobial polymers:
Another approach to achieve antimicrobial activity is antimicrobial polymers. Such polymers are designed with low surface energy and are engineered to release active agents from their surfaces. Antimicrobial polymers can be produced by synthesis of monomers having bio-cidal moiety and their polymerization or it can be co-polymerized with another monomer also.
Figure 2 explains the low surface energy of antimicrobial coating which reduces the microbial attachment and bio-cidal moiety kills the attached microbes subsequently.
Examples of antimicrobial polymers are N-halamine based polymers, quaternary amino functional polymers. These polymers can be used as binder or in an additive form.
Research work in this area is receiving more attention for their application in textile, food, biomedical industries. Synthesised polymers using N-halamine monomers, silver sulfadiazine which can be used as polymeric biocidal agents. N-halamine is a compound which contains one or more halogen-nitrogen bonds. This bond formation can be done by halogenation like bromination or chlorination of imide, amide or amine groups. Reaction for Halogen transfer from halamine in microbial cells results in disruption of cell and finally cell death.
This patent invented the novel method of polymerization to incorporate halogenated monomer for textile application. B. Qian and coworkers studied micro/nanocapsules based on pickering emulsions showing dual self-healing and antibacterial properties. Polysulfone/silica hybrid Pickering microcapsules have prepared using linseed oil. Silica particle modification have done with quaternary ammonium salts to make their surface active against bacteria. Modified silica surface can be adsorbed on negatively charged bacterial cell wall, resulting in cell death.
Polymers containing reactive groups like trimethylsilyl (TMOS), (dimethylamino) ethyl methacrylate etc. have modified with silica particles. Resulting polymer showed superhydrophilicity and antimicrobial performance against E.coli due o presence of quaternary ammonium group. Modified glass surfaces prepared using composite polymer showed high stability, antifogging property, easy cleaning and antibacterial efficiency. “Green” antimicrobial agent has synthesized by S.B. Aidarova and coworkers. Sio, nanoparticles as a shell and (trimethoxysilyl) propyl methacrylate (TMSPMA) loaded with 5-Dichloro-2-n-octyl-4-isothiazolin-3-one (DCOIT) as a core were synthesized as “submicrocontainers” and studied their antimicrobial activity. Pickering emulsion prepared using silica aqueous solution and oil phase added along with emulsifier for stabilisation.
In second stage photoinitiator used to carry out polymerization. Submicrocontainers with DCOIT showed 72% growth inhibition for A. niger, A. awamori and B. cereus, whereas free DCOIT showed 52% in 5 days duration.
Essential oils:
Essential oils are plant originated compounds that have antioxidant, antimicrobial, antiradical properties and hence are widely getting used in fields like paper, food, medical, cosmetics from centuries. Essential oils are basically complex mixture of volatile & non-volatile constituents which are produced by plants as secondary metabolites. They consists of terpene and terpenoids, aldehydes and phenols as a major functional groups.
Mechanism of antimicrobial action of essential oils is not fully understood yet, but few possible theories have explained the interaction of their different constituents with microorganism cells. For example, cinnamaldehyde, Carvacrol, Eugenol interact with intracellular protein and enzymes to disrupt protein synthesis and ATP production resulting in cell death. Whereas, Thymol, p-Cymene disturb cell membrane.
Figure 3 illustrates possible antimicrobial action of essential oils as per constituents.
Some of the essential oils (Os) have antibacterial and antifungal activity and can be used in combination with other essential oils. Hydrophobic nature of EOs is the main constraint for their incorporation & stability in water-based products.
Emulsion polymerization is one of the best methods, adopted in food, textile, cosmetics industries, through which EOs can be stabilized in water-based systems.
To stabilize most of the essential oils, oil-in-water emulsion technique used with stabilizers like triglecerol, triglecerides, Tween 80, Span 80, Tween 20. Chitosan is second most abundant polysaccharide and non-toxic, antibacterial, chelating biopolymer obtained by deacetylation of chitin (37).
Property improvement have observed in chitosan film when it infused with Eucalptus oil nanoemulsion, Tween 80 surfactant was used to stabilize the oil in emulsion form using ultrasonication method. In agar diffusion method, EO modified chitosan film (for 5% concentration, ZOl = 15 0.1 mm) showed better inhibition than chitosan film (ZOl = 7+0.1 mm) against clinical pathogen Staphylococcus aureus (37). Similarly mint oil, parsley extract, cinnamon oil, clove oil can be stabilized using these surfactants or triglycerol. Their composite with biopolymers like chitosan, alginate improved antimicrobial efficiency than chitosan and alginate individually.
Conclusion:
Demand of sustainable products is increasing tremendously in all industries due to worldwide stringent laws and legislation for betterment of environment and all living beings. The use of natural alternatives for preservation is not as simple as an ingredient substitution; there are many practical issues to consider in maintaining safe, effective, and stable consumer products that are free from microbial contamination.
Conventional preservatives annot be completely replaced by nano-composite, essential oils, or halogenated/quaternary amino functional polymers. The problematic organisms of concern studied in ood, health, textiles, biomedical industries are common to hose encountered to be causing problems in paints and oatings. Hence these methodologies can be explored as co-friendly and safe alternatives in niche products.
By: Subhadip Sikdar, Manager – R&D, Apcotex, Taloja
Subhadip has completed his M.Sc. degree in Chemistry from IIT Kharagpur and MBA in Marketing from Welingkar College. He has total 10.5 years of experience in polymer industries. Earlier he has worked with Asian Paints Ltd. and Huntsman India Pvt. Ltd.