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Smart release of biocides in finishing materials for the sector of construction

Final Report Summary - AXIOMA (Smart release of biocides in finishing materials for the sector of construction)

Executive Summary:
The project was defined by the partners and formulated in a report act as a guideline for partners in the consortium and summarizes the basic information needed for research and the challenges faced by the consortium. End-user applications are specified covering both indoors and outdoors. The ‘AXIOMA’ project focused at investigating fungal and algal growth on coating and plasters in both indoor as well as outdoor environment and growth of fungi on tiles and grout in two indoor environments: bathroom and kitchen. In indoor applications the highest risk of attack by such micro-organisms was shown to be the bathroom environment, which was concluded from extensive testing. Furthermore, cold cellars with thermal bridges were shown to form a risk. In the outdoor situation the focus was put on algal growth on coatings and plasters, since the main concern is algal growth, especially on north facing facades. The growth rate of micro-organisms is sensitive to relative humidity and temperature and depends on the organism. To be able to investigate the effect of environmental conditions, a specific data-set of weather conditions (relative humidity, temperature, days of rain, etc.) through Europe was created, with data covering the last 10 years. The micro-organisms present in each end-user application were identified. Enzymes secreted by these organisms were identified, except for algae in which no enzymes are excreted, however other organisms will grow in the biotope of this organism. The choice of biocides (especially zinc pyrithione) reflects considerations of cost, conformity to the relevant regulations and effectiveness against a broad spectrum of organisms (including bacteria).

For efficient protection biocides should be released at a rate reflecting the growth rate of the organisms. Thus any ‘slow release’ technology (controlled release by moisture) should be tuned to ensure that biocide release maps the growth of organisms in the substrate being protected. Ideally a switching mechanism would be most advantageous, since that would act only when needed. This strategy is called ‘BioSwitch’. To achieve this BioSwitch biopolymers can be used that decompose on an action of the enzyme, freeing the active that prevents biological growth. To achieve this suitable biopolymers should be chosen that are compatible with coating and plaster formulations, and should be able to retain the biocide. Issues related to preparation and application, such as batch-sizes, maximum particles size of biocide containing additives, etc. For paints and plasters the behavior of the new materials in terms of rheology, gloss, setting-time etc. should differ little from that of conventional materials used in the building trades. For ceramic materials two approaches to achieving resistance against fungi were investigated. A suitable found working solution was submitted as a patent.

BioSwitch - Three bio-polymers were selected as BioSwitch materials, celluloses, proteins, and fats. After screening cellulose, they were not considered option in the formulations to be used. The biocide was inserted in the polymer to investigate the triggered effect. In both cases from capsules triggering was observed by addition of artificial enzymes, also confirmed by experiments on colored actives acting as model compounds for biocide. The particle size still needs to be further optimized. Also in coating and plaster formulations the BioSwitch materials were tested. However, unfortunately either too much retention or early leaching was observed, using the agar based medium tests. The high amount of nutrients may affect the testing. In all cases chemical analysis was difficult due to the low concentrations, and due to interference effects of other compounds. Fortunately, it was clear from the chemical analysis that the leaching and triggering can be observed, and may be optimized using a combination of biopolymers. Additionally, it was shown from a diffusion cell experiment that enzymes could pass the coatings used, allowing them to reach the biopolymers.

Nanoclay technology - Different nanoclays were produced with different surface modifiers. Different synthesis routes were investigated as well as different modifiers and clays. Discoloration effects were observed and subsequently solved. An improvement of a factor of 14 was observed in biological experiments, showing the potential of the nanoclay technology. However, the testing (performed by new test methods concerning artificial rain events and subsequent biological testing) conditions affects the release of the biocide. Differences in coating formulations also greatly influence the release rate. In most cases the nanoclay performed better (never less!) than the ordinary used biocide and/or zinc pyrithion. The latter has a very low solubility and showed already a slow release due to its chemical nature. Consequently, more in depth knowledge is required to optimize the technology.

Modelling - To understand the differences in coating formulation, environmental conditions, an empirical an deterministic model was built to describe the release of the biocide from the materials. The input parameters were determined from specially selected experiments, such as temperature, moisture transport, diffusion cell, wet cup and Magnetic Resonance Imaging (MRI) experiments. The resulting model calculations, verified by experimental tests, predicted a far slower release of biocides. We found that the humidity in the rain experiments is the reason, a high relative humidity after the rain event affects the release in fact it accelerates the release. Furthermore, the typical timescales of the process are identified and can be used for more targeted material development and release vehicles. Furthermore, a parameterized model was developed to describe the data obtained from the bathroom, cellar and kitchen. A specific software tool, based on experimental tests and empirical models, was designed and is available online through the AXIOMA website. It assists in the prediction of fungi and algae risk in both indoor and outdoor situations.

LCA and risk assessment - LCA was carried out to compare the environmental footprint of AXIOMA paints, plasters and tiles. A comparative analysis of different scenarios were made by taking into account the release of the biocide, the lifespan of the product, as well as, the amount of paint used in intermittent repaints for the reference surface. The results of LCA on the base paints have been compared with similar studies that were performed in the past in other European countries as a benchmark for the base material. The LCA shows a clear impact and positive of the AXIOMA technology for the products investigated.

Project Context and Objectives:
The acronym “Axioma” of this project is based on “Active eXternally Induced release in cOnstruction MAterials”, which is our “Axioma” for innovation of finishing materials for the sector of construction.
1.2.1 INTRODUCTION
Bio-resistancy of building and finishing materials usually requires addition of dedicated bioactive chemicals, so-called biocides. Two main reasons exist why biocides are added to materials for constructions. First of all, micro-organisms negatively affect the durability of material, i.e. induce biodegradation. Secondly, biocides are also used to prevent the growth of micro-organisms (algae, fungi and bacteria) that create unhealthy indoor environments or affect the esthetic value (Figure 1).
Traditionally, the action of biocides in materials, (e.g. coatings, plasters) is based on a passive and uncontrolled release principle, i.e. molecular dispersion of the active ingredients in the material matrix. As a consequence these bio-active agents have a high and inherent mobility in the matrix, which causes an initial boost in biocide activity and a steep decrease when time proceeds. Consequently, we are facing two challenges:
- Short bio-resistance of materials leads to early replacement
Coatings in the built environment usually exhibit biocidal functionality between 0.5 and 2 years, whereas the desired service life in building practice is at least 10 years. Application of an increased biocide concentration only results in a minor prolongation of the material service-life. Besides the environmental impact of early replacement of such functional coatings, an inherent disadvantage of the inefficiency of the traditional release concept is the emission of a relatively large amount of biocide molecules into the environment. In addition, the limited bio-resistance of the material will allow the growth of micro-organisms, which negatively affect health, leading to hypersensitivity and allergic reactions such as rhinitis and asthma. This is a problem prevalent in homes and occupational buildings worldwide [ , ]. A total of 11% of the entire global burden of disease has been attributed to unhealthy buildings. A recent pan-European housing survey by WHO clearly indicates a link between present-day housing conditions and human health and well-being. Therefore, the performance of biocide containing materials needs to be improved.
- Environmental legislation restricts use of biocides and chemicals
Growing ecological demands and international environmental legislation increases even more the pressure on the materials’ performance. First, the application of active agents for materials’ bio-resistance is regulated by the Biocidal Product Directive 98/08/EC, which sets strict conditions with respect to the use of bio-active agents [ ]. In line with that, the European Commission launched mid 2004 the 6th Environmental Action Plan [ ] in which the restriction of chemical emissions and consequent risks for human health have become a first priority. And second, the industrial trend towards eco-friendlier building products is often accompanied by an increase in the biodegradability, requiring even more biocides to compensate.

1.2.2 CONCEPT OF THE PROJECT
Our answer to the above challenges is:
To develop, adapt and apply smart release concepts of eco-acceptable biocides to extend service life of finishing materials substantially.
Figure 2: Three release mechanism; uncontrolled release, slow release and responsive release. The minimum inhibition concentration (MIC) denotes the concentration necessary to inhibit biological growth.
The smart release concepts will be based on induced or triggered release, giving bio-inhibition only when needed. This approach is new for materials in the construction sector, and requires considerable research effort to make the technologies applicable for finishing materials in the construction sector. Figure 2 shows three release principles. The left figure shows the uncontrolled release process which is the current situations for coatings and plasters in which biocides are added. The slow release mechanism is shown the middle, in that case a retarding step is used to release the biocides. However, the ultimate situation is the situation in which release is initiated in case of trigger.
In our case the responsive material will be able to react to variations in moisture level and presence of micro-organisms. We will focus on biocide release in response of secretion of enzymes by the micro-organisms (ultimate solution). To achieve this we will use a BioSwitch technology in combination with surface modified nano-clays. The BioSwitch matrix consists of a biopolymer which is destructed when in contact with enzymes secreted by micro-organisms. By combining this BioSwitch technology with surface modified nano-clays technology, a high loading of the BioSwitch material with biocides can be achieved. In addition the nano-clay technology itself can be used as a responsive release technology which reacts upon high moisture levels (can be considered a first step solution). We will focus on mainly on two finishing materials, plasters and coatings. In addition we investigate the possibility of applying a post-treatment (a coating) on ceramic materials.

Project Results:
The project has focused on developing the BioSwitch technology, which is a triggered release of biopolymers and slow release ‘nano-clay’, which can be combined with the biopolymer BioSwitch technology.

- To investigate the BioSwitch technology the AXIOMA project has focused on testing different biopolymers. We have shown that cellulose based biopolymer could not be used because of the incompatibility with the coating matrix. Clearly, the combined approach of research on fundamental and application level proved to be an advantage. Proteins and fat based biopolymers showed to be more suitable biopolymers. It was shown in lab experiments that triggering of biocides could be achieved when triggered with enzymes. Furthermore, we have shown several methods and strategies to “tune” the release within these biopolymers, by addition of several compounds to influence the network structure and increase binding with the biopolymer. Different methods of preparation were investigated, including spray drying. The most challenging was to obtain particles sizes below <40 µm, needed for paint applications. For application in plasters this was not a restriction. Experiments have also shown that the enzymes can permeate the commercial coatings used.

- In case of the nanoclay technology we have shown a potential protection of the nano-clays up to a factor of 14, using a rain setup (developed within the framework of the AXIOMA project) reflecting more realistic and practical leaching conditions. In the demonstrator we have shown that nano-clay containing materials were always amongst the best performing.

- New test methods have been developed. Furthermore, the experiments showed that the test method greatly influences the release behavior of the material. Slow release technology based samples outperforming others, showed equal performance with other samples in other tests. In those cases, these were standard testmethods, having an extreme regime, and as such those methods currently in use in the biocide industry, cannot be used to assess the performance of triggered release. Agar based test have an excess of spores and nutrients, and as such the timescale of growth is much shorter than in the application. In case of leaching tests, 24h dipping tests showed that this is a very aggressive tests. Consequently, it was concluded that this has shown to greatly affect the results of the tests. This indicates the timescales of the processes are very important, and a more deep insight in the fundamental processes is needed. Also the coating formulation has a major impact on the release, e.g. binder content, porosity, will influence the behavior. Furthermore, we concluded that even accelerated tests took a relatively long time, extending the time needed for a feed-back loop essential in development to several months for each step taken.

- We have demonstrated the concept materials in a demonstrator, since it is necessary to demonstrate the effectiveness of the proposed new products in comparison with existing practice. However, unexpected difficulties were encountered in attempting to establish the growth of microorganisms on the surfaces to be examined in the demonstrator. Controlling the climate proved to be difficult, several months were needed setting up, running and improving various test rigs and the protocols for using them. However, it was found that:
o The materials containing the nano-clays were always best performing, and outperformed the standard biocide used.
o The ceramic tiles protect the grouts, which directly relates to the experiments performed in the laboratory.

- To develop a suitable release system, models are needed to determine the typical timescales and obtain the corresponding parameters which can be used for the design of the release technology. The model built, within the framework of AXIOMA and TNO internal programs, can be used to predict the release, based on determined parameters. As a result of the model we have obtained more insight in the working process and timescales thereof. The model has also provided considerable insight in the processes and timescales involved.

- LCA has shown that release technologies have a considerable reduction in the environmental impact. Moldy conditions have a major impact on the indoor unprotected environment, due to airborne emissions and can increase the human health impact up to 89%. Conventional antimicrobial materials have significant higher impact compared to products using encapsulated technologies in all scenarios studied. Conventional antimicrobial materials have a higher impact on human health compared to products using encapsulated technologies, when assessed in view of ecosystem quality.

- During the course of the project the landscape with respect to biocides in Europe has changed considerably, with the biocidal product directive and the biocidal product regulation (BPR) . The number of biocides that can be used in products have reduced considerably. Additionally, labelling regulations will also lower the threshold for pot labelling. The focus of the AXIOMA has been on several biocides, but especially zinc pyrithione. At an early stage in the project it was decided to concentrate on zinc pyrithion for practical reasons: one of the beneficiaries is a supplier of the material, the product is known to be effective against a broad spectrum of micro-organisms and is widely used in toiletries such as anti-dandruff shampoo. Furthermore, it was likely to be acceptable in the context of building products. Zinc pyrithion has some advantages: Its low solubility in water makes it an inherently slow-release biocide. However, this makes the task of demonstrating the benefits of slow and triggered release compared to ‘normal’ release more difficult. It must be stressed that the AXIOMA concepts are ‘generic’ and potentially applicable to any permitted biocide compound.

- In addition, some fast and relatively low-cost assay techniques were difficult to apply to low concentrations owing to spectral masking by other materials. In all it was difficult to determine the low concentration of biocides, especially in case of more practical release circumstances. The rain test method developed has the disadvantage that although it is more realistic, more spreading in the results exists. Consequently, duplicates for the chemical and biological tests are needed. A good relation has been observed between chemical and biological tests.

- Making products acceptable to manufacturers of surface coatings at a price attractive to buyers in view of improved performance: the choice of materials and processes for AXIOMA is closely circumscribed, notably in terms of particle size, stability, viscosity, colour, ageing, temperature sensitivity, fire, health and environmental hazard, compatibility with a host of ingredients used in product formulation and finally cost - finding a route through such a maze without a map & compass proved frustratingly slow and almost brought the project to a halt around mid-term. It then became necessary to shift resources from new product development to screening processes and materials towards techniques for modelling biocide and enzyme transport mathematically.

- The mathematical model and tool will facilitate resumption of work after the project’s end in different combinations and permutations of parties, target product, release system and choice of biocide (or combinations thereof). In case of the ceramic products, the technology applied can be used as an active trigger to protect the grout. A patent was written. Furthermore, it was shown that a route towards the market is realizable, not only based on the technical results, but also based on costs and gains.
- Furthermore, for the nanoclays it was shown that it can be an economically feasible route, when taking certain commercial attractive clays and modifiers.
- For the bioswitch materials it was shown that some biopolymers are quite expensive, however, some particular based bioswitches seem to be the most commercial attractive material.

Potential Impact:
A complication has since arisen with the arrival of the Biocide Directive and Biocide Regulations now coming into force. This will have a major impact on the building industry:
1) the high cost of compliance will further restrict the choice of biocide available for product formulation; and
2) the need for compliance will shift concern from the present overriding pre-occupation with carbon emission and energy to bio-protection and safeguarding health and environment.

In the distant past building design concerned aesthetics, structural integrity and durability, today’s regulations force attention on energy efficiency and sustainability, but from now on the Biocide Directive will force attention onto concerns with environment, health and safety. In this milieu it is not structural integrity and comfort that plays the key role but chemistry – a subject about which the building industry (comprising the regulators, architects, contractors and building managers) knows next to nothing while facing a lamentable lack of reliable data.
Past experience with the ozone layer and global warming shows that it takes at least a couple of decades to pass from a regulatory crisis to established practice. In due course, the emerging crisis in bio-protection will provide fertile ground for new scientific services, R&D, innovation and new product development but in the meantime there is the problem of how to operate in this nascent environment, affecting particularly those parties in the value chain, e.g. like partners in AXIOMA, who possess only modest means.
Best placed will be AXIOMA partners offering scientific/technical services – they can already promote themselves unhindered by publishing papers, appearing at conferences, lobbying regulators and cultivating those who control funds (something they do in any case). This should put them into a prime position for creating and subsequently exploiting the new opportunities emerging in the wake of the Biocide Directive.

In this context attention to the Simplified Active Substances scheme of the Biocides Regulation 528/2012 might be helpful: it offers both opportunities for lobbying (as ‘discretionary powers’ to modify Annex 1 are implied) and the pursuit of new ideas and innovations meeting the requirements of the Simplified Active Substances regime. Much more difficult will be the position of those offering, or planning to offer, biocidal products. Again, there are several routes to exploitation that merit further consideration:
• As zinc pyrithion is already widely used in toiletries, there is an opportunity to lobby for its inclusion under the Simplified Active Substances scheme, though this will no doubt take time and much effort to bear fruit.
• In the meantime it may be possible to incorporate zinc pyrithion in formulations with¬¬out making any biocidal claims for it. This could be particularly promising if the smart release technologies developed under AXIOMA allowed the use of very low levels of biocide in producing effective and long lasting formulations. Promoting such products to justify the extra cost will require an imaginative approach and considerable care.
• Finally, the situation should stimulate a search for other solutions to the problems addressed by AXIOMA that more readily fall into the simplified active substance category. Guidance Notes on the subject being drafted by the Commission will cover, among others, substances such as ‘food additives, pheromones and other substances considered to have low toxicity such as weak acids, alcohols and vegetable oils used in cosmetics and food’. The wording, seems to invite creative interpretation.

The coming years it is clear that the biocide producers will move from getting their products legislated through the complex regulatory system towards new product development, in a next step to keep or increase their market share via innovations. Steps towards stabilizing and decrease the biocide release from products is to be expected. As indicate the challenges related to measurement and long cyclic testing, requires new approaches. It seems that the modelling aspect is a great opportunity to aid the market by focusing their product innovation.

List of Websites:
The project website of the AXIOMA project is: http://www.axioma-project.eu
Contacts:
Dr. ir. Bart Erich
Coordinator
TNO - Material Solutions
van Mourik Broekmanweg 6
P.O. Box 49, 2600 AA Delft
Tel: +31 88 866 3291
Mob: +31 6 13587645
E-mail: bart.erich@tno.nl
Website: http://www.tno.nl

Maarten Weide
Project manager
Uniresearch
Elektronicaweg 16C
2628 XG Delft – The Netherlands
Tel: +31 15 275 4000
Mob: +31 6 15 87 4004
E-mail: m.weide@uniresearch.com Website: http://www.uniresearch.com