SubscribeA hygiene coating system is a multi-functional new surface applied to a building substrate.
The coating must be tested, certified, safe to use/apply, not contaminate the environment locally or overall, provide defined physical functions as well as decorative benefits and be capable of use in a variety of environments and on virtually any substrate. It is also to be understood that the film will protect itself from micro-organisms whilst neutralising them in a way which will not allow resistance to develop.
The needs of clean rooms
These are those areas housed in structures where it is essential that micro-organisms are not allowed to proliferate. The infrastructure being coated – ceilings, walls, (and floors) – are diverse, as old buildings are refurbished and new buildings are constructed. The demands on a coating are to cope with anything from old, possibly damp substrates, to modern synthetic surfaces.
They must also allow for moisture vapour pressure, give excellent adhesion even if steam-cleaned daily, be resistant to cleaning chemicals or contamination from chemical splashing and continue to provide a seamless film-protected environment with no hiding places for deposited matter.
To ensure this, the coatings must be part of a system approach which recognises that substrates – particularly old, previously uncoated ones – will be contaminated with micro-organisms and require sterilising before a hygiene system is applied.
Following this, the in-use situation may demand that the coating can be reinforced to resist impact damage and that a fully seamless approach is provided over joints and cracks, which may already exist or can be expected to occur.
The coatings need to include elastomers and possess a rheology to allow high enough film build to include local or overall reinforcement systems, this differentiates them from normal paints.
Industry will ask that the system gives long-term protection from the growth of micro-organisms and this will usually be seen as the main function. A further requirement is that the coating system will do this without exuding contaminating chemicals. If biocides which leach to the surface are used to protect the film, they will be both short-lived and potentially contaminating.
There are additional speciality requirements. Pharmaceutical industries demand clean rooms whose surface finishes will not attract and retain dust and/or are easily cleaned. Rooms can be prone to condensation and this emphasises the point that film-protective agents must be non-leaching.
The environment in which the system is to be applied is often not helpful to the application process. The pace of modern life dictates that industrial environments are ‘in process’ 24 hours a day, seven days a week.
Coatings must not contaminate the working atmosphere and offer fast turn-around times. The needs in this respect are for water-borne systems which can be applied and allow continuation of processes within 24 to 48 hours.
Legislation
It is clear that manufacturing industries will have in-house demands from their own hygienists and environmental managers. The other driver is that of legislation.
When considering what legislation prevails at a given time, we truly enter the field for experts. A simple look at some relevant extracts from the European Directive (93/43/EEC) is useful:
- Surfaces must be maintained in a sound condition and they must be easy to clean and, where necessary, disinfect. This will require the use of impervious, non-absorbent, washable and non-toxic materials, and require a smooth surface up to a height appropriate for the operations …
There are other relevant legislative issues. The European Biocidal Products Directive demands that any biocide or product containing a biocide, such as a hygiene coating, shall be subject to regulation.
Most European countries have adopted this position, and the coating manufacturers have to have the reassurances that all biocides added will be supported by their respective manufacturers through ‘examination’ and toxicological data.
Finally VOC regulations are decisive. It is highly desirable to use water-borne coatings which have as low a content of VOC’s as possible.
Pre-Treatments, Coatings and Systems
What then of the coating systems which are available? What are the key benefits and properties and what advantages are there over conventional paints?
Coating Systems
The choice is often made from remarkably few criteria:
- Is the system perceived as safe to use and does it carry any approvals to verify efficiency?
- Is the system tested, and if so, by a third party and to recognised standards, against specific elimination of micro-organisms, for example?
- Can it be used in the given process area?
- Is the system a complete package?
- Can it be applied by brush, roller or spray as may be convenient or dictated by process considerations?
The package approach is important to guarantee the co-ordination of system, and should include the following products:
Repair Mortars
A comprehensive range of one or two-pack materials based on the most modern liquid or spray-dried polymers in conjunction with cement and micro-silica technology.
Sealants
Specific water-based acrylic sealants are available with excellent compatibility both above and below the hygiene coating system. These are internally film protected in a similar way to the coatings, in sealing around door/window frames, and between coatings and skirting boards, for example.
Biocidal washes
Typically a biocidal wash is water-based and supplied as a concentrate for use as-is in known areas of contamination, or let down with water as a sterilising wash. In the author’s experience, the use of a biocidal wash below a coating system provides long-term eradication of spores, rather than just a quick kill.
This is important if the substrate has been contaminated, as spores may be deeply embedded in the substrate and a soak-away eradicator prevents regrowth below the coating which otherwise may be pushed off.
Primers
Water-borne primers are available to provide the functions of substrate binder (where friable), functionality where needed (eg metal primer), and to ensure top coats strongly adhere. Not all top coats need a primer and selective priming can be useful.
For example, a mainly dry process area may have a critical area subject to regular steam cleaning or frequent washing. The added adhesion of a primer would assist here whilst not being necessary elsewhere. Conversely, a humid or wet process, or one subject to regular overall wet cleaning, would benefit from overall priming.
Top coats
These may be used alone, in combination with each other as defined by the manufacturer, or as part of a reinforced system. Table 1 shows details of five top coats, all water-borne with low VOC levels and defined physical test data. In most cases, two coats can be applied in a single day if necessary.
All are low VOC, water-borne, have good water vapour transmission and are highly fire retardant. These features allow a full range of substrates to be coated, though some demand a system approach rather than any single coating. For example, a tailor-made system would include:
- Lining board substrate
- primer to kill suction;
- product A into which is set glass-fibre woven reinforcement;
- two finish coats of product B
This gives a tough impact resistant system which covers the joints in between the boards to give the required seamless finish.
Table 1: Properties of hygiene topcoats for different application
| Product | A | B | C |
| Chemical Type | Single-pack water-borne acrylic | Single-pack high-build water-borne acrylic | Single-pack water-borne PU/acrylic |
| Key Features | High-build, non-leachable protection, crack-bridging, interior and exterior use. Matt finish | Very low VOC (7g/L), non-leachable protection, hard – resists mechanically aggressive cleaning. Sheen finish | Gloss finish, non-leachable protection, fast drying, resists mechanically aggressive cleaning. Gloss finish. |
| Water Vapour Transmission1 | 4.8g/m²/day at 516µm | 22g/m²/day at 140µm | 44g/m²/day at 72µm |
| Scratch Resistance2 | N/A | 2.5Kg | 3.0Kg |
| Tensile Load/ Elongation3 | Unreinforced 3N/mm²/200% | Unreinforced 9.3N/mm²/40% | 13.2N/mm²/20% |
| Reinforced 13.9N/mm/10% | Reinforced 18.2N/mm²/40% | ||
| Fire Retardancy4 | Class O | Class O | Class O |
| Drying Time5 | N/A | 45 mins | 45 mins |
| Product | D | E | |
| Chemical Type | Two-pack water-borne PU/acrylic/ siloxane | Single-pack water-borne acrylic copolymer | |
| Key Features | Direct adhesion even to tiles, ideal for kitchens, showers etc, impact and scratch resistant. Gloss finish | Very low VOC (13.7g/L), direct application to plaster boards and renders, economic for use in corridors etc. Matt finish | |
| Water Vapour Transmission1 | 22.3g/m²/day | 21.2g/m²/day | |
| Scratch Resistance2 | 4.2Kg | 700g | |
| Tensile Load/ Elongation3 | N/A | 6N/mm²/114% | |
| Fire Retardancy4 | Class 1 | Class O | |
| Drying Time5 | 30 mins | 45-60 mins |
1. Under temperate conditions. In tropical conditions figures may be different. 2. Resistance to full penetration. 3. For a 1cm wide strip. 4. BS476 Part 6 (for Class 1) and Parts 6 & 7 for (for Class O). 5. At 20ºC/55% RH or 23ºC/50% RH.
Other reinforcements include knitted nylon reinforcement tapes for specific joint treatments, which may be used locally as shown in Figure 1 – in this case the crack could be static or dynamic.
Figure 1: Procedure for producing smooth crack-bridging finish
Hygiene coatings compared with traditional materials
Traditional materials include conventional fungicidal paints; tiles and wall cladding systems. To look at these individually:
Conventional fungicidal paints
These are designed to give ‘quick kill’ of micro-organisms with biocides which are in excess on the surface of the dried paint. The drawbacks are that they leach potentially toxic chemicals into the environment and that washing cycles dilute these so that they are efficient for only two to three years.
The paints also tend to be thin, hard, prone to flaking and can lose adhesion. They can therefore only be used on dry static substrates.
Ceramic Tiles
These are very hard, easy to clean and impermeable. However the grouting is the weak link with growths usually seen in that area. They can only be used on ideal (flat) substrates.
Wall Cladding Systems
These are inert, but expensive and require specialist installation. They do not provide a seamless finish and usually have no inherent ‘surface protection’, unlike hygiene coatings.
However, they can be used as the basis of a seamless system, as shown in Figure 2.
Figure 2: Construction of a seamless hygiene cladding system
Microbial problems and hygiene coatings
Pharmaceuticals and clean rooms
The threat here is from the contamination of products and cultures from (mainly) airborne bacteria. Equally it would be disastrous to have any fungal/mould growth giving potential cross-contamination. It requires only low levels of opportunist pathogens to cause spoilage.
Examples of spoilage micro-organisms are; Salmonella SP, Pseudomonas SP and various gram negative bacteria/pyrogens.
Combating the proliferation of micro-organisms
What is needed from a hygiene coating to prevent proliferation and how is this achieved?
First the active ingredient in the coating must be non leaching. A comparison with a ‘leaching quick kill paint’ is shown in Figure 3.
Non Leaching Anti-Microbial Protection Versus A Quick Kill Paint
Figure 3: Limitations of ‘quick-kill’ coating systems
The lack of any zone of inhibition with fungal growth right up to the edge of the hygiene coating shows that no leaching is occurring. The quick kill material leaches out its active and diminishes in its effectiveness very rapidly.
A comparison on a life-cycle graph further illustrates the point in Figure 4, in which:
- On the unprotected film, bacterial proliferate until, in the closed test system, they run out of space and food.
- The contact/quick kill paint swiftly kills bacteria until the active runs out and growth recommences.
- The correctly formulated hygiene coating shows a growth pattern as it neutralises more slowly, but then eliminates growth to virtually zero proliferation.
Figure 4: Changes in bacterial count over time
(arbitrary units, bacterial count x106)
Any individual organism must be neutralised by a process which is non-site specific (ie multi-site) to avoid the risk of resistance developing. This can be simply summarised as:
Mode of Action: The biocide chemically disrupts the metabolism of the micro-organisms preventing life processes.
Site of Action: Respiration; spore forming; protein/enzyme synthesis; reproduction; energy assimilation.
This results in the following neutralisation process:
- surface is contaminated;
- assimilation at multi-sites;
- disruption by biocide;
- denatured – neutralised;
- non-proliferating – non sporing.
Approvals and test certification
The hygiene coatings illustrated in this article have been tested to a variety of challenge tests from European sources:
Fungi/yeasts
The recognised test in the UK is BS3900 Part G6 “Anti-microbial Surface Properties”. This calls for resistance testing to a minimum of nine mixed fungal/yeast inoculum. More can be tested. Product A in this paper was subject to a mix of 39 species, successfully neutralising all.
Bacteria
The search for a universally accepted test method continues in this area. There have been several attempts by the IBRG (International Biodegradation Research Group) to produce both a European and an ISO standard method.
Current thinking centres on the Japanese Standard JIS Z2801 which is a surface anti-microbial challenge test. Hygiene coatings act as a secondary line of defence to known cleaning regimes. Tests are also commonly carried out on a wide range of additional bacteria.
Conclusion
In meeting industries clean room requirements, a holistic approach is needed in selecting a hygiene coating:
- The coating must offer a range of physical properties to perform a number of associated tasks dictated by both the substrate onto which it is being applied – as well as the use environment with decorative aspects being covered by a range of matt to gloss finishes.
- The anti-microbial ‘film protected’ coating must be demonstrated to be multi-site specific in its action, as safe as possible to use and offer guaranteed long-life performance.