Robert Stanfield

Clean rooms are areas in which it is essential that microorganisms are not allowed to proliferate because they could contaminate pharmaceuticals and directly affect human health. 

The infrastructures being coated are diverse, creating demands on a coating 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. 

To ensure this, the coatings must be part of a system approach that recognizes substrates — particularly old, previously uncoated ones — will be contaminated with microorganisms and require sterilizing before a hygiene system is applied. Following this, the specific situation may demand that the coating can be reinforced to resist impact damage and that a fully seamless approach is provided over existing or potential joints and cracks. 

Industry requires the system to give long-term protection from microorganism growth and this will usually be seen as the main requirement. 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 short-lived and will potentially contaminate the environment.  

There are additional specialty requirements. Pharmaceutical industries demand clean rooms whose surface finishes will not attract and retain dust, and are easily cleaned. Rooms can be prone to condensation and this emphasizes the point that film-protective agents must be nonleaching. 

 Table 1 Properties of hygiene topcoats for different applications.			

Time restraints mean a room must be ready to be used 24–48 hours after the application of a hygiene coating. To comply with this, water-borne agents are best as they can be applied without contaminating the environment with high levels of volatile organic compounds (VOCs).			

Manufacturers will have in-house demands from their own hygienists and environmental managers, and external legislation regarding validation and performance of hygiene measures. When considering what legislation prevails at a given time, we truly enter a 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."

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. 

Finally, VOC regulations are decisive. It is highly desirable to use water-borne coatings, which have as low a content of VOCs as possible.  

What coating systems are available? What are the key benefits and properties, and what advantages are there over conventional paints? 

 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? 

 Has the system been validated to recognized standards by an independent third party?  

 Can it be used in the given process area? 

 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 coordination of the system, and should include the following products: 

 One- or two-pack materials based on the most modern liquid or spray-dried polymers in conjunction with cement and microsilica technology.  

 Specific water-based acrylic sealants are available. 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. 

 Typically a biocidal wash is water-based and supplied as a concentrate for use as-is in known areas of contamination, or diluted with water as a sterilizing wash.  

 Water-borne primers are available to provide the functions of substrate binder (where friable), for metal primer and to ensure top coats strongly adhere.  

 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 types of top coats, all water-borne with low-VOC levels, fire retardancy and defined physical test data. In most cases, two coats can be applied in a day. 

Figure 1 Procedure for producing smooth crack-bridging finish.			

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, which incorporates  						

 primer to kill suction and prevent absorption of the coating resin binder into the substrate, thereby ensuring film integrity 

 product A set with glass-fibre woven reinforcement

This gives a tough impact-resistant system, which covers the joints in between the boards to give the required seamless finish. 

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. 

Hygiene coatings versus traditional materials. 

Traditional materials include conventional fungicidal paints, tiles and wall cladding systems.  

 These are designed to 'quick kill' microorganisms with biocides that are in excess on the surface of the dried paint. The drawback is that they leach potentially toxic chemicals into the environment. 

Figure 2 Construction of a seamless hygiene cladding system.			

Grouting is the weak link with growths usually seen in that area. They can only be used on ideal substrates. 

 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 (Figure 2). 

The threat here is from the contamination of products and cultures from (mainly) airborne bacteria. Equally, it would be disastrous to have fungal/mould growth giving potential cross-contamination. It requires only low levels of opportunist pathogens such as salmonella and pseudomonas to cause spoilage. 

Figure 3 Changes in bacterial count over time (arbitrary units, bacterial 	count 3106). 			

Combating the proliferation of microorganisms. 

What is needed from a hygiene coating to prevent proliferation and how is this achieved? A comparison on a life cycle graph illustrates the point in Figure 3. On the unprotected film, bacteria 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 neutralizes more slowly, but then eliminates growth to virtually zero proliferation. 

Any individual organism must be neutralized by a process that is nonsite specific (for example, multisite) to avoid the risk of resistance developing. This can be simply summarized as: 

The biocide chemically disrupts the metabolism of the microorganisms preventing life processes. 

 Respiration; spore forming; protein/enzyme synthesis; reproduction; and energy assimilation. All these are sites or areas where the reproduction processes are attacked and this results in a neutralized microorganism by the following process:  

Articles

Clean rooms are areas in which it is essential that microorganisms are not allowed to proliferate because they could contaminate pharmaceuticals and directly affect human health.