Selecting materials for cleanroom
Choosing which materials to use for a cleanroom construction is increasingly complex due to the wide variety of options now available. Jorge Nuero, Telstar, looks at key considerations for optimising cost and ensuring safety and quality
When deciding on the materials and elements to be used in the construction of a cleanroom, it is necessary to keep in mind the specific requirements and working conditions of each area. Only through appropriate design will the cost of installation be optimised and the resulting facility offer long-term safety and quality standards that meet user requirements.
The traditional building methods – where the walls and ceilings are constructed on site from raw materials, followed by the application of a PVC or epoxy finish coating – are rapidly being replaced by the use of modular elements built using self-supporting, factory-made sandwich panels.
This transition has occurred due to the many advantages offered by the use of modular elements, including:
- rapid and clean installation
- improved tracking of materials
- reduction in waste and construction residue
- reduction in variability introduced by those performing the installation
- increase in quality (due to the fact that the materials are factory manufactured and finished to precise, pre-determined specifications)
- better defined values of mechanical and chemical resistance and lower permeability to air and water
The main disadvantage of the modular method is that it is less flexible when the time comes to make changes in design, since it needs to be fully specified and designed in advance, which also requires that joints be added between existing elements to provide transitions between exiting and new construction.
There are a wide number of options when considering the use of sandwich panels, which in turn enable an optimal solution to each specific set of requirements to be attained.
The fundamental parameters applicable to defining a panel are as follows: thickness; core material; material and thickness of the skins (exterior surfaces); external finish; type of jointing or fastening system; whether or not a perimeter frame is required, and behaviour and resistance to flames. It is important to distinguish between ‘behaviour’ and ‘resistance’, as failure to do so can result in specification errors.
The behaviour of a material in the presence of fire or in reaction to fire is classified in European Standards according to Norm UNE-EN 13501-1:2007 and is based on three fundamental parameters:
- Fire certification, from Class A (inert materials) to Class F (highly combustible materials)
- Level of smoke production, from S1 (low smoke production) to S3 (high smoke production)
- Level of production of liquid droplets, from d0 (does not produce droplets) to d2 (high number of droplets)
The fire resistances (previously RF) of elements that comprise an enclosure are regulated by Norm UNE-EN-11501-2:2009, which defines that fire resistance is primarily based on three parameters: bearing capacity (R), integrity (E) and insulation (I). For example, a material with a resistance of REI-60 will maintain each of these three characteristics for at least 60 minutes.
This norm also states the types of tests to be performed for partitions that separate two independent fire sectors, which is not common in cleanrooms, except in certain instances (ATEX rooms, partitions for separation from other areas, such as storage areas, offices, technical zones, etc.).
The norm that should be followed when working with insulating sandwich-type portable, double-faced metallic panels is Norm EN 14509:2005.
From a process perspective, the most critical element to consider is the material used for the surface or skin that will be in direct contact with the room’s inner environment. In this respect, it is important to know the cleaning and sanitation methods that will be employed and to select the finishes accordingly.
For example, for a washing area where pressurised water is used daily, partition surfaces in stainless steel can be justified. In addition, use of slip-resistant flooring with appropriate slope towards the drain is required. Such considerations would not apply to a packaging area, where an impact-resistant panel is more appropriate.
For areas where hydrogen peroxide is used as a disinfectant or bio-decontamination agent, the use of special insulation (type PET, PVDF) and surfaces of epoxy laminated resins intertwined at high pressure (HPL) or the use of stainless steel needs to be considered. In this case, decisions would be influenced not only by cost but also by the sanitising method employed, the frequency of application and the concentration of the particular chemical agent.
The purpose of the room also needs careful consideration. For example, it is not necessary to achieve the same level of air tightness in a laboratory for quality control, working under positive pressure as that in a zone where vaccines or cultures with biological hazards are present. In this latter case, it is necessary to perform tests that demonstrate the absence of leaks in ceilings and walls, secondary barriers of the containment area, and to design with double sealing of all panels, utility, electrical and mechanical penetrations, light fixtures and filter assemblies. In addition, doors may require inflatable joints and drains may require hydraulic seals.
Floors that are typically installed in cleanrooms are of two basic types: epoxy resin or PVC. The selection of which floor material to employ depends on several factors. Resin floors are recommended for rooms where water is present or high humidity is expected, i.e. washing areas, steam outlets, etc. Another factor to consider is the mechanical load that will be presented when objects are moved. In this respect resin floors provide a higher resistance and strength.
PVC floors in a tile format or laid from a roll (welded joints in both cases) are considerably more economic and easy to install and replace. They are recommended for labs, hallways, or rooms that do not handle a high volume of traffic or movement of heavy objects.
In certain areas where humidity is low or where there is an ATEX classification, conductive floors (made of resin or PVC) are used to prevent static electricity. Following installation, it is necessary for a certified contractor to perform some conductivity measurements to confirm the correct installation and grounding connection.
Critical points that need to be taken into consideration during the design and installation phases include:
- Use of a vapour barrier that prevents any humidity infiltration that could produce bubbles of gas or water (in the case of PVC) or cracks (in the case of resins). The relative humidity in the existing slab should be measured before installation and an interior humidity of less than 3% is recommended.
- The surface should be smooth, clean and level, due to the fact that the finished surface will replicate the defects found in the base, including any slope, cracks or irregularities in the surface. The use of a self-levelling substrate or surface levelling compound can correct these defects to a degree, but can use large amounts of material.
- Particular attention should be paid to the proper treatment and sealing of the expansion joints and the connection with other critical elements such as panels, drainage, etc., which tend to be weak points.
The European norm for PVC floors is EN 14041 and EN 13813:2002, for industrial multi-layer pavements (epoxy resin). Floors should also comply with the requirements established by 2033/94/ECC relating to GMP recommendations.
Locating viewing panels
The design and location of the viewing panels within the cleanroom is important, not solely from an aesthetic perspective but also for providing user comfort and operational efficiency. For example, the use of view panels in a partition separating a filling line from its respective packing line enables operators in both rooms to have visual contact and provide simple instructions without having to leave their post. For this reason, considered location during the design phase is critical.
From the aesthetic point of view, frequently the viewing panels are located in wall panels of visiting and inspection halls, which permit the facility to be visited or supervision tasks of the operators and equipment to be carried out without the need to disrupt the operation and without needing to follow the strict protocols for admittance into controlled areas. The use of view panels to the exterior is also very helpful to the operators when working on repetitive tasks because it enables them to better take brief periods of relaxation while working without having to leave their area.
Usually, the requirements are for viewing panels to be flush on both sides of the wall and the void between each pane should contain drying agents to prevent humidity condensing. In addition, the glass used should be tempered, so that in the case of breakage, it shatters into small pieces, not shards or sharp fragments. In special cases, such as evacuation routes, laminated glass should be utilised. In ATEX zones or walls dividing different sectors, windows must be reinforced and fire resistant. In the case of photosensitive products, where decontamination is performed through ultraviolet methods, the windows are usually protected with coatings or films that filter the respective wavelength.
Choose easy-to-clean doors
The design of cleanroom doors should be kept as simple as possible, avoiding the use of elements that are difficult to clean, that have areas difficult to access or utilise rails or complex mechanisms. Swing doors are recommended for most applications; they are simple and more easily cleaned than roll-up or sliding doors. However, with swing doors, it is not recommended to use perimeter joints due to the fact that they deteriorate with time and are often placed on rails that are very difficult to keep clean.
For the same reason, the use of electrical, mechanical or hidden hinge devices to close the doors is not recommended. Both sides of the door should be flush with the wall, to avoid uneven surfaces when it comes time to clean.
In conclusion, selecting materials of construction for cleanrooms is becoming increasingly complex due to the wide variety of options that are now available. Understanding the requirements of the end-user and the specific processes that will be carried out in the cleanroom environment are crucial to a successful installation. Key factors to be considered are cost, layout and ergonomics, safety and fire regulations, energy consumption and standards and norms. For these reasons it is important to involve an experienced supplier at the very onset of the project, to be a partner from concept design to handover.