Don’t be floored by contamination

Contamination in a cleanroom environment can be disastrous – in order to avoid particulate contamination, says Gerry Prout – you have to get the floor right

Particles of 10µm or less, with which cleanroom operators are particularly concerned, are mainly invisible to the naked eye, are of differing shapes and derive from a wide range of sources. They range from human hairs 50-150µm, dust 1-100µm, bacteria 0.5-10µm. Generally particles greater than 40µm can be seen by the human eye.

People are a major source of contamination through body regenerative processes (skin flakes, oils, hair), behaviour (rate of movement, sneezing, coughing) and attitude (work habits, communication). Personnel activity rapidly accelerates the rate of generation of particles.

Control of particulate contamination from personnel movement is thus a critical factor in manufacturing operations undertaken under cleanroom conditions affecting product yield, productivity, cost, quality, reliability.

Studies of current practice in the semi-conductor industry suggest that particulate contamination can reduce product yield by as much as 20%. In other sectors of industry, such as pharmaceuticals and medical device manufacture, the additional control of viable or biologically active particulate is of critical importance in preventing active contamination entering the cleanroom or causing cross-contamination between working areas.

The reduction of particulate contamination from people is thus of paramount importance for the operation of cleanrooms and is normally achieved in a progressive manner. Large numbers of both viable and non-viable particulate can be carried on the feet of operators or on cart-wheels. The systematic removal of foot-borne small particulate at this stage, is essential. In normal industrial practice, control of foot-borne contamination is attempted by the use of adhesive peel-off disposable mats or, increasingly and more effectively, the use of polymeric contamination-control flooring.

Flooring products for control of foot and wheel-borne contamination must be simple - requiring minimum overt action by personnel, allowing continuous flow of traffic and maintainable within existing cleaning schedules - and effective, being large enough to accommodate personnel and carts and capable of removing and holding the finest (and most numerous) particles.

By comparison with the use of adhesive peel-off mats, a greater efficiency of particulate removal is achieved over a much larger control area, with a consequent increase in product yield. Major cost savings can be achieved over the service life of polymeric flooring compared to peel-off mats.

In an increasingly resource-conscious world, the polymeric products are economical and relatively environmentally friendly. They avoid the waste of resources associated with the manufacture and disposal of adhesive peel-off mats. On completion of their service life, the polymeric flooring products may be readily recycled into less critical uses.

The mechanism of particulate control by polymeric flooring was shown to be attributable to the short-range electromagnetic forces acting over the optically flat, flexible surface of the product and their ability to retain particulate over a wide range of particle sizes. Polymeric flooring is also very effective in the control of viable, biologically active, particulate under circumstances where adhesive peel-off mats can be almost totally ineffective.

Above 25µm the performance of both polymeric flooring and peel-off mats is largely similar, with both systems recording percentage reductions of particulate in the range 80-95%. For particulate of 10µm and below is has been shown that polymeric flooring is significantly more effective (2 to 5 times) than peel off mats.

The overall efficiency of contamination control in a practical operating situation is clearly dependent on a number of variables other than the inherent properties of the control surface. These include:

• Cleaning or mat replacement procedures:  Cleaning of the flooring is essential in order to remove contamination and to renew the control surface; this can normally be accommodated at no extra cost within existing cleaning schedules.  Replacement of peel-off mats, however, is frequently undertaken on an irregular basis “when the mat appears dirty” but, as noted earlier, most of the important small particulate is invisible to the naked eye.
• Other variables:  Research undertaken to date, together with theoretical considerations of particulate control developed using this period, had suggested that the type of soling used on footwear could be a significant factor and had suggested that footwear with smooth soling would offer advantages, especially in the removal of small particulate.

Polymeric flooring is largely effective with all soling types, although most effective when smooth-soled shoes are employed. Peel-off mats are significantly less effective with all soling types and performance is highly variable, being most effective with smooth soling and almost entirely ineffective with heavily patterned or ridged soles.

In an operating industrial situation, consideration will also be given to operating costs. In an age that is increasingly resource conscious, the environmental impact of the products in use will also be reviewed by responsible cleanroom operators. The flooring products described avoid the waste of resources associated with the manufactured and disposal of adhesive peel-off mats, since on completion of their service life, the polymeric flooring products may be readily recycled into less critical uses.

While the balance of cost and environmental impact will clearly vary significantly between different industrial applications, broad conclusions can be drawn from the case study of a large US installation in which polymeric flooring at full-floor coverage replaced peel-off mats at 10 control points (Table 1).

Table 1: Cost/ecobalance study comparing polymeric flooring and peel-off mats. Basis for comparison: US plant with installation of polymeric flooring replacing peel-off mats at 10 locations over two years.

Item	Polymeric flooring	Peel-off mats
1. Contamination-control area 2. Cost     Material cost     Maintenance cost     Total cost 3. Materials     Product     Packaging     Total Materials 4. Energy used in manufacture     4.1. per kilo     Materials     Conversion     Total energy per kilo     4.2. Total energy/2years 5. Greenhouse gas emissions (kgCO2)     Manufacture     Disposal (incineration)     Total	2,800ft2 $50,000 $15,000 (clean) $65,000 700kg 35kg 735kg 75Mj 75Mj 105,000Mj 3,500kg 1,120kg 4,620kg	80ft2 $250,000 (approx 600,000 mats) $125,000 (replace/dispose) $400,000 18,500kg 500kg 19,000kg 88Mj 100Mj 188Mj 3,500,000Mj 70,000kg 55,000kg 125,000kg

Table 1 shows that over two years the use of polymeric flooring to replace peel-off mats provided 35 times the contamination-control surface area, saved over $300,000, used 18 tonnes less of raw materials and reduced emission of greenhouse gases by over 120 tonnes.

Acknowledgements:

I wish to express my grateful thanks to the late Dr Geoffrey Barrett who researched most of the environmental and ecological data presented in this paper. His knowledge about and enthusiasm for, the field of contamination control was unsurpassed and his presence is greatly missed. I also wish to acknowledge the contributions made by Eva Paraskevaides and Will Prout for their assistance in ensuring that the paper is correctly formatted.

References  1) Barrett, G.F.C.: “Polymeric Flooring Demonstrates Particle Retention Properties”; CleanRooms, November 1996. 2) Whyte, W.; Shields, T.: “Cleanroom Mats; An Investigation of Particle Removal”; Journal of the Institute of Environmental Sciences, July/August 1996. 3)  Prout, G.; “A comparative study of two floor cover materials in control of foot and wheel-borne contamination”; European Journal of Parenteral Sciences, Vol.2 1997. 4) Clibbon, C.; “An Evaluation of the Effectiveness of Polymeric Flooring compared with peel-off mats to reduce wheel and foot-borne contamination within cleanroom areas”; Eur. Parent. Sci. Vol 7 No.1 2002. 5) Sandle, T.; “A Final Floor show for Bugs”. Cleanroom Technology April 2006. 6) Russell, T.; “How Clean is your floor”. Cleanroom Technology November 2007