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Floor Plan

Gerry Prout at Dycem explains why polymeric flooring is the most effective solution to cleanroom particulate contamination

THE NATURE OF PARTICULATE CONTAMINATION

Of all the impediments to effective cleanroom operation, particulate contamination stands out as one of the largest, yet is probably also the most essential to deal with. Of course, only certain types of particles can be readily detected by the human sensory system. Broad bench-marks include human hair (100 microns in width), particles visible in normal light (50 microns), particles visible on a reflective surface (10 microns), particles visible in intense light (5 microns) and particles visible with an optical microscope (0.5 to 0.8 microns). Particles of 10 microns or less, with which cleanroom operators are particularly concerned, are thus mainly invisible to the naked eye, are of differing shapes and come from a wide range of sources. Spherical particles include smokes and pollens, while irregular and cuboid particles include most minerals. In addition, there are flakes, including human epidermal tissue, and fibres such as lint from clothing.

If we consider the comparative sizes of this variety of particles, we see wide variations. Human hairs are typically between 50 and 150 microns, whereas tobacco smoke is far smaller, ranging from 0.1 to one micron. In between these extremes, floor dust can be from one to 100 microns across, and bacteria are typically between 0.5 and 50 microns.

Obviously humans are a major source of contamination. Our habits and body processes produce a wide variety of particles. Body regenerative processes produce skin flakes, certain oils and hair. Certain behaviours and attitudes also come into play, including rate of movement, sneezing and coughing, along with general working habits and communication. Personal activity rapidly accelerates the rate of generation of particles, as demonstrated in Table 1.

Table 1: Relative sizes of partiles visible to the human eye  
Activity Particles per minute (0.3 microns and larger)
Motionless - standing or seated  100,000  
Walking - about 2mph 5,000,000 

Walking - about 3.5mph 

7,000,000 
Walking - about 5mph  10,000,000

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

Studies of current practice in the semiconductor industry suggest that particulate contamination can reduce product yield by as much as 20 per cent. To remain competitive, continued research must be directed towards the progressive reduction and control of particulate contamination from all potential sources. In other sectors of the 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. There is no published data relating to product yield loss from either viable or non-viable particulate contamination.

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, from the point at which personnel enter the building through to the critical areas of the gowning room and subsequent entrance to the controlled production area itself. At the point of entry to the gowning room where gowns, gloves, hoods and overshoes are donned, any gross contamination of footwear will normally have been removed.

Nonetheless, large numbers of both viable and non-viable particulate can be carried on the feet of operators or on cart wheels; research has demonstrated that over 20,000 particles per cm2 of two micron particles can be measured on the feet of operators under controlled experimental conditions. A more systematic removal of foot-borne small particulate at this stage, most of which cannot be seen by the naked eye, is essential and at the point of entry from the gowning area to the cleanroom itself controlled procedures to reduce carry-over of foot-borne particulate should be unavoidable within normal movement of personnel and wheeled traffic.

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 not only must be inherently effective but also must be used in a disciplined management regimen directed at contamination control as a whole. A management regimen must:

  • Require minimum overt action by personnel
  • Allow the continuous flow of traffic
  • Be maintainable within existing cleaning schedules
  • Be large enough to accommodate personnel and carts
  • Be capable of removing and holding the finest (and most numerous) particles
  • Be able to handle large personnel movements at shift changes

These requirements are fulfilled by the use of polymeric flooring when used as full floor coverage in the gowning area, prior to air-showers and air-locks and at the entrance to the cleanroom area. In many cleanroom situations, the flooring may be employed between areas as an additional aid to the control of small particulate or crosscontamination from viable particulate.

Installation of up to 100m2 of polymeric flooring may be specified for gowning areas and provide the only fully effective means of control. By comparison with the use of adhesive ‘peel-off ’ mats, 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, polymeric products are economical and relatively environmentally friendly. Polymeric flooring products 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.

CONTROL OF FOOT & WHEEL-BORNE CONTAMINATION

The mechanism of particulate control by polymeric flooring has been 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 (1).

Earlier laboratory research had suggested that the efficiency of particulate removal by polymeric flooring was greater than that which could be achieved by adhesive peel-off mats, particularly for the smaller and most numerous particle sizes, and the results of a research programme undertaken under practical operating conditions in a cleanroom were described (2).

This investigation was undertaken in a Class 10,000 cleanroom suite in the Centre for Drug Formulation Studies at the University of Bath, England (3). The investigation demonstrated that, in comparison with peel-off mats:

  • Polymeric flooring shows a significantly higher removal of particulate of all particle sizes, especially with small particulate (see Table 2)
  • Polymeric flooring is effective in the control of viable, biologically active, particulate under circumstances where adhesive peel-off mats can be almost totally ineffective (see Table 3)

Table 2: Foot-borne particulate collection as a function of particle size   
Particle size (microns) Polymeric flooring (percentage of particles removed) Peel-off mats (percentage of particles removed) 
2 57.3 10.9 
10  67.8  31.8 
20  69.3  36.8 
50  85.3  61.7 
100  >80  >70 
Source: University of Bath 1996  

Table 3: Foot-borne and wheel-borne viable particulate control   
Viable counts  Viable counts after   Percentage reduction  
Before  Polymeric flooring  Peel-off mats  Polymeric flooring   Peel-off mats 
Foot-borne >1,000  567  967  43 
Wheel-borne >1,000  17  764  98  23 
Source: University of Bath 1996     

During the course of these investigations a number of ‘rogue’ results were obtained, particularly with peel-off mats, where the number of particle counts after treading on the control surface was greater than the count before. This somewhat surprising result has been attributed to a proportion of operators picking up additional contamination from areas of the mat where operators had previously trodden and has been investigated in two further programmes at different locations. A subsequent study showed that after a year of arduous use, the performance of the polymeric flooring was shown to be superior to that of new peel-off mats for all particle sizes, and particularly so for the smaller sizes.

The total count of small particles after passing over the peel-off mats was found to be higher than the control. This apparently surprising result can be attributed to particulate from the feet of operators at the early stages of the trial being transferred back to the feet of later operators. The different results between the polymeric flooring and peel-off mats can be considered to be similar to the relative performance of the flooring between cleaning operations and to the peel-off mat between mat changes. Actual results supporting this finding are shown in Table 4.

Table 4: Total reduction in particle count    
Particle size  Polymeric (>1 year)  Polymeric (New)  Peel-off mat 
2 microns  4,708 8,504  -3,967 
10 microns   5,051  5,948  -1,208 
25 microns   543  639  552 
Source: University of Bath, 1997    

Taking the mean of the total for each operator, it is clear that the polymeric systems remove literally thousands of particles at the 10 and two micron level at which the peel-off mats are totally ineffective. A further series of tests were undertaken in the demonstration cleanroom suite in Strasbourg, France, comprising Class 10,000, Class 1,000 and Class 100 areas.

In these tests undertaken in April 1997, similar analytical procedures were employed to those used earlier at Bath University to measure retained particulate from the shoes of 20 operators, each making four footfalls over the polymeric flooring or over a peel-off mat prior to entering the cleanroom suite. Particulate counts were measured as particle sizes of two, five, 10, 25, 50, 100 and 125 microns.

Above 25 microns, the performance of both polymeric flooring and peel-off mats is similar, with both systems recording percentage reductions of particulate in the range 80 to 95 per cent.

For particulate of 10 microns and below the results are radically different and are broadly in line with those of previous work, as shown in Table 5.

Table 5: Reduction in particle count (per cent)    
Control medium Particle size  
  2 microns  5 microns  10 microns 
Polymeric flooring  71.1  64.9  68.4 
Peel-off mat  15.2  43.1  38.1 
Source: Strasbourg, 1997    

In examining these figures, which clearly demonstrate the superiority of polymeric flooring over peel-off mats within this range of particulate size, it should also be borne in mind that the mean figures quoted are based only on the number of observations in which an actual reduction of particulate was observed, discounting the cases where an increase took place. For the peel-off mats, particularly, a significant number of observations at each particle size displayed an increase in particulate counted after walking over the control medium; the number of observations of this type as a percentage of the total is illustrated in Table 6.

Table 6: Observations showing increase in particle count (per cent)
Control medium Particle size  
  2 microns  5 microns  10 microns 
Polymeric flooring  Nil Nil 10
Peel-off mat  15 45  35
Source: Strasbourg, 1997    

In a total of 60 observations within this particulate range, almost one third of the observations on peel-off mats showed an increase in particulate count to offset an almost identical average percentage reduction in particulate on the remaining 40 observations. The clear inference is that, as a means of control of particulate less than 10 micron in cleanrooms, the use of adhesive peel-off mats provides little significant benefit. The performance of polymeric flooring within the range of two to 10 microns of particulate has been shown to be consistently effective. The control of biologically viable particulate was also evaluated and the flooring found to be very effective. Small variations of results on the polymeric flooring between observers and locations can almost certainly be attributed to other variables in the trials.

OPERATIONAL VARIABLES IN CONTAMINATION CONTROL

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, reviewed in the previous sections. These include:

  • The effective area of the control surface – it is recommended practice that a full-floor coverage of between 20-30m2 will replace a peel-off mat of around one square metre, such that the control area of the flooring is approximately 25 times greater than that of the peel-off mat
  • Cleaning or mat replacement procedures – regular 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 during 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

Using the well-established procedures developed for use in the cleanroom suite at Bath University, particle counts before and after polymeric flooring and peel-off mats were undertaken, in which participants wore varying types of footwear in common use within industrial cleanrooms as follows:

  • Commercial cleanroom shoe with smooth sole
  • White overshoe with light textured pattern
  • Blue overshoe with heavy textured pattern
  • Grey shoe with checked patterned sole
  • Shoe with heavy ridged sole

Using polymeric flooring as the control surface, the highest level of particulate reduction was obtained from the smooth soled shoe, but a generally high level of particulate control was achieved with all soling types (see Table 7).

Table 7: Polymeric flooring with varied shoe soles     
Sole type  Percentage particle reduction   
  2 microns  5 microns  10 microns 
Smooth  92.6  93.5  86.1 
White overshoe 85.0  87.8  86.6 
Blue overshoe  76.8  79.6  76.1 
Grey pattern  74.3  82.2  87.2 
Riged sole  83.7  86.4  87.6 
Mean  82.5  85.9  84.7 
Source: University of Bath, 1998    

Using peel-off mats as the control surface, the highest level of particulate reduction was also obtained from the smooth soled shoe, but at a lower level of particulate control than that which was achieved with polymeric flooring. Other soling types showed extreme variability but a uniformly adverse affect on particulate removal; for the heavy ridged sole, control of particulate by peel-off mats was almost entirely ineffective (see Table 8). A comparison of performance of the two systems based on the average particulate reduction for all soling types is shown in Table 9 and Figure 4.

Table 8: Peel-off mats with varied shoe soles     
Sole type  Percentage particle reduction   
  2 microns  5 microns  10 microns 
Smooth  77.1 78.9  57.4 
White overshoe 78.4  64.2  32.8 
Blue overshoe  37.7  45.1 39.2 
Grey pattern  25.4  19.0  11.6 
Riged sole  0.0  5.0  0.0 
Source: University of Bath, 1998    

Table 9: Polymeric flooring versus peel-off mats - average all soling types
Sole type  Percentage particle reduction   
  2 microns  5 microns  10 microns 
Polymeric flooring 43.7 42.4  28.2 
Peel-off mats 82.5  85.9  84.7 
Source: University of Bath, 1998    

COSTS & ENVIRONMENTAL CONSIDERATIONS

In an age which is increasingly resource-conscious, the environmental impact of the products in use will also be reviewed by responsible cleanroom operators. In the form of the flooring products described, their use avoids the waste of resources associated with the manufacture 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 a case study of a large US installation in which polymeric flooring at full floor coverage replaced peel-off mats at 10 control points. Over two years, the use of polymeric flooring to replace peel-off mats provided 35 times the contamination control surface area, while saving over $300,000. Using 18 tonnes less of raw materials, polymeric flooring can save over three million megajoules of energy in manufacture and use, reducing greenhouse gas emissions by over 120 tonnes.

References

  1. Barrett GFC, Polymeric flooring demonstrates particle retention properties, CleanRooms, November 1996 
  2. Whyte W and 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 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 7(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

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A biochemist and microbiologist, for the past 21 years Gerry Prout has been a consultant to the pharmaceutical and medical device industries. He is a founder member of the Parenteral Society (now PHSS) and a Qualified Person, as defined in Directive 94/2003. He is also a registered Biomedical Scientist with the Health Professions Council, a Chartered Quality Professional and a Fellow of the Royal Society of Medicine. Gerry is the founder of Kennet Bioservices Limited, specialising in pharmaceutical microbiology.
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