Polymeric Flooring Demonstrates Particle Retention Properties
Polymeric flooring represents a small segment of the flooring market. This articles examines the particle removal properties of this type of flooring material.
By Dr. Geoffrey F.C. Barrett
Following their introduction in 1974, contamination control flooring products based on polymeric compositions have been adopted by a range of industries as an efficient and cost-effective means of
controlling particulate and microbial contamination from feet and cart wheels at the entry to cleanrooms.
The mechanism of particulate retention is demonstrated to be due to the high surface energy associated with the optically smooth, flexible surface which retains contamination over a wide range of particle sizes between cleaning operations which, in turn, result in the regeneration of the original properties of particulate retention. Between cleaning, retention of airborne particulate on the floor surface prevents re-circulation of particulate and resultant contamination.
The mechanism of particulate retention
The particle retention properties associated with the polymeric compositions being reviewed can be connected with a formulating and manufacturing technology that results in an exceptionally flat and level surface–comparable with that of a liquid or of float glass. Such surfaces have high levels of surface energy associated with short-range electromagnetic (van der Waals) forces. Previously reported studies at the University of the West of England [1], have resulted in the development of apparatus for measuring adhesion forces acting between a standard test foot and a range of flooring surfaces. The resulting value, expressed in kilo Pascals, gives a comparative value of the adhesion forces that are derived from the high surface energy and the observed properties of particulate retention. (See Table 1)
In practice, these properties effect the transfer of foot and wheelborne contamination to the polymeric flooring surface in a cleanroom changing area or airlock. In addition, these adhesion forces prevent the contamination from being tracked into the cleanroom, as described in further sections of this article.
However, it has been demonstrated that the high surface energy associated with these types of flooring compositions is effective in trapping residual airborne particulate contamination, and holding it at floor level between cleaning operations. Thus, it prevents the re-distribution of contamination into the atmosphere by air turbulence caused by moving personnel or carts. Modified formulations of this type can also be employed to offer a combination of particulate retention and static dissipation.
Clearly, the effectiveness of particulate retention is a function of both the surface energy of the collecting medium, the density and size distribution of particulate contamination, and the contact area between the particulate and the collecting surface. These factors effectively determine the adhesion force acting between the particulate and the collecting medium.
In 1990, the University of Glasgow [2] demonstrated the effectiveness of particulate collection under varying conditions of contaminant density and particle size using both the defined polymeric flooring and peel-off mats as the particulate collecting media. The tests involved preparation of a heterogeneous particulate media made up from silica particles varying in size from 1.3 to 83 microns in defined proportions; known particle densities of the media were then deposited onto microscope slides.[5]
Samples of the polymeric flooring and the peel-off mats were taped onto a brass wheel. The wheel`s weight and surface was designed to simulate as accurately as possible the load-bearing surface and rolling motion of the average foot-fall. Then, the wheel was rolled over the microscope slide. The remaining particles were counted using a special optical microscope.
In one series of tests, the efficiency of particulate collection using an uncontaminated collecting medium was studied as a function of particulate size and density of particulate contamination.
The polymeric flooring compositions` degree of flexibility allows for a high rate of efficiency in collecting smaller particles less than 5 microns. This flexibility lets the polymeric flooring conform to the particle shape and provide more intimate contact with both larger and smaller particles. Bridging and loss of contact with the smaller particles, as occurs with a flat, rigid medium or a medium of irregular surface, such as peel-off mats, is prevented. The results are also shown in Figure 1.
The polymeric flooring also exhibits a much higher particulate collection efficiency than peel-off mats after progressive contamination of the collecting surface area as illustrated by the results of tests carried out in a similar manner, but using the same sample of collecting medium in five successive tests.
As the particulate contamination of the collecting surface increases, the efficiency of removal decreases. This occurs, however, much more sharply with the peel-off mat surface, whereas the polymeric flooring remains consistently more efficient in particulate collection and retention.
Tests of polymeric flooring (like the kind of flooring discussed in this article) carried out in NASA`s Materials Science Laboratory have demonstrated that the product complies with industry standards on outgassing of volatile constituents. [3]
Quantitative studies of particulate and microbiological control
Kennet Bioservices performed research in a Class 10,000 cleanroom suite constructed for the Centre for Drug Formulation Studies at the University of Bath in England, a facility sponsored by Glaxo. The primary use of the cleanroom suite is to provide hands-on experience for undergraduates in cleanroom operations and to provide industry with a living laboratory for research on cleanroom technology relevant to pharmaceutical production or as an actual production area for small batches used in clinical trials.
The suite consists of a conventional turbulent vertical-flow room with vertical-flow Class 100 cabinets and low-level extract for recirculation. Access to the cleanroom is from a preparation area that is accessed from a gowning area. On the “white side” of the gowning area, the polymeric flooring composition described was installed.
This cleanroom facility represents many cleanrooms in the pharmaceutical industry which have conventional turbulent flow in the general cleanroom area, with critical product exposure areas further protected by Class 100 laminar flow in either vertical or horizontal configuration. With this type of airflow pattern, particle levels at or near floor level can be expected to be higher than at work level. Footborne or wheelborne contamination can significantly increase the prevailing particle levels and provide a mechanism for the ingress of viable and non-viable particles unless careful control is exercised.
The primary objective of the research was to develop quantitative data on the control of footborne and wheelborne particulate and microbiological contamination in a practical cleanroom situation. Experiments were carried out in a number of stages, the results of which are summarized in the following paragraphs.
Footborne contamination
Swabs were taken from the left foot cover of each person participating in the test before taking a minimum of two steps over the polymeric flooring. Swabs were taken again from the right foot cover after each person stepped over the contamination control surface.
Both sets of swabs were then analyzed in the following manner:
Non-viable particulates were examined by particle counts over a range of particle sizes from extracts of the swabs using a liquid-borne particle counter.
Viable particulates were analyzed by colony counts on agar plates of three compositions, to reflect different microorganisms and incubated for five days.
As shown in earlier laboratory work, efficiency of particulate collection increases with particle size; what is of greater significance, however, is the very large number of 2-micron particulates collected–over 6,500 for each foot of one operator. For viable particles, the three culture media employed are described as follows.
MacConkey Agar: semi-specific for molds and yeasts
Tryptic Soy Agar: non-specific
Sabouraud Dextrose Agar: semi-specific for gram-negative bacteria.
The effectiveness of polymeric flooring controlling footborne contamination is clearly demonstrated over a range of microorganisms.
Wheelborne contamination
A similar series of experiments were performed using swabs taken from cart wheels before and after four rotations over the contamination control surface. As with footborne contamination, efficiency of particulate collection increases with particle size, what is also of greater significance, however, is the very large number of 2-micron particles collected–over 3,000 for each cart wheel, which in practice would mean over 12,000 for each pass of a cart.
It is clear that cart wheels can be a very significant carrier of microbial contamination and that the polymeric flooring is virtually 100 percent effective in controlling a wide range of microorganisms.
Comparison of polymeric flooring and peel-off mats
Another series of experiments, also carried out at the University of Bath facility, was performed as a direct comparison of polymeric flooring with peel-off mats under test conditions comparable to those used in practice. The test protocols were similar to those in the previous tests with the following exceptions:
A length of polymeric flooring, allowing a minimum of four footfalls, was employed as being representative of the best industrial practices.
The peel-off mat tested was of a widely used commercial type incorporating a bactericide.
To comply more closely with operational practice, the carts were weighted with a load of 25 kg compared with the unloaded carts in the first experiment.
The results, averaged for all participants in the footborne contamination experiments, are shown in Table 2. The results confirm the high degree of efficiency that the polymeric flooring has removing particulate in a wide range of sizes. As anticipated, the efficiency is shown to improve with an increasing number of steps. By comparison, the peel-off mats tested have a much lower level of efficiency in particulate removal, particularly for smaller particle sizes, allowing thousands of additional 2- and 5-micron particles to pass into the cleanroom from the gowning area.
This has a major bearing on the control of microorganisms because the majority of microorganisms attach to small particulates within this size range and any of the particles allowed to pass into the cleanroom may be biologically active. This finding is borne out by the study on viable particulates shown in Table 3.
As Table 3 suggests, the polymeric flooring is again demonstrated to be highly effective. By comparison, the results suggest that peel-off mats are not as effective in controlling microbiological contamination, both for footborne and wheelborne contamination. n
References
1. Fitton, P, Journal of the Society of Environmental Engineers, March, 1987.
2. Whyte, W., Unpublished data, University of Glasgow, Scotland, 1990.
3. NASA, Material Science Laboratory, Kennedy Space Center, 1988.
4. Barrett, G.F.C., Seventh International Confederation of Contamination Control Societies, Paris, 1984.
5. Whyte, W.; Shields, T.; “Cleanroom Mats: An Investigation of Particle Removal,” Journal of the Institute of Environmental Sciences, July/Aug. 1996, pp. 19-27.
Dr. Geoffrey F.C. Barrett is a Senior Consultant with Corporate Development Consultants Ltd. He graduated with a BS in chemistry from the University of Leeds, UK, and subsequently went on to obtain his Ph.D. in physical organic chemistry. He developed an interest in the field of high polymers, which has led to over 40 years of work associated with the plastics industry and its markets.
For the last 10 years, Dr. Barrett has a consultant for Dycem, Ltd. in the United Kingdom.
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Figure 1. Polymeric flooring demonstrates a high degree of efficiency in collecting particles smaller than 5 microns. Whereas, peel-off mats demonstrate less efficiency in particulate removal as the size of the particles decreases.
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