THE ROLE OF POLYMERIC FLOORING IN THE CONTROL OF PARTICULATE

Particulate contamination within cleanrooms and controlled manufacturing areas affects product quality, yield, reliability and safety - all of which can have a major impact on costs.

Studies of current practice suggest that up to 80% of particulate is carried into clean environments on feet or wheels and that the resulting contamination can reduce product yield by as much as 20%. It is therefore vital that effective measures are taken at floor level to control contamination.

Particles which are of greatest concern to cleanroom operators are generally of the order of 1 0 microns or less. These small particles are also the most numerous and are mostly invisible to the naked eye under normal illumination (see Figure 1).

In the fields of microelectronics, micromechanics, semiconductor and medical device manufacture control measures will typically be designed to prevent particulate from entering a clean area. In pharmaceutical manufacturing and research environments control measures are also frequently required to prevent the movement of compounds within and between clean areas.

It should also be noted that it is often at the "dirtier" end of the scale that contamination control flooring can have the greatest impact. In cleanrooms of Class 10, 000 through to unclassified "clean" areas, a lower level of capital investment in air filtration (and sometimes looser cleanroom disciplines) mean that flooring may be the final and only barrier to contamination.

Systems to be considered range from adhesive peel-off mats to mat and flooring products made from washable polymeric compounds.

Polymer systems do not contain adhesives. Their exceptionally flat surfaces remove and retain particles by virtue of high surface energy associated with short-wave electromagnetic forces. This principle is simply illustrated by considering the force which is required to separate two sheets of glass when they are placed face to face. In addition to being very flat, polymer surfaces are also slightly soft, allowing particulate of differing sizes to be pressed into their surface.

One very important factor is often overlooked in comparisons between competing systems. Quite simply, the more steps or wheel rotations which can be achieved on any control surface the better the end result. Current wisdom suggests that at least three footsteps will be required with both feet to ensure adequate decontamination.

In many cases the approach to the cleanroom may not be large enough to allow an adequate number of footfalls. In such instances it should not be forgotten that personnel take many steps while changing into cleanroom clothing. Installing contamination control flooring in the gowning room can therefore provide a highly efficient solution.

Small control areas, whether peel-off mats or small polymeric mats, can and frequently will be avoided by cleanroom personnel. In areas of heavy traffic the use of small mats also means that contamination will be concentrated, necessitating frequent peeling or washing to maintain a receptive surface.

A polymeric flooring system, installed in the same way as vinyl flooring, can be designed to be large and unavoidable. Contamination is spread over a much larger area, thereby minimising the cleaning requirement.

Polymeric surfaces are also very practical for areas of wheeled traffic, offering little resistance to the passage of trolleys. If restricted to straight-line traffic, it is often possible to run pallet trucks or even fork lifts across polymeric flooring.

In terms of cost polymeric systems represent a higher initial capital outlay than peel-off mats, but without the ongoing costs of replacement sheets. This outlay generally buys an area which will often be significantly larger than it is possible or practical to cover using any other system. Subject to correct cleaning, the effectiveness of polymeric systems may be guaranteed for two years, and they will typically have a lifespan of three to four.

Several research projects have been undertaken to quantify the relative effectiveness of peel-off mats and polymeric flooring.

W. Whyte at the University of Glasgow.
This work developed laboratory techniques for simulating the effect of contamination from footfalls. Polymeric flooring was shown to be slightly more effective than adhesive peel-off mats at collecting 50 micron particulate, but more efficient as particle size was reduced.

Kennet Bioservices
Research undertaken in 1996 and 1997 in the Glaxo sponsored, Class 10,000 cleanroom at the Centre for Drug Formulation Studies at the University of Bath.

Kennet Bioservices
Research undertaken in 1997 at the Class 100, 1000 and 10,000 Demonstration Suites at Clestra, Strasbourg.

The microbiological studies carried out at the University of Bath will be of particular interest to the pharmaceutical and medical device industries and clearly demonstrate the extent to which feet and trolley wheels are carriers of microbial contamination.

Polymeric flooring was shown to be very efficient at removing viable contamination under conditions where peel-off mats were shown to be almost totally ineffective (see Table 1).

The collection efficiency of both types of product was shown to decrease as particle size reduced. Against this background, and of particular interest to semiconductor manufacturers, the efficiency of polymeric flooring at removing 2 micron particulate was shown to be five times greater than that of the peel-off mat (see Tables 2 and 3).

Similar results were obtained in the trials at Clestra, Strasbourg (see Table 4).

The University of Bath and the Clestra tests also highlighted a high proportion of instances in which the levels of 2-10 micron particles on operators' shoe soles, measured after passing over peel-off mats, was greater than the starting, control count. This surprising result can be attributed to particles from the feet of personnel in the early stages of the trial being transferred back onto the feet of subsequent users.

Of the 60 observations made for 2-10 micron particulate during the Clestra tests almost one third of the observations on peel-off mats showed an increase in particulate count after trafficking.

As a means of control of viable and non-viable particulate of 10 microns or smaller, polymeric flooring offers a significantly more efficient option than adhesive peel-off mats. This is probably not only a function of its particle collection mechanism, but also of the larger control areas which are possible with polymeric flooring and which enable more adequate decontamination.