CLEANROOM MATS

In a recent edition of the Monitor we published an article by Dr Geoffrey Barrett that extolled the virtues of the Dycem cleanroom mat. This was a controversial article, particularly amongst those selling or using the peel-off type of mat. We also have in this edition of the Monitor a letter from Dycem.

Knowing that Bill Whyte of Glasgow University had recently published research on the efficiency of cleanroom mats, the Monitor invited him to contribute to this debate. We found that he had, infact, contributed to an article written on cleanroom mats and published last year in the magazine 'CleanRooms'(1). We reproduce part of that article here.

In his research published in the Journal of the Institute of Environmental Science and Technology (2), Bill Whyte investigated the particle removal efficiency of cleanroom mats using six surfaces (five peel-off and a Dycern mat). He first of all settled on a test method. This method used a step wheel designed to produce the pressure and movement similar to a person stepping on and off a mat (see Figure 1). The two types of mats were tested for removal efficiency of silica dust particles, sized 1.6, 6.5 and 57 microns, which were distributed homogeneously on a surface. The variables that influence the efficiency of particle removal were found to include particle size, particle surface concentration, mat softness and surface adhesive strength.

Overall, it was found that when surface concentration was low and particles well apart the removal efficiency of either type of mat was high and vice versa. A likely explanation of this is that as the particles come closer, the larger particles will stop the mat removing the smaller particles (see figure 2). The Dycem mat was found to he more efficient in removing large particles (57 microns), despite the fact that the adhesive strength of the polymeric mat was substantially less than the peel-off mats. However, the peel-off mat was more efficient with the 6.5 micron particles and there was no statistically significant difference between the two types with the 1.6 micron particles.

Figure 2. Removal of widely and closely spaced particles

The reason for the polymeric mats higher efficiency with larger particles is thought to be due to its softness compared to the peel-off mat. The 57-micron particles would sink further into the polymeric mat than into the adhesive of the peel-off mat, giving the smaller particles a greater contact with the mat and thus, a greater likelihood of removal (see figure 3).

Figure 3. Removal of closely spaced particles with Dycem and a plastic film adhesive surface

The influence of surface adhesive strength to remove particles sized 6.5 microns was studied with two peel-off mats, which were identical except for their adhesive. The mat with the higher adhesive strength had greater particle removal efficiency. Although this result is expected, Bill Whyte believes that it is unwise to assume the greatest particle collection efficiency would be obtained from the peel-off mat with the greatest adhesive strength, although the property has a significant bearing on removal efficiency. The adhesive type of thickness, as well as the softness of the carrier of the adhesive, would also affect particle removal efficiency.

Removal of heterogeneous particles

Next, Bill Whyte studied a heterogeneous mixture of silica particles of a size distribution similar to naturally occurring floor dust. The weight of dust studied was equivalent to a "very light" and a "very heavy" weight of naturally occurring contamination. The test used the step wheel method, rolling the wheel several times over the test area.

It was found that if the surface particle weight is very low and the particles well distributed over the surface, practically all of the particles could be removed by one application of either type of mat. However, the peel-off mat appeared to be more efficient that the polymeric mat at these low concentrations. Because the particles are well spaced out, both types of mat surfaces are able to make contact with the smaller particles (see Figure 2), but the peel-off mat's greater adhesive strength ensured greater removal efficiency.

With higher particle concentrations, such as those normal levels of soiling found outside a cleanroom, two rolls were required from both mats to remove the particles, and the first roll was less efficient in removing particles than the second. The likely reason is that the large particles prevent the mats from contacting and removing the smaller particles; the closer the particles are together, the less efficient the removal (see Figure3). On the first roll, the Dycem mat was more efficient than the peel-off type, presumably because more particles can be reached at this higher concentration. Both types of adhesive surfaces were generally found to remove particles more efficiently on the second roll, presumably because the larger particles had been removed and the rest of the particles could then be more easily removed.

When the soiling was high, three rolls were required to remove practically all of the particles. It is clear that if all particles are to be removed from a surface that is heavily loaded with a heterogeneous distribution of particles, particles must be removed in "layers" - the dirtier the surface, the more applications of the adhesive surface that are required. At the high particle concentrations, the polymeric mat removed almost all of the particles one less roll than the peel-off mat. The reason is likely due to the polymeric mat's softness, which allows the larger particles to be pressed into the mat material and the smaller-sized particles to be picked up.

Gravimetric removal efficiencies

Bill Whyte continued his investigations into removal efficiency by confirming his laboratory results with experiments on actual floors. These were published in Environmental Engineering (3). Wearing a test shoe, with a concentration of particles on it similar to the low concentration tested in the previous lab test, a volunteer stepped onto three test mats (a Dycem and three peel-off). The weight of particles removed by one step was generally between 82 and 88 per cent, the two peel-off mats with the higher adhesive strength performing best and the polymeric mat coming third.

Further tests were conducted studying the gravimetric particle removal efficiency of a single step on four surfaces (a Dycem and three peel-off mats). This method showed that the weight of particles removed by one step on the mats studied was generally between 82 and 88 per cent and related to adhesive strength. However, it was found that the polymeric mat demonstrated removal efficiency greater than anticipated given its adhesive strength.

A study of three consecutive steps on the test surfaces was also performed. Results showed that the third step onto the most adhesive peel-off mats gave a test shoe weight less than at the beginning of the experiment. The mat removing rubber from the sole and heel caused this. Three steps onto a cleanroom mat were sufficient to remove all removable dirt from a shoe bottom, but with more efficient mats, only two steps were necessary.

Conclusions

It was concluded that the removal of particles by cleanroom mats is more complex than first thought. The removal efficiency of mats was generally found to be greater if the mat was:

• soft;
• the particles smaller;
• the particle size distribution more homogeneous;
• the distance apart of the particles greater;
• the adhesive strength of the mat surface greater.

These variables interact in complex ways to influence particle removal. This complex interaction highlights the problems that could arise if simple comparisons are made between mats. Different results will be obtained under different conditions and it is likely that peel-off mats will appear to be more efficient in some conditions and the Dycem type of mat in others.

Based on his findings, Bill Whyte surmises that the Dycem mat will work better in situations where there are larger particles, more heterogeneous particles and where the concentration of particles per surface area is high. However, peel-off mats will work best with smaller particles, more homogeneous panicles, and at lower concentrations of particles per surface area. Dycem mats may work best in dirtier areas, such as those approaching the cleanroom change area, and the peel-off mats may be better in areas within the cleanroom.

Bill Whyte’s final conclusion is that if the mats are used correctly, which means if shoes are applied three to the mat, then it does not matter which mat is used, because for all practical purposes, both are close to 100 per cent efficient.

References