Peel-off and washable mats compared, contrasted
Peel-off mat manufacturers respond to a previously published article on washable, polymeric flooring, while tests show comparative particle removal capabilities of the two types of mats.
By Sheila Galatowitsch
Editor`s Note: An article published in the November 1996 issue of CleanRooms suggested that peel-off, disposable mats are not as effective as polymeric flooring in controlling contamination, both for footborne and wheelborne contamination. The study used research dating from 1984 to 1996, and tests conducted at the University of Glasgow and the University of Bath to prove its thesis. This article will serve as a follow-up to the previous article. It will attempt to compare and contrast peel-off, disposable mats with washable, polymeric mats and flooring; allow manufacturers to respond to claims made in the previous article; and give guidelines on applications for both products. It will also summarize the most recent research conducted on cleanroom mats.
However, this article is not intended to be a definitive, conclusive study. It is intended simply to provide a starting point for users who choose to initiate their own investigation into cleanroom mats.
Mats have been used for decades to successfully control particulate and microbial contamination from feet and cart wheels at the entry to gowning areas and cleanrooms, and in air showers. Peel-off, disposable mats are the most widely used. The category has captured approximately 90 percent of the marketplace, according to manufacturers` sales figures.
Peel-off mats are known by numerous names, including generic terms such as adhesive floor mats, disposable mats and contamination control mats, as well as trademarked names such as ALMA Inc.`s First Step; Controlled Environment Equipment Corp.`s Cleanline Sticky Mat; Liberty Industries` Tacky Mat; and Markel Industries` Trim-Tack mat.
The mats consist of 20 to 60 layers of laminated plastic film. Individual layers range from 1.2 mil to 4 mils in thickness, and are coated with a pressure-sensitive adhesive. Most mats are secured to the floor with a fully coated adhesive bottom or double-sided tape, while some brands slide into or sit inside of a specially designed frame.
Mat sizes vary by manufacturer, with ranges from 18-in. ¥ 36-in. to as large as 6-ft ¥ 15-ft. When the top layer, or sheet, becomes contaminated, a user peels it off and exposes a new clean sheet. The used sheet is discarded. The life span of a single sheet depends on the amount of foot and cart traffic and contamination levels. Usage can vary from one sheet a day to one sheet an hour.
Several companies manufacture washable, reusable polymeric mats. The largest and best known manufacturer is Dycem, which supplies both a polymeric mat and a polymeric flooring system. Dycem constructs its products using proprietary formulations of polyester polymer with a nonmigratory plasticizer. Other manufacturers use proprietary formulations of polyvinyl chloride (PVC).
Dycem`s polymeric mats and polymeric flooring are the same product in two different forms. Both are constructed with the same material, but differ in size. The mat is a solid piece of polymer that stays in place on its own. It is 7mm thick and available in sizes ranging from 2-ft ¥ 4-ft to 13-ft ¥ 4-ft. While the mat is loosely laid in a small area, the flooring is installed in large areas to any length or width.
The flooring, trademarked Clean-Zone, is a completely sealed system, secured to the floor with a self-adhesive flooring panel, which is typically used to install any type of standard flooring. It has a thickness of 2.1 mm, similar to a vinyl tile. The average replacement time for polymeric flooring is 5.2 years, according to company figures.
Polymeric mats and flooring are reusable because they are washable with a damp mop and detergent, then dried with a squeegee. A Dycem spokesman says the company installs a flooring system sized sufficiently so that it can accommodate the designated number of personnel and carts entering and exiting a room, as well as be maintained on the same schedule as other flooring in the area. Whether it is cleaned once per shift or once per day, the size of the flooring is related to the frequency of cleaning. According to the spokesman, the company`s philosophy is to promote the large-area polymeric flooring systems over the polymeric mats, as much as it is to advocate the value of washable surfaces over disposable ones. The flooring can be installed wall-to-wall, inlaid with other flooring, coved up walls, or placed on top of existing flooring.
Peel-off mats and polymeric mats and flooring can be compared on their inherent adhesive strength, thickness and softness; particle removal efficiencies; effectiveness after multiple steps; maintenance; cost; and environmental impact. However, as some manufacturers have indicated, comparing peel-off, disposable mats with polymeric mats and flooring is like comparing apples with oranges. The two types function differently and are difficult to analyze on equal terms. Even constructing a fair test has proven formidable, given the variables involved.
While most peel-off mat manufacturers have conducted tests comparing different brands of disposable mats — an apples-to-apples comparison — few have initiated comparison testing between the peel-off mat and the polymeric mat. One reason is that peel-off mat manufacturers say they do not view polymeric mats as a direct competitor, given the type`s small market share. Most of the research studies comparing these two mat types, therefore, have been initiated by the polymeric mat manufacturers. They maintain that the two mat types must be compared, since they are designed for the same purpose.
Particle removal comparisons
Bill Whyte, a well-known contamination control researcher based at the University of Glasgow, and the General Secretary for the Scottish Society for Contamination Control, has conducted a wide range of contamination control research, including studies of cleanroom mats dating over several years. His most recent work in this area was published last year in two periodicals and is summarized below [1-3].
In his most recent research, Whyte first investigated methods suitable for measuring the stickiness or adhesive strength of mat surfaces. Adhesive strength is important because it has a direct relation to particle removal efficiencies. The principles behind five preliminary methods chosen are detailed in the previously published articles. Whyte ultimately settled on a method that measures the edge tension at a block placed on and then pulled off the mat material. Based on this method, six surfaces (five peel-off and one polymeric mat) were measured. It was clear that the polymeric mat had less adhesive strength than the peel-off mats, and that one of the peel-off mats had twice the adhesive strength of another.
After settling on a method for testing adhesive strength, Whyte next studied variables that influence particle removal, using a test method also detailed in the referenced articles. In brief, the method uses 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, that were distributed homogeneously on a surface. The variables were found to include particle size, particle surface concentration, mat softness, and surface adhesive strength.
Overall, Whyte found that when surface concentration was low and particles well apart, the removal efficiency was high and vice versa. A likely explanation of this is that as the particles come closer, the larger particles of the distribution prevent the smaller particles from coming in contact with the adhesive surface, and thus being removed (see Figure 2a and Figure 2b). It was also demonstrated that the polymeric mat was more efficient than the peel-off mats in removing large particles (57 microns), despite the adhesive strength of the polymeric mat being 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.
The reason for the polymeric mat`s 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 particles a greater contact with the mat and thus, a greater likelihood of removal. The peel-off mat`s greater efficiency in removing 6.5-micron particles is likely to be explained by different interactions of the same phenomena.
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 a greater particle removal efficiency. Although this result is expected, 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 and thickness, as well as the softness of the carrier of the adhesive, would also affect particle removal efficiency.
Removal of heterogeneous particles
Next, 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.
Whyte found that if the surface particle weight is very low and the particles well distributed over the surface, practically all of the particles can be removed by one application of both mats. However, the peel-off mat appeared to be more efficient than 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, but the peel-off mat`s greater adhesive strength ensured a 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 Figure 2b and Figure 2c). On the first roll, the polymeric 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 in 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
Whyte continued his investigations into removal efficiency by confirming his laboratory results with experiments on actual floors. Wearing a test shoe, with a concentration of particles on it similar to low concentration tested in the previous lab test, a volunteer stepped onto three test mats (one polymeric and three peel-off). The weight of particles removed by one step was generally between 82 and 88 percent, 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 (one polymeric 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 percent and related to adhesive strength. However, Whyte found that the polymeric mat demonstrated a 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 second step onto the most adhesive peel-off mat and the third step on the other peel-off mats gave a test shoe weight less than at the beginning of the experiment. This was caused by the mat removing rubber from the sole and heel. Three steps onto a cleanroom mat was sufficient to remove all removable dirt from a shoe bottom, but with more efficient mats, only two steps were necessary.
Whyte 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; and the adhesive strength of the mat surface greater. These variables interact in complex ways to ultimately impact particle removal.
Based on his findings, Whyte surmises that polymeric mats will work better in situations where there are larger particles, more heterogeneous particles and where concentration of particles per surface area is high. However, peel-off mats will work best with smaller particles, more homogeneous particles, and at lower concentrations of particles per surface area. Polymeric 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.
Whyte`s final conclusion is that if the mats are used correctly, which means if shoes are applied three times to the mat, then it does not matter which mat is used, because for all practical purposes, both are close to 100 percent efficient.
Maintenance and cost
Whyte`s scientific study did not venture into the even more complex areas of maintenance and cost differences between the two types and their impact on performance. Manufacturers of both types claim superiority in cost-effectiveness and ease of use. But since these factors do not lend themselves to testing in a laboratory environment, the claims of both sides will simply be listed for consideration.
Peel-off mat manufacturers say that keeping cleanroom mats clean should be the top priority, because if mats are not kept clean, they will cease to remove foot and wheelborne contamination effectively. Peel-off mats are easy to keep clean because the top sheet can be peeled off in seconds, exposing a new clean sheet. Peel-off mat manufacturers say the need for washing and drying the polymeric products is time-consuming, costly and could interrupt the flow of personnel into and out of the cleanroom.
Polymeric flooring manufacturers say that most products can be maintained on the same schedule as other flooring, usually washed and dried during off-peak hours, or in sections, to avoid closing off a room to personnel. In addition, they say that even though specialty detergents are available for the polymeric products, the same cleaning agents used to clean standard flooring can also be used to clean the polymeric flooring.
The manufacturers also question whether peel-off mat users always follow the proper protocol in peeling off the top layer of the disposable mat. They say that improper peeling can result in particle dispersion in clean areas. They also claim that changing out a peel-off mat with a replacement can sometimes create a cleaning problem if the adhesive used to attach the mat to the floor sticks to the floor.
Cost comparisons of the two mat types depend entirely on the size of the mat or square footage of the flooring, the traffic flow in an area, the application`s cleanliness requirements and the mat`s life span, among other variables. Users are urged to make their own cost comparisons, factoring in the original purchase price with the daily labor and supply costs necessary to properly maintain the mats.
Peel-off mat manufacturers say that over its longer life, the polymeric mat may be less expensive than a peel-off mat on a square-foot basis. However, the higher cost of cleaning, potential downtime during cleaning, the frequency of cleaning and the possible tendency to delay an involved cleaning procedure must be considered. In addition, the cost of the mats vs. the value of the product in the cleanroom being protected is also an important criteria in deciding which type to use.
On the other hand, polymeric mat manufacturers say the environmental impact of disposing of peel-off mat sheets on a regular basis is unacceptable in a time when most companies are trying to reduce their landfill waste. Peel-off mat manufacturers respond that an entire year`s worth of disposable mats would be considerably less waste than the volume of water and cleaning chemicals used to maintain a polymeric product.
More than anything, polymeric mat manufacturers stress mat size. They believe that mats should be large enough to accommodate the traffic and allow sufficient steps and cart wheel turns to remove contamination. Larger coverage will ensure that when a particle touches the floor, it will remain anchored on the mat surface until cleaning, helping to reduce contamination above floor levels on benches, racks, gowns and overshoes.
User experience
One disposable peel-off mat user is Roger Fournier, site contamination control representative at IBM`s Burlington, VT, facility. The mats are used at the entrance to garment rooms and other in/out areas, primarily where tooling comes off the line. Personnel typically walk on the mats in street shoes, then use a shoe-clean device, don a shoe cover and then a bootie.
Site facilities managers tried polymeric products several years ago, Fournier says, and liked that the mat was durable and didn`t have the replacement costs of the peel-off type. But they found that the manpower and chemicals required to keep the product clean became burdensome. They also found that the polymeric mats became soiled more quickly than the peel-off type, and that the polymeric mats were slippery when wet.
Fournier says that the peel-off mat approach has disadvantages as well. A cleaning vendor is required to monitor the tacky mats after, and/or during a shift change, to peel off the top-most layer.
Angelo Caruso, aseptic filling coordinator for Roche Diagnostic Systems (Belleville, NJ), helped make the decision to replace peel-off mats with polymeric flooring about two years ago at his facility. While the peel-off mats were easy to clean, and Caruso was happy with the size, availability and service he was getting from his distributor, the decision came down to one of cost. The peel-off mat was significantly more expensive to use than the polymeric system managers ultimately settled on, he says, considering the polymeric product`s life span and other factors. The product was installed at the airlock, entry to cleanrooms and gown-up areas.
Caruso believes the polymeric product is especially effective for Roche because of the heavy pallets that are moved daily over the flooring, which in some areas measures as large as 30-ft-long ¥ 9-ft-wide. The flooring is cleaned every night on the regular cleaning schedule.
Both mat types clearly have application in many situations. Some suggest that peel-off mats may work best in areas where traffic is low and cleanliness requirements are at their most stringent, and that polymeric products may work best in large, high-traffic areas — while others suggest just the opposite guidelines. It soon becomes obvious that there are no fixed rules and that users must weigh all the variables for a specific application before choosing between the two types. n
References
1. Whyte, W.; “Summary of Work on Cleanroom Mats,” Letter to the author, Dec. 27, 1996.
2. Whyte, W., Shields, T. and Prvan, T.; “Cleanroom Mats: An Investigation of Particle Removal,” Journal of the Institute of Environmental Sciences, July/Aug. 1996, pp. 19-27.
3. Whyte, W., Shields, T; and Wilson, I.B.; “Cleanroom Mats: An Investigation of Adhesive Strength and Soil Removal from Shoes,” Environmental Engineering, March 1996, pp. 21-29.
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Figure 1. A step test wheel was used to measure the particle removal efficiency of peel-off and polymeric mats.
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Figure 2. a) Removal of widely spaced particles using an adhesive film; b) Removal of closely spaced particles using an adhesive film; c) Removal of widely and closely spaced particles using a resilient surface.