Garments at risk
The increasing requirements for contamination protection in controlled areas have led to the development of cleanroom fabrics with an emphasis on leakage reduction. As a result, the important attribute of comfort of the cleanroom garment has decreased. However, comfort is a crucial factor in the acceptance of cleanroom garments by employees. A lack of acceptance or unreadiness could, for example, lead to erroneous utilisation of the cleanroom garment or other mistakes that might result in very high follow-up costs.
Personnel risk Employees have to accept that product protection against the greatest contamination risk – the human being – should have highest priority over all others. Since employees in Europe traditionally have more influence on company business than elsewhere in the world, very early on the aim was to combine product protection with personal comfort for cleanroom personnel. In this context, it is important to realise that comfort is a very subjective judgment, which can also be influenced by other parameters not necessarily related to the cleanroom garment. It is therefore not surprising that the cleanroom garment material in Europe was found to be more comfortable than that used in North America and parts of Asia. In addition to the smoother grip of these materials, the air permeability, the size of the pores and quite often the water vapour (humidity) impermeability are the most striking differences. Because of properties such as the higher air permeability, the more comfortable fabrics were considered problematic for critical areas (e.g. cleanroom Class ISO-5 and higher). However, this is not always the case as demonstrated by the often-used cleanroom fabric ION-NOSTAT-VI.2, which shows a relatively high air permeability and outstanding particle filtration efficiency. This was demonstrated by an independent investigation by the Institut fuer Textil- und Verfahrenstechnik (ITV) in Denkendorf, Germany. After 50 decontamination cycles the particle filtration efficiency of 95-97% was measured for a particle size of 0.5µm. These statements consider cleanroom fabrics in their unprocessed condition and not the complete garment or the cleanroom garment system. Most of the testing methods used are only suitable for pieces of fabrics. Nevertheless, the investigation of the whole system is at least as important as the analysis of individual textile parameters. What good would a very good cleanroom fabric be if, due to other factors, it posed a possible contamination risk to the product that it should be protecting? There is the so-called Body-Box-Testà, a testing method, which allows the investigation of complete cleanroom garment systems. In this test the influence of the laminar airflow due to the test person and the location of the measurement probe have, in most cases, been neglected.
Pump effect The so-called pump effect is one of the most important properties of a complete cleanroom garment system. Hottner refers to the pump effect and pressure differences in the garment system and points out the resulting possible risksá and in 1990, Iti and Sugita referred to the pump effectâ. This effect is mainly the result of body movement under the cleanroom garment, which builds up a higher pressure compared with environmental pressure. This pressure attempts to balance through areas of the cleanroom garment that have the least resistance. These are usually openings for the arms and legs, as well as the seals for the neck and head. At these points, the release of contaminated air is at its highest and also very highly concentrated. The denser the fabric, the less the pressure release over its large surface area and it therefore has to escape through an opening in the garment. This inevitably leads to the higher risk of the highly concentrated particle clouds, which could contaminate the protected product. In addition to the pressure differences, thermal effects support the release of contaminated air at the very critical places such as the head/neck region. During initial investigations into the efficiency of cleanroom-appropriate intermediate garments at ITV-Denkendorfã, pressure differences of 30 Pascal could be measured. For that particular test a pressure sensor was mounted at the back of the intermediate garment and the differential pressure was continuously measured during the movement program. The position of the sensor, the volume around it, and the sequence of movements influenced the measurements, showing that the measured values are not absolute values. In any case, it can be shown that with body movements, high pressure builds up under the cleanroom garment. During these investigations, it was shown that the laminar airflow around the test person in the cleanroom and the position of the particle counter could influence the results when comparing different cleanroom fabrics. Therefore, a very simple experimental set-up was used subsequently to reduce these effects. (For support during the tests we wish to thank Mr. v. Kahlden, of CCI von Kahlden). In addition, the aim of these new investigations was to measure areas that potentially had the highest release of contaminated air.
Goals of these tests The important goal of these tests was to demonstrate the possible difference between air permeable and air impermeable cleanroom fabrics with regard to the pump effect. If the obvious assumption that the pump effect on air impermeable fabrics is increased compared with air permeable fabric proved to be true, then the common belief that denser is better must be critically scrutinised. Three cleanroom fabrics with differing air permeability were compared with cleanroom Class ISO-5 and better. One of the fabrics (C) showed a much higher air permeability than the other two (A and B), which showed similar results. For fabric C the measured values were around 29 l/min x dm2 at 200 Pa and the other two measured 4.9 l/min x dm2 and 10.7 l/min x dm2 at 200 Pa, respectively. Overalls (coveralls) with the identical cut, hoods and knee-high boots were produced from these fabrics. Underneath, a pure cotton jogging suit with the same cut was worn by test personnel. The particle counter was hooked up to a hose, which was mounted on the upper edge of the collar of the overall. In order to exclude any influence by breathing of the person and the facemask, a uniform face protection system was used. The textile facemask was always produced from the same material and fixed via a press button inside the hood. Underneath this, a disposable facemask of the classic type from Tecnol/Kimberly & Clark was used. As the gloves needed to be powder free, 12 inch cleanroom gloves made from latex were chosen. The tested cleanroom garment was cleaned several times at the start and decontaminated by the German company Microclean. After each test interval the cleanroom garment was again cleaned. For the particle measurements, a six channel-laser particle counter from Met One was used. After entering the cleanroom (comparable with the cleanroom Class ISO-4/ISO-5, with a vertical airflow of 0.45m/sec, at a temperature of 21-22°C with 45% relative humidity) the test person should, in order to acclimatise, move around for 15 minutes. The hose for the particle counter was then fixed to the test person, which was always located at the same position in the cleanroom.
Moving ahead Before beginning to move, four measurements (at 15 seconds) were carried out without any movement. The average of these obtained values served as a basic value for the particle count against which the values obtained during the pauses between the moving cycles were compared. After four background measurements, eight measurements (at 15 seconds) were performed in which the test personnel were walking. After each moving cycle, at least eight measurements were performed without the test person moving. However, in the case of the more air impermeable fabric, the pause between the moving exercises had to be extended as more particles were released than at the beginning of the test phase. The complete experiment (i.e. moving and resting cycles) was repeated 10 times with two complete sets of cleanroom garment for each fabric and test person. A technician from ITV-Denkendorf was co-ordinating and overseeing the exercises and cycles as well as the attachment of the hose and the operation of the particle counter. The results obtained from this experiment were straightforward. The particle concentrations obtained at the measurement point for the air impermeable fabrics (A and B) were many times greater than those for the more air permeable material (C). Even while pausing between the movements, this difference was clearly visible. However, it is important to note that the measurements for the materials B and C with test person 2 wearing the first garment set was accidentally terminated before the eighth measurement.
Results The missing values in this figure (for material B the 10th measurement, and for material C the measurements 7 to 10) were chosen to be constant. The differences between the particle emissions of the more air impermeable fabrics and material C are in the order of factor 10. Even though the particle release can differ from person to person (test person 2 released more particles of all tested materials than test person 1), the measured values in the moving and resting phases are more or less constant and for material C the pump effect was always lower. This result was obtained for the shown particle size of 0.5 µm as well as the sizes of 0.3, 1.0, 3.0, and 5.0µm. The results of this investigation are unique and cannot be compared with other results from literature, since this was the first measurement of its kind in which the pump effect was directly linked to the air permeability of cleanroom fabric. Nevertheless, it seems logical that the measured higher particle concentrations of fabric A and B are the result of the lower air permeability. The tests at ITV-Denkendorf have demonstrated that for some cleanroom fabrics the pump effect is significantly reduced without the danger of more airborne particles diffusing through the fabric. Cleanroom fabric C showed that after 50 decontamination cycles there was a particle restraint of 95-97% for a particle size of 0.5µm (at the ITV test stand). Even after 100 cleaning cycles, the same particle restraint was measured. Assuming an increasing pump effect, using a fabric with the properties of material C could greatly reduce the particle contamination. The discharge in front of the neck is in this case very critical, since the particle flow due to the pump effect aims in most cases towards the product, which the cleanroom worker is usually facing. This flow towards the product is probably also enhanced by the laminar airflow, which is usually from top to bottom. The assumption that an air permeable cleanroom fabric is more suitable for cleanroom applications is as wrong as the statement "the denser the fabric the better". For example, cleanroom fabrics exist which have higher air permeability than material C, but their restraint for airborne particles is only in the range of 50-70%. The same holds for some fabrics, which show similar results for the air permeability as fabric C but with much smaller particle restrain values.
A positive ending In the end a cleanroom fabric should be a well-balanced compromise between particle restrain, air permeability, and size or the pores in order to protect the product as best as possible. A positive side effect, as for material C, could be better comfort and a higher acceptance by the employees. The importance of these aspects was discussed at the beginning of this article. It would be interesting to extend the research to further influences in the development of cleanroom fabrics. Since these kinds of experiments are time consuming and involve substantial costs, it is not possible for one company alone to carry on with this kind of research.