Cost, Comfort and Contamination Control Drive Cleanroom Garment Trends
A cleanroom garment system is the primary barrier between a product or process and the greatest particle generator in the room–the operator. As such, it is as important as the HEPA filters in the ceiling.
By Susan English
Lower-cost, user-friendly–and environmentally friendly–garments with improved filtration and barrier capabilities are all driving the newest trends in cleanroom garment materials. Users are now turning back to more breathable reusable polyester fabrics and the recycling of disposables to achieve contamination control, wearer comfort and environmental responsibility, all at lower cost.
When a garment processor sits down with a user to recommend a specific garment program, many factors come into play. Cost is obviously a principal concern, but fabric density, pore size, air permeability, frequency of change, and moisture vapor transmission rate (MVTR)–the rate of water vapor passing through a fabric–are also controlling variables in determining comfort levels, balancing the inevitable trade-off between contamination control and operator comfort. To be truly cost effective, a garment must be comfortable enough to work in without the use of excessive air-conditioning. Experts say fabrics should have a “good hand” (feel soft to the touch), be durable enough to withstand repeated washing and drying without shrinking or puckering, dense enough to trap particulate, but breathable enough to permit the transfer of body moisture through the garment.
Major fabric categories
The major types of fabric used in today`s controlled environments are: synthetic continuous filament materials such as polyester and nylon; spun-bonded polyolefins, which are designed for limited use or disposable garments and may include polyethylene, polypropylene or coated polyester; and polymeric film material, including Teflon, butyl rubbers, urethanes and PVC or latex-laminated materials. A fourth category consists of materials designed to be inherently antistatic, typically interwoven or knit with a conductive filament in either a pin-stripe or a grid pattern.
Fabric technology and garment system design have taken the traditional 100 percent polyester garment and added splash resistant coatings and conductive yarns, transforming them into static-dissipative, high density barrier fabrics that are also–to differing degrees–user friendly. Susan Routt, president of garment manufacturer Fibra-Tek (Longmont, CO) says, “The vast majority of usage–90-95 percent–now is in wovens. All the Class 100 products are basically made up of 100 percent continuous filament polyester. Then they take it from there and do a couple of things to it: They either do a taffeta weave or a herringbone weave. Then they play with the density of the fibers per square inch and all sorts of finishes, calendering it–flattening it with a big steam roller–and that`s where the differences start.”
Expensive fibers such as conductive yarn or special laminates and coatings are then added. A variety of polyester herringbone and taffeta weaves are widely used in Class 100 or higher cleanrooms. These include fabrics like Burlington`s C-3 (Burlington Klopman Fabrics, Greensboro, NC)–one of the most commonly used in the industry– the Selguard line from Teijin Shoji (USA) Inc. (New York, NY); ChemStat 909 and 909A by Stern & Stern (NY, NY) and PFG`s (Precision Fabric Group, Inc., Greensboro, NC) F107. The other category is the membrane fabrics–PTFE compounds like Gore-Tex (W.L. Gore & Associates, Inc., Elkton, MD) and Genesis II (Tetra-Tex, Philadelphia, PA)–where a membrane is laminated to the fabric and a topical ESD coating placed on it for use in Class 10 or better environments.
In applications demanding full-body coverage and particulate control to 0.3 µm or lower, the cleanroom garment system functions very much like a high efficiency membrane filter. First and foremost, there must be low particulation from the fabric itself. Selection of low-linting fabrics with low residuals after laundering is essential right from the start.
A garment`s particle capture efficiency is related to mean pore size of the fabric and garment construction, i.e., how well apertures and seams at the throat, wrists and ankles are sealed. High density barrier fabrics with inherent liquid repellency (splash resistance), and Tyvek, a spun-bonded polyolefin from DuPont Fibers (Greensboro, NC), are widely used in pharmaceutical and biotech applications. Made of very fine, high density polyethylene fibers deposited as a mat in a tortuous path, then fused to minimize linting, Tyvek is extremely durable. But barrier fabrics have a low MVTR and tend to be much less comfortable for the wearer than the 100 percent polyester garments generally used in Class 10 to 100,000 cleanroom applications.
W.L. Gore & Associates, Inc. (Elkton, MD) manufactures state-of-the-art garments for semiconductor applications from Class 10 to Class 1 and sub-Class 1. Constructed of polytetrafluoroethylene (PTFE), or Teflon, expanded to create micro porosity, it is laminated to a polyester knit fabric. In filtration efficiency tests conducted by Gore on cleanroom garment fabric swatches, the fabric was found to have a filtration efficiency of 99.999 percent at 0.1-micron particles, compared to a 90 percent rating for spun-bonded polyolefin and a 20 percent rating for polyester fabrics.
Although tests indicate operator-generated particulate in a wafer fab is much less than that emitted by process equipment and gases, pharmaceutical and medical device man ufacturers see operator bioburden as the major source of microbiological contamination. Where fab operators are more apt to be wearing special static-dissipative garments to protect against device-destroying ESD, in medical, pharmaceutical or biotech cleanrooms, technicians don spun-bonded polyolefins or laminated polyester garments that form a protective barrier against splashes and potential exposure to highly toxic substances. Tyvek is still chief of the spun-bonded polyolefins, well-known in the cleanrooms industry for its barrier capabilities, high tensile-tear strength, splash resistance and inherently static-dissipative properties for Class 100 to Class 100,000 applications. According to Penny Stein, product development engineer at DuPont, the company has been investigating some of the newer fabrics on the market–high density, fine denier, calendered-type wovens with increased barrier properties. Although, she concedes, there has been some advancement, DuPont`s tests have concluded that Tyvek still outperforms even the highest density, high denier, calendered wovens used in pharmaceutical, sterile environments. “From the studies done, it`s about one and a half times as effective as the high density [fabrics] at blocking out particulates and twice as effective against microorganisms,” says Stein.
Reusable vs. disposable
In highly regulated industries, such as medical/medical device, pharmaceutical, or aseptic processing, where the main hazard is microbial contamination, “limited use” or disposable garments are constructed of special barrier fabrics which must be sterilized and tested for bioburden. They also have lower operating costs than reusables and are often used in Class 100 to 100,000 semiconductor and other applications. Not so much “disposable” as “limited use,” Tyvek garments can generally be worn from two to three days, less in sterile areas requiring three to four garment changes a day. With mounting concern over management of waste streams, disposables do pose an environmental challenge, marking a trend back to reusables for some applications or toward more environmentally responsible alternatives.
In line with trends toward resource conservation and cost reduction in garment systems, DuPont has been working with industry reprocessor Miller`s Precision Enterprises, Inc. (Schererville, IN) to recycle used Tyvek as building structural materials and in automotive paint spray rooms. The ultimate disposable garment is manufactured by Isolyser-White Knight Industrial (Norcross, GA). Its OREX biodegradable, disposable cleanroom garments are made from polyvinyl alcohol and were exhibited at the CleanRooms `96 East show in Boston. The OREX garments actually dissolve in water when heated to temperatures above 200°F. (See “Wear and Wash-away Garments Debut at CleanRooms `96,” CleanRooms, March 1996, p.1.) Bill Papciak, the company`s vice president and general manager, comments on the difficulties of manufacturing environmentally responsible products which are also cost effective. “Our customers say, `we want quality, service, performance and we want cost reduction.` And it`s a cost if it impacts their waste stream: they`re paying for it until it hits the final destination–a landfill, incineration or just general waste. I think companies are now providing ways for a positive environmental impact with the knowledge that it`s got to be economically feasible.”
Because cleanroom laundries, or garment processors, are large buyers (and vendors) of cleanroom garments, it is no coincidence that they are major drivers of advanced fabric technology. An example of this is the recent development of the Integrity line of garments to service both the medical and micromanufacturing markets. The new fabric, “designed as a woven made to function like a membrane,” was developed by Precision Fabrics Group, Inc., who tested it at Micron Clean Uniform, Inc. (Newburgh, NY). PFG pioneered the reusable medical fabrics market with the development of Compel, the first 100 percent continuous filament polyester fabric from which reusable surgeon`s gowns are manufactured. The company took the technology and adapted it to micromanufacturing applications. According to Brad Whitsel, Micron Clean`s vice president of technical operations, the company was looking to develop that single “dream” fabric that would address both segments of the industry, combining the antimicrobial characteristics of a pharmaceutical/medical garment with the antistatic, splash-resistant features demanded by microelectronic applications and at a competitive price.
The result was the Integrity 1800 and Integrity 2000 fabrics currently being wear-tested by garment processors throughout the country. Integrity 1800, designed for the medical/pharmaceutical/biotech market segments, is virtually identical to the 2000, except that it features a conductive stripe instead of a static-dissipative grid. (The grid is actually an interwoven conductive filament made of a carbonized yarn, encapsulated in polyester). The Integrity 2000 fabric, designed for semiconductor applications, not only has a Teflon finish cross-linked around the fiber to give the garment splash resistance, but a microbial agent to stop multiplication of bacteria and control odor in garments usually worn continuously for five days at a time in many fabs. “We know people wear these garments for a long period of time, so we wanted to offer the potential of controlling bacterial growth that is also non-migrating, which means that it`s not going to volatilize out when bacteria comes in contact with it,” says John Smith, who has market responsibility for PFG`s medical and protective fabrics.
To make the Integrity 1800 fabric competitive in its targeted market, PFG devised a single conductive stripe instead of a grid pattern. “We know that people wear four or five garments a day in the pharmaceutical/medical/biotech industries, so we needed to come up with a way to create a more economical product. A stripe is more economical than a grid fabric,” says Smith. “We knew if we wanted a product that would generally have more uses, it would have to fit into a Class 10 environment and also work in a Class 1,000 or 10,000. So we decided we`d make a product with a very small pore-size. The Integrity fabric is the first woven fabric to perform in use like a membrane, because it has such a low air permeability and pore size, but a remarkable capacity to pass moisture vapor.” PFG is also developing a twill fabric for automotive paint spray applications and worked with DuPont Fibers (Greensboro, NC) to introduce the Nega-Stat antistatic fiber for medical applications.
PuriTech (Vestal, NY) supplies Korean-made garments from Dongyang Purity Materials Co., Ltd. (Seoul, Korea). PuriTech is one of the few companies that supplies both the fabric and the completed garment, handling fabric for people “who would like to sew the fabric in the U.S.,” says Puritech president David Farnetti, as well as garments that are actually sewn in Korea. PuriTech`s HiChek Class 1 polyester garment is specially treated with a substance that shrinks the pore size of the material to 0.01 micron and contains no silicone. The material is then calendered, as are all the company`s fabrics from Class 1 through 100,000. The garment`s name is derived from its tighter than normal carbon check pattern–an ESD grid pattern only 5/16 of an inch square. Company specifications indicate its Class 1 twill garment has a filtration efficiency of 99.9 percent down to 0.1-0.3 microns, a moisture permeability of 7,600 Gr/Cm sq./24-hr., and an air permeability of approximately 1.2 cc/cm/sq./sec).
Farnetti sees the cleanroom garment industry as having changed quite a bit from five years ago, when garments were “a lot more show than substance.” He says, “Many individuals bought garments for the wrong reasons, and I think that`s changing now.” He decries the escalating costs of micro-increments of cleanliness, which cannot be related to yield. “If you don`t need to go that extra mile for that extra billion dollars and it`s not affecting your yield, why do it?”
In MR head technology, the focus is on ESD performance, followed by the ability to filter out ionic and organic contaminants, says Dom Carradero, contamination control officer for Quantum Corp. (Milpitas, CA). Since the majority of disk drives are manufactured in the Far East, a big challenge, Carradero says, is monitoring the integrity of cleaning processes long distance. “We use herringbone in some of our semiconductor processes–manufacturing wafers for the MR heads.” For Class 1 applications Quantum uses Gore-Tex and Genesis II garments, while in Japan, workers are primarily garbed in the Selguard line of polyester garments with static grid from Teijin Shoji (USA) Inc.
Another trend is microporous film laminates–typically, polypropylene microporous film laminated onto a spun-bonded polypropylene substrate. Kappler (Guntersville, AL) offers its IsoClean line of disposable garments made of Tyvek, but the company is also currently in development on a reusable cleanroom-compatible fabric as part of its ProShield line. The ProShield II fabric offers a composite material consisting of a non-woven substrate and a microporous film, says Lee, making it ideal for use in Class 10 or above semiconductor applications, in cleanup areas of the fab or in the photoresist area. The company is currently doing particulate testing on the new product, which, says Kappler`s Tony Lee, look favorable compared to other limited use products on the marketplace. Lee says an important way of addressing the customer`s need for comfort is through engineering of garments. “If you don`t sew it correctly or put the right patterns in, no matter how breathable the fabric is, if it`s too tight or binding, the person wearing it is not going to be comfortable.” Kappler`s new cleanroom fabric utilizes a more roomy pattern for its cleanroom coverall. Lee sees “a lot more exciting materials out there.” He says, “Many years ago, there wasn`t the technology to develop something better than Tyvek. Now there is.” He doesn`t see the market ultimately gravitating to only one kind of material because of the continuing diversity of customer requirements over the years to come. “Besides the cleanroom market in the U.S., there`s an emerging market in the Pacific Rim, so it`s really a global opportunity.”
“People don`t wear fabric; they wear garments,” says cleanroom garment guru Bob Spector, currently of Prudential Overall Supply (Irvine, CA), one of the top two cleanroom laundries and garment reprocessors in the country. Spector says the garment, not the fabric, is used to contain particles, although the fabric is a part of the garment. “I`m not comfortable with test results based on fabric swatch testing. I have to see it as it`s used, dealing with the stresses and strains with the body inside, as well as the particle-loading that comes from the garment or the skin or the hair or whatever might be exposed to the inside of the garment–abrasion, what are things treated with, etc.”
A fabric manufacturer who designs and sells its own garments all over the world and who believes that design is key to contamination control is American Protective Clothing, Inc. (Lorain, OH). Its AmTex line is manufactured in the company`s new clean production facility with its own specially designed air systems. “The key is to blend the design with the fabric,” says Founder Bob Jordan. Producing garments in a clean facility “cuts laundering dollars in half” on garments leaving his facility–the only new one in North America, he says. AmTex, a membrane sandwiched between two layers of carrier fabric is 100 times stronger than the leading membrane fabric at a fraction of the cost, Jordan claims. AmTex Class 10 garments are built around the Mil-452088 spec, and American certifies that every raw piece of material prior to manufacturing is either sealed or serged before the garment is put together. The company also creates hybrid garments tailored to customer requirements, which can also be worn inside-out with equal protection against contamination. n
A Gore-Tex garment, described as “an ULPA filter for the body.” The cleanroom garment is sometimes described as the most neglected area of contamination control in the cleanroom.
Kappler`s new ProShield cleanroom-compatible garment touts itself as a more comfortable microporous film laminate with a non-woven fabric base.
A Tyvek garment from Kappler`s IsoClean line. Tyvek has superior barrier properties and is also recyclable.
PFG`s Integrity 2000 garment, designed for semiconductor and other Class 10 and under applications, features a pore size of about one micron–the smallest for a non-laminated fabric-air permeability of < 1 cfm, but an MVTR similar to that of fabrics with much poorer filtration properties.
SEM cross-section of a coated lightweight polyester fabric.
SEM cross-section of the Integrity 2000 fabric, finished, not coated. The Teflon finish gives splash-resistance; the Microbe Shield is an antimicrobial compound polymerically bonded to the 100 percent acron polyester filaments, which inhibits the growth of bacteria and fungi. It also aids in the control of bacterial colonies which create additional particulates. The Teflon finish, Microbe Shield and small pore size (1 µm) are the essence of the triple-barrier protection for Integrity 1800 and 2000 fabrics.
5,000x magnification of the EPTFE (expanded polytetrafluoroethylene) (Teflon) Gore membrane.