by Sheila Galatowitsch
DEMAND IS INCREASING FOR FABRICS THAT COMBINE BOTH PARTICLE AND STATIC CONTROL.
Fabric and garment manufacturers say they're up to the task.
As part of an effective electrostatic control program, garmentsand the fabrics that make themare starting to get more attention. While most cleanroom users require contamination control more than static dissipation and choose their garments accordingly, other segments insist on garments that offer both a high degree of cleanliness and static control.
A prime example of this small, but growing, group is the disk drive industry, the first to demand higher electrostatic discharge (ESD) qualities in a cleanroom garment.
Other key users of combination cleanroom/ESD fabrics are in the semiconductor, pharmaceutical and auto spray paint industries. But the bulk of ESD fabrics are made into garments for non-cleanroom electronics assembly operations, such as circuit board, cell phone and pager assemblies.
The fabric surface is shown from an angle perspective of the Milliken & Company Perimeter Grid with a splash-resistant finish. On the fabric is a droplet of colored water showing how the fabric prevents penetration.
There's a reason why garment materials emphasize one quality over the other. Cleanroom fabrics use continuous, tightly woven polyester filament yarn for particle control, which is inherently less static dissipative than the polyester-cotton blend and other fabrics used for non-cleanroom ESD applications. Due to its ability to absorb moisture, cotton is more static dissipative than polyester, but it produces lint.
In addition, fabrics headed for cleanrooms must incorporate durable, noncontaminating conductive yarn for static dissipation. Belltron-branded carbon yarn from Kanebo Ltd. in Tokyo, Japan, is the most widely used for cleanroom fabrics, which have an average surface resistivity of 107 to 1011 ohms per square. A typical ESD-specific fabric, using more conductive but sometimes contaminating ESD yarn, has a surface resistivity of 105 to 107 ohms per square.
Still, several manufacturers have developed fabrics that provide both cleanroom and ESD properties. Precision Fabrics Group Inc. (Greensboro, NC) offers a lineup that includes the Integrity 2000 fabric for ISO Class 3 (Class 1) applications, the Integrity 1800 for ISO Class 4 (Class 10) pharmaceutical and biotech cleanrooms, and the Integrity 1700 for ISO Class 5 (Class 100) areas.
This spring the company introduced the Integrity 1750 fabric for ISO Class 5 applications and the Integrity 1650 for undergarment sitewear. These fabrics have tight pore sizes, down to 1 micron in the case of the Integrity 2000, and feature a Belltron carbon grid or stripe for static dissipation.
Another fabric line incorporating particle and static control is the ChemStat 909 Plus Series, manufactured by Stern and Stern Industries Inc. (New York, NY) for ISO Class 5 and ISO Class 4 cleanrooms. In fact, all seven Stern and Stern fabrics (with the exception of a face mask material) contain some level of ESD control, from the ChemStat 969Z Plus developed for sterile cleanrooms to the 959A Plus for areas that demand extremely high levels of particle and static control. That fabric has a surface resistivity of 105 ohms per square.
Each of the vendor's fabrics was designed for a specific application and feature different yarns, construction, finishes and static dissipative properties, but they all have a patented raised grid above the plane of the fabric. The grid, composed of conductive yarn woven into the base fabric, acts as a wick to draw static electricity out of the fabric and dissipate it into the surrounding ionized air of a cleanroom or through garment grounding.
The ChemStat fabrics also feature a two/one twill weave (two yarns floating over one yarn) for durability and softness. “It makes for a quieter fabric, and that's especially important in noisy cleanrooms where hoods are worn,” says Stern and Stern spokesman Thomas Hogan.
All fabrics headed for cleanroom or ESD applications strive for performance, affordability and comfort. The Teijinselguard line of contamination control and ESD fabrics from N.I. Teijin Shoji (USA) Inc. (New York, NY) allows air and moisture permeability and a greater degree of suppleness to enhance worker comfort. The T85675 fabric, introduced this year, is suitable for ISO Class 4-and-above cleanrooms.
For more stringent applications, the vendor offers the Selguard IV S85930, with improved comfort, and the Selguard 110 T85935, a thin, lightweight undergarment fabric. Because Teijin Shoji manufactures its own proprietary polyester and carbon fibers, it can develop custom fiber and fabric combinations, says Takashi Yoshimura, sales and marketing manager.
Teijin Shoji has been a global cleanroom and ESD fabric supplier for more than 15 years, but new to the cleanroom/ESD market is Milliken & Company (Spartanburg, SC), a 135-year-old fabric manufacturer. Its recently launched Perimeter line includes four fabrics for particle control and two that target ESD.
Milliken's Perimeter Grid C, a 100 percent polyester filament fabric suitable for ISO Class 3 and 4 areas, also includes Belltron carbon yarn to bleed off static. The Polystat ESD fabrics, which contain cotton and carbon yarns to control static build-up from bodily movements, are designed for assembly operations above ISO Class 6 (Class 1000).
Under development for five years using the vendor's proprietary microfilament yarn and moisture management technologies, the fabrics were tested on a “sweating” manikin at North Carolina State University and with human high-tech workers across the country. “The result is a patent-pending fabric to help eliminate the moisture vapor that builds up between skin and garments,” says Tom Eckles, senior account manager.
Of course, fabric ultimately is made into garments, and garment construction is just as important as the fabric in particle and static control. When they set out four years ago to design a cleanroom/ESD garment system, Kay Adams and Michele McSwain of TW Clean Inc. (Carlsbad, CA) weren't satisfied with the ESD properties of the cleanroom fabrics they tested. So they created their own fabric with ESD performance, durability and ease of ESD testing as key features. The resulting Class 100-compatible fabric is used solely for TW Clean's Ground Zero Personal Grounding System product line, which grounds both the worker and the garment simultaneously.
Other garment manufacturers choose a variety of fabrics to meet the end-user's need for performance, comfort and cost. “We assess all the fabric options on the market to come up with the right offering to the right customer base,” says Tim Reasoner, territory manager for Fibrotek Industries (Longmont, CO). “In the mix of fabrics we have chosen, each has a differentiating characteristic. One has a small percentage of cotton in it, which adds more bulk and which some users perceive as more comfortable. Another has a higher percentage of carbon than most, which leads to superior ESD performance, and one is extremely lightweight. Our mission is to have a diverse mix on the shelf in finished goods for immediate delivery.”
Fabric makers continue to improve the conductivity of fabrics headed to cleanrooms. In the past two years, Burlington Barrier Products Div. (Greensboro, NC) has introduced two new fabrics to complement its C3 product, which has been on the market more than a decade for ISO Class 4 areas. The C4 and C5 fabrics provide ISO Class 3 cleanroom garment protection if used in conjunction with an undergarment.
Demand is increasing for fabrics that combine both particle and static control, says Anderson Hostetler, Burlington Barrier Products' vice president. Greater demand means a better supply of fabrics, and that's good news for static-sensitive cleanroom users.
Cleanroom fabrics use continuous, tightly woven polyester filament yarn for particle control, which is inherently less static dissipative than the polyester-cotton blend and other fabrics used for non-cleanroom ESD applications.