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Addressing environmental issues of silicone technology in nonwovens applications.

"Silicone" is a generic name for a diverse group of polymeric materials that are used as both performance enhancers and process aids by nonwovens producers. Performance-enhancing additives based on silicone technology can be either reactive or nonreactive. They are designed to improve the physical properties of nonwoven products, including water repellency, softness and abrasion resistance.

As process aids, silicones often function as wetting agents, lubricants, highly-effective antifoams and release agents. A significant advantage of using silicone process aids with nonwovens is their ability to provide more than one benefit with a single product.

Common applications of silicone enhanced nonwovens include both durable and disposable goods in a wide range of industries, such as hygiene products, surgical apparel, filtration media, carpet backings, roofing and geotextiles. Nonwoven materials may also serve as applicators for products containing silicone. Automotive, personal care and household uses are the most common examples; in these cases, a cleaner, polish or moisturizing agent is delivered to a surface via a nonwoven applicator.

There are many possible variations of silicone molecules and this discussion has been limited primarily to those common in nonwoven applications. Most are based on polydimethylsiloxane (PDMS) technology, and can be either volatile (usually low-viscosity liquids that evaporate easily) or nonvolatile materials (typically high-viscosity liquids, resins and coatings).

Nonwovens producers can take advantage of the versatility of the silicone molecule to increase both efficiency and performance. For example, lowviscosity silicone fluids can be used as fiber lubricants for carding operations. Silicone elastomers and resins are used as coatings to provide resistance to water, chemicals and abrasion in geotextiles and other industrial nonwovens applications.

Polydimethylsiloxane can also be combined with many other technologies to produce a wide range of copolymers with a controlled hydrophobic/hydrophilic balance. Silicones are synthetic engineering materials that combine certain characteristics of both organic and inorganic materials, frequently offering the most desirable properties of both.

Environmental Fate

Silicones are typically very stable compounds, due in part to the siloxane bonds that are also found in sand (silica). Polydimethylsiloxane has undergone some of the most extensive toxicological testing of any materials class and has been determined to be extremely inert, without significant potential for negative environmental impact.

Extensive studies with marine life, fowl and mammal species have shown PDMS to be non-toxic, even when tested at concentrations much higher than anticipated from any type of environmental exposure. PDMS has a long history of safe use as heat transfer fluids, electrical transformer liquids and many other applications, often serving as replacements for organic materials found to present health or environmental hazards. Copolymers based on methyl silicone and polyether chemistries are also popular.

The ultimate fate of silicone products used in nonwovens processing depends on the chemical and physical properties of the specific formulation employed, as well as the mode of environmental entry.

Non-volatile Materials. Non-volatile silicone materials used with nonwovens may end up in the plant's waste water stream in the form of tiny dispersed droplets. Because the solubility of non-volatile PDMS fluids is extremely low (below the current limits of detection), these materials will become a minor part of the sludge in a treatment plant. The fate of these products depends on how the municipality handles its sludge.

If the sludge from water treatment is incinerated, the silicone content reverts to silica, which presents no further environmental consequence. In treated sludge used as fertilizer, very small levels of silicone may be introduced to the soil environment, where it is subject to further degradation.

Because it is biologically inert, PDMS does not inhibit the microbial activity by which waste water is treated. In addition, extensive testing on aquatic plant and animal life has revealed no ecologically significant adverse effects from these methyl silicones, even under highly exaggerated conditions of exposure.

The mobility of individual silicones depends on the size of the particular molecule under consideration (Table 1). The molecular size of the nonvolatile methyl silicones renders them too large to pass through biological membranes.

The fact that non-volatile methyl silicones cannot pass through the membranes of fish or other animals is very significant, as it prevents bioconcentration. More important, the inert nature of these silicones poses no known hazard to the environment and they are not classified as hazardous wastes.

Laboratory testing of both volatile and non-volatile dimethylsiloxanes has shown these materials are not skin irritants or sensitizers. No studies to date have demonstrated adverse effects from inhalation, oral ingestion or dermal exposure when the products are used in their intended applications.

Volatile Materials. Volatile silicones may enter the environment during a spraying operation, as in the case of an aerosol product applied to processing equipment as a release agent. Some non-volatile silicones may also contain volatile components that can be released to the environment through exhaust fans or HVAC equipment. These volatiles enter the atmosphere, where they are broken down by photochemical oxidation. The partially oxidized degradation products will eventually be transported from the air by rain to the earth's surface, where they are further diluted and degraded. The final products of this oxidation process are naturally-occurring substances: silicic acid, carbon dioxide and water.

Volatile silicones at present are considered as VOCS; however, they appear to exhibit very little smog-forming potential and therefore would seem to lie outside the intent of the legal VOC definition. Researchers have found no evidence that these materials contribute to ground-level ozone formation.

In fact, in a recent series of tests conducted at the Statewide Air Pollution Research Center at the University of California (Riverside), it has been found that volatile polydimethylsiloxanes actually inhibit smog formation as a result of their tendency to lower the rate of photochemical ozone production. Further, their short atmospheric lifespan (10 to 30 days) prevents travel to the stratosphere, precluding any effect on the stratospheric ozone layer or global warming. Efforts are currently underway to petition the EPA to exempt volatile PDMS from air quality regulations that restrict the use of VOC'S.

Very small amounts of volatile silicones may enter the environment via the treatment of waste water, but these will partition to the atmosphere as a result of their low water solubility and tendency to evaporate. For this reason, they are not expected to accumulate in the earth's aquatic environment. Because of the low density of the materials, they will remain at the water's surface, where they can be recovered by conventional techniques. Remaining amounts can be allowed to evaporate, where they will degrade as already noted.

Supplier Initiatives

The efforts of silicone suppliers to improve manufacturing efficiency and minimize any effect on the environment will help to ensure that nonwovens producers can rely on the availability of silicone products in the years to come. Rather than concentrating on end-of-pipe waste management, the current focus at Dow Corning is on waste prevention.

In 1991, the firm became one of 300 companies to make an initial commitment to the EPA'S Industrial Toxics project, also known as the 33/50 project. The voluntary program has challenged participants to reduce total environmental releases of 17 targeted chemicals by 33% in 1992 and by 50% in 1995, using 1988 figures as a baseline. Today the program has grown to include more than 700 manufacturers.

Packaging is another area of focus for environmental improvements. Many silicones are shipped to nonwovens producers in steel drums, which serve the marketplace well as durable containers that do an excellent job of preserving the purity of the materials inside. But until 1992, no formal program existed for the recycling or disposal of steel drums that had been used to ship silicone products.

A Packaging and Environmental Task Force was established to examine the problem and Dow Corning and Van Leer Containers have introduced a co-sponsored recycling program that addresses the concerns of nonwoven producers about potential liability for drum disposal. Regardless of the number of drums involved, silicone users can now arrange for pickup of empty steel containers, which are returned to service in one of two ways: some are cleaned and reconditioned for refilling, while others are crushed or shredded, then used as high-grade scrap in the production of new drums. Participants cite lower costs, better efficiency and reduced landfill waste as key benefits.

The shift "upstream" for environmental policy includes a specific focus on transportation and distribution. As part of the Chemical Manufacturers' Association (CMA) "Responsible Care" Program, a documented approach to transportation safety has been undertaken to examine current handling practices.

The Responsible Care Program is indicative of the management practices now being employed to identify improvement opportunities and provide a meaningful approach for implementing changes. Adopted by the CMA in 1988, the initiative fosters a commitment to conscientious environmental performance throughout the industry, one that encourages dialogue between manufacturers, suppliers, employees and the public.

Nonwovens customers have responded positively to environmental responsibility initiatives and suppliers continue to offer support in traditional and nontraditional ways. They provide extensive product safety information, continuous environmental testing and new methods of reducing waste.

On some occasions, an unusual request will send researchers back to their labs to find a new approach or even a new material variation. In one nonwovens operation, a manufacturer requested assistance in tracking potential mobility of silicone materials in the plant. In order to do so, a fluorescing material was added to the silicone formulation, which allowed inspection with a "black light" to determine exactly where the silicone resided during the operation and whether any residuals were left behind.

The partnering concept is strong in nonwovens production and emphasis is on close cooperation with individual customers, determining their specific needs and finding ways to meet them. The versatility, efficacy and environmentally friendly nature of silicone technology is clear evidence of its utility. Ongoing efforts by suppliers to eliminate manufacturing by-products and offer case-by-case technical support indicates the likelihood of a sustained presence for these products in the nonwovens industry.
Table 1
Application Type Of Silicone Molecular Weight
Process aids:
 Wetting agents PDMS polyether Low to high
 Antifoams PDMS Mid to high
 PDMS polyether Mid to high
Release agents PDMS High
Lubricants PDMS Low to mid
Melt additives PDMS High
 Softeners PDMS Mid to high
 Functional PDMS Mid to high
 treatments PDMS polyethers Low to high
 treatments PDMS Mid to high
 (incl. coatings)
Delivery products:
 Personal care PDMS Low to high
 Household care PDMS Low to high
 Functional PDMS Mid to high
 Cyclic silicone Low-volatile
COPYRIGHT 1992 Rodman Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Buckingham, Anne
Publication:Nonwovens Industry
Date:Oct 1, 1992
Previous Article:Survey of raw material suppliers to nonwovens.
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