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Silicone additives in nonwovens applications.

Silicone Additives in Nonwovens Applications

Silicone chemistry has brought many benefits to nonwovens producers through the last three decades, most notably in the form of water repellents, softeners and process aids such as antifoams and lubricants. But in recent years the versatility of the silicone molecule has permitted development of new technologies, some of which can provide physical characteristics that appear to contradict the advantages for which early silicone products were brought to the market.

Three basic classes of silicone materials, involving distinct chemistries, are frequently used in nonwovens applications. Nonreactive silicones are based on polydimethylsiloxanes (PDMS), which impart desirable properties as a result of their flexible polymer backbone, stable bonds and low intermolecular forces.

These products are commonly used as lubricants, antifoams and other process aids and some softening agents. The nonreactive materials lack durability to washing, however, and it may not be possible to maintain their effects throughout the life of the final product. In some situations, this is an advantage; in others, nonwovens producers may prefer durability (Table 1).

Table : Table 1
 TYPE OF TECHNOLOGY
 PDMS Cyclics Reactive Amino Glycol Phenyl
Lubricity X X X X X X
Softener X X X X
Emollient X X X X X


Water
 Repellent X X X
Antifoam X X X


Wetting
 Agent X
Durability X X X X


Conventional reactive silicone technology became commercially available about 20 years ago and is based on polydimethylsiloxane polymers that have been modified with silanic hydrogen or silanol functional groups. In the presence of water and an organometallic catalyst, these polymers form a crosslinked network on the fabric surface. These polymers are film-forming systems, providing improved softness as well as gains in stretch and recovery. Used primarily as water repellents and softeners, they cure to a very high molecular weight elastomer and exhibit far greater durability than nonreactive materials.

Organofunctional silicones are also modified polydimethylsiloxane polymers and form the newest class of silicone polymers designed as textile and nonwoven finishes. Replacing the silanol group used to modify conventional reactive silicones, organoreactive groups are introduced along the polymer backbone. These might include amines, epoxies or alcohols (Table 2). [Tabular Data Omitted]

Organofunctional emulsions offer a premium softness unavailable with organic products. In subjective hand evaluation studies and mechanical evaluations of reactive silicone and organic softeners, aminofunctional silicone polymers have demonstrated superior softening capabilities over other agents tested. They are based on the same chemistry as that included in many shampoos, conditioners and other personal care products.

In general, there are two types of silicones as they relate to nonwovens: those that impart some physical change to the substrate and others that act as process aids. The water repellents and fabric coatings are part of the former group, which also includes softeners and effective antimicrobial agents. As process aids, silicones are most commonly used as antifoams, lubricants and release agents. One of the advantages of utilizing silicones in nonwovens processing is the fact that a single product many provide more than one benefit.

Water Repellents

The hydrophobic nature of conventional silicones is well known and silicone remains one of the most effective water repellents available. Specifically, methyl hydrogen polysiloxane is employed because it is a highly crosslinkable material, forming a durable, invisible film that surrounds fibers with a hydrophobic sheath.

The water repellency is due to the orientation of the polysiloxane molecule. Hydrophobic methyl groups are forced out from the substrate, while the siloxane backbone is turned towards it. Some finishes are durable and withstand repeated insults without losing their resistance to moisture. Nonreactive silicones can be removed from the fiber for subsequent dyeing and finishing processes.

Silicone elastomers are also employed as waterproofing agents when applied to roll or sheet goods. Like fabric coatings in textile manufacturing, liquid silicone rubbers (LSRs) for nonwoven applications provide a durable, waterproof surface that remains pliable over long periods of time. Silicone water-based elastomers can also be used to form waterproof films without the use of solvents.

There Are Such Things As Hydrophilic Silicones

Perhaps the single most common misconception about silicone, however, is that all of its derivatives are hydrophobic in nature. In fact, the molecule can be manipulated to develop hydrophilic products as well, which act as a wetting agent for most substrates, including polypropylene. This can be accomplished through post-treatment of the nonwoven or by incorporating the silicone into the chemical binder system. Some of these hydrophilic crosslinkers will react with the binder material to produce physical property changes; others are designed to wash out.

Hydrophilic silicones are critical in many applications because, unlike their water repellent counterparts, these products should not inhibit dyeing operations. The materials also serve as thread lubricants in high speed operations, able to reduce friction and quickly dissipate heat without building up on equipment. They are frequently used in the manufacturing of carpet and printed toweling products, for example, where a hydrophobic product would be unsuitable. Some softening advantages can also be realized through the use of hydrophilic silicones, though in general the softening effect of this group of products is not at the same level as that of the hydrophobic materials.

In addition, fiber-to-fiber lubrication with silicones (either hydrophobic or hydrophilic) can improve the tear strength of the substrate; although some organic materials may offer the same benefits, the concentrations necessary to achieve the effects are much higher. Further, organics generally cannot withstand the higher operating temperatures associated with many drying processes and the resulting fumes and buildup on equipment are both undersirable (Table 3).

Table : Table 3
 COMPARISON OF SILICONES VS.
 CONVENTIONAL ORGANICS
Performance Organic Mineral Dimethyl Silicone
Requirements Ester Oil Silicone Glycol
Emulsifiable Yes Yes Yes Yes


Antistatic
Behavior No No No Yes
Scourability Good Good Poor Excellent


Conventional Application

Methods Yes Yes Yes Yes

The Advantages Of Chemical Softeners

Chemical softeners offer some advantages over mechanical methods, since manufacturers retain the flexibility to alter the process through additive selection rather than investing in dedicated equipment. Although some organic products can deliver both softening and release properties, silicones provide these benefits at much lower usage levels. The premium performance of silicones, such as high-heat stability and superior softening capabilities of the organofunctional group, is a significant factor in their selection as a softening agent.

The range of silicone products available to deliver softening and/or release benefits is extremely board. Matching the right product to the specific application is critical to the success of the operation and a detailed analysis of the bath is an important part of the process. Compatibility with other bath components is an essential consideration.

The Role Of Antimicrobial Agents

Antimicrobial agents based on silane chemistry are highly effective products that can be applied easily in a nonwovens process. Although the EPA classifies these materials as pesticides because of their intended use, they differ from conventional biocides by the way they control the growth of microorganisms.

Their effectiveness is the result of two factors. First, these silanes chemically bond to the surface of fibers and other substrates, making them extremely durable and longlasting. Secondly, this class of silane carries a positive electrical charge, which it imparts to the surface, causing the cell membranes to rupture and interrupting the life cycle of the organism without "using up" the treatment. Due to the chemical bonding, silane-based antimicrobials will not migrate to other surfaces and this tends to give them a very favorable safety profile.

The Variety Of Release Agents

Like the softeners and lubricants, a number of release agents are available, some offering added slip, others that are non-slip, depending on the requirements of the application. In some operations, processors will apply the release agent directly to equipment surfaces and reapply only periodically. Once a conveyor belt is coated, for example, it will not require reapplication for some time.

In order to maximize processing efficiency and hold down costs, most nonwovens producers prefer materials that can be added to an existing process bath. Silicone release agents can be incorporated as part of the existing process formulations or they may be applied directly to equipment by spraying techniques. In some applications, release agents are applied directly to the nonwoven fabric via kiss rollers.

Antifoams: From Food To Paper Applications

Like softeners, silicone antifoams have been widely used in a number of industries, including pulp and paper, textiles and nonwovens. Their performance in low concentrations is well documented and risks from incidental contact are so low that many products have attained food grade status from the FDA for use in food processing (Figure 1).

In some nonwoven applications, controlled foaming is a necessary part of normal processing. For these situations, there are silicone products whose cloud points can be manipulated to produce an antifoam that is inactive below a certain temperature. Below the specified point, they may offer lubrication or softening benefits but permit foaming activity. Once the cloud point temperature is reached, the material begins to separate from the bath and defoaming characteristics become apparent. Returning the bath to a temperature below the cloud point allows processors to rinse the silicone from the fabric completely, so dyeing operations will be unaffected.

Silicone antifoams exhibit excellent stability at extreme temperatures and will withstand processing under high or low pH conditions. They can be supplied as 100% silicone or as waterbased emulsions.

Trends: More R&D, Earlier Involvement

More than ever before, chemical formulators and textile/nonwovens producers have an extremely wide range of choices from which they may select or develop specific products to meet the needs of individual processes and equipment. The role of the raw material suppliers has grown to include technical service on a scale never before available to these manufacturers and the use of laboratory and testing facilities during initial phases of product development has provided significant advantages.

Research and testing facilities dedicated to textiles and nonwovens are an indication of this change in the industry, as suppliers of silicones and other materials recognize the need to concentrate on product development in close cooperation with manufacturers. The current trend towards earlier involvement of the material supplier, often working directly with nonwovens producers, serves as an advantage for both parties as they seek to enhance the physical properties of final products or improve system performance.

New material formulations serve as an example of this increased cooperation between formulators of textile and nonwovens finishes, the manufacturers they serve and the chemical suppliers who provide the building blocks for new products. From that codevelopment process will come the new technologies that will shape the future of nonwoven products, both in the U.S. and worldwide.

PHOTO : Figure 1 PERFORMANCE EVALUATION: ANTIFOAM EFFICIENCY
COPYRIGHT 1991 Rodman Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Buckingham, Anne
Publication:Nonwovens Industry
Date:Nov 1, 1991
Words:1766
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