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Bonding technologies: chemical bonding.

Nonwoven fabrics are broadly defined as web structures made by mechanically, thermally or chemically bonding or entangling fibers. Chemically bonded nonwovens are held together by using binders as the bonding agent. The properties of the finished nonwoven products are a function of both the fiber and bonding system. Therefore, design of the binders used for the demanding end use applications of nonwovens is Of major importance. Since the binder plays a key role in performance, the user needs to understand how the binder contributes to the finished nonwoven product.

In nonwoven technology, binders provide the main mechanism by which the fibers are locked together. Binders have a major impact on the performance properties of the finished nonwoven fabric.

Many types of binder systems are in use today. Rubber and synthetic rubber are used, as are vinyl polymers and copolymers and acrylic ester polymers and copolymers. Aminoplast resins, starches and even hot melt adhesives add to the total. These systems usually exist as aqueous dispersions but can be added to the fiber structure as solutions or solids in thermoplastic form. Typical application methods include saturation, printing and spraying. Binders are needed to improve various qualities of the fabric (Table 1).
 Table 1
 Tensile Strength Permeability/Porosity Loft
 Tear Strength Elongation Hand/Drape
 Burst Strength Flammability Adhesion
 Light Fastness Cost Filtering Properties
 Heat Resistance Washability Tackiness
Chemical Resistance Delamination Durability
Abrasion Resistance Sealability Printing
Pilling Resistance Crease Recovery Laminating
Rewetting Properties Absorbency Fiber Tie Down

Of all of the binder systems currently available to the nonwoven producer, the most common is a latex or emulsion polymer. Although the words are used interchangeably, there are subtle differences between them. The chemical dictionary defines latex as a milk-like fluid in which small particles of natural or synthetic (plastic) are suspended in water.

Emulsions are described as substantially permanent mixtures of two or more liquids that do not normally dissolve in each other but are instead held together in suspension. Therefore, most of the water based polymers used as nonwoven binders are latexes made by emulsion polymerization. Like milk, which is an emulsion of fat dispersed in water, binders are simply emulsions of polymer particles dispersed in water. The major reasons for their wide appeal in nonwovens manufacture range from their variety and versatility to their ease of application and cost effectiveness.

Emulsion Polymerization

Polymers, in emulsion form, were produced as early as 1909 by Farbenbabriken Bayer labs. Butadiene was used in a potential route to form synthetic rubber. Today, emulsion polymerization is used to make high molecular weight polymers using the free radical vinyl addition reaction mechanism.

Four techniques are used in vinyl addition. These are bulk, solution, suspension and emulsion polymerization. Although emulsion polymerization involves the most complex polymerization procedure and the most ingredients, it is the main method used to produce nonwoven binders. Free radical emulsion polymerization takes place through the carbon-carbon double bond of vinyl monomers to yield high molecular weight, straight chain (linear) structures. This mechanism is used to produce vinyl acetate homopolymers, vinyl acrylic copolymers, ethylene/vinyl acetate copolymers, acrylic polymers and copolymers and styrene/butadiene or styrene/acrylate polymers.

The components used in emulsion polymerization include water, monomer, surfactant, initiator and buffer. The individual components are described in detail below.

Water. Water is the main ingredient in an emulsion polymer. The solids content of commercial binders ranges from 45-65%. Water acts as the continuous phase of the emulsion as well as being the polar medium. The heat capacity of water allows for the control of the fast exothermic reactions associated with emulsion polymerization.

Monomers. The basic building blocks of the nonwoven binder are the monomers used to form the backbone of the polymer. The most important properties of the finished polymer, such as strength and hand, are affected by monomer choice. The choice of monomer or monomers to use in the polymer is of critical importance. Monomer choice is based on manufacturing capability, end use performance requirements and cost structure.

Surfactant. Another important criterion of polymerization is choice of surfactant type and level. Not only is the surfactant necessary to disperse monomer in the aqueous phase, it also controls particle size, particle charge, surface tension, adhesion, film formation and emulsion stability. Anionic surfactants are often recommended to control small particle size latexes. Nonionic surfactants improve the mechanical and feeze-thaw stability of latexes.

Surfactants come in four major types based on ionic charge: anionic, nonionic, cationic and amphoteric. For emulsion polymers used as nonwoven binders, anionics and nonionics are most common. An anionic surfactant has an anionically charged hydrophilic head and a hydrocarbon-based hydrophobic tail. A good example of an anionic surfactant is sodium lauryl sulfate.

A nonionic surfactant is one in which the hydrophilic group is a covalent polar functional group that dissolves without ionization. Ethoxylated alkyl phenols are good examples of nonionic surfactants used for emulsion polymerization. The selection of surfactant is an important one. Even after the backbone and functional monomers have been chosen, a variety of different latexes can be produced to serve a range of end use performance parameters based on choice of surfactant type and level and by modifying polymerization conditions. For example, anionic surfactants are typically used in absorbent nonwoven applications and aid in wetting the fiber substrate.

Initiator. The actual polymerization reaction starts with the addition of a catalyst or initiator. Emulsion polymerization of nonwoven binders is dominated by initiators of the water-soluble, free radical type. These initiators are compounds that decompose to form single electron species known as free radicals. This decomposition is accomplished thermally or via oxidation-reduction reactions. This latter technique is referred to as the redox method. Typical oxidizing agents include persalts such as the alkali metal and ammonium persulfates, hydrogen peroxide and organic hydroperoxides such a tert-butylhydroperoxide. Reducing agents include water soluble thiosulfates and hydrosulfites and their salts, such as the sulfates of metals such as iron, nickel and copper.

Buffer. Buffer is used to control the pH of the polymerization reaction. Monomers used in the manufacture of nonwoven binders, such as vinyl acetate and some acrylates, may hydrolyze if pH is not tightly controlled. One of the more common buffers used in emulsion polymerization is sodium acetate.

Fiber Bonding

Film Formation. Once the binder is made, proper film formation of the polymer is necessary to effectively bond nonwoven fibers. Once the wet binder has been applied to the fiber web, capillary action forces the binder to migrate to the crossover points between fibers where it dries to form a "spot weld." For a latex to form a continuous film at this intersection, the particles must coalesce. Coalescence occurs as the water is driven off during drying. Proper coalescence provides for good adhesion to the fiber and influences overall nonwoven strength. Also, if the particles do not fuse together to form a good film and cracks form in the binder, the strength will be reduced. The presence of chemical crosslinks at the molecular level will be ineffective if film formation is not adequate.

Crosslinking. Once the film is properly formed and adhered to the fiber, the strength of the polymer bond can be increased by crosslinking the polymer. Crosslinking serves to link the linear carbon chains of the polymer, thereby increasing molecular weight and tensile strength. Self crosslinking systems require a crosslinking moiety that is randomly distributed throughout the polymer. This functionality is usually introduced into the polymer through the copolymerization of N-methylol acrylamide (NMA). Since most vinyl polymers are linear, the crosslinking provided by NMA is ideal. Crosslinking is an add catalyzed, condensation reaction that can be affected by heat.

Crosslinkable polymers are ones in which the functional monomer can be reacted with post-added crosslinking agents. The common functional monomers contain hydroxyl or carboxyl functionality. Carbamide resins, most frequently urea formaldehyde and melamine formaldehyde resins, have been most widely adopted as the post-added crosslinkers.

At this point, the nonwoven binder has been designed, polymerized to include major and functional monomer and characterized. Nonwoven binders are seldom ready-for-use as supplied. Formulation with a variety of products depending on processing conditions and desired end use performance is a common practice. The most common formulation ingredients are defoamers and wetting agents. Catalysts are used in crosslinking systems. Other ingredients include external crosslinkers, thickeners, colorants, flame retardants, water repellents, brighteners, fillers and dilution water.

Dilution water is added to control the weight of dry polymer solids applied to the nonwoven being bonded. Surfactants are added to improve stability, wetting and penetration of the binder mix into the fiber matrix. Foam may result from the addition of surfactant and typical processing. Defoamers are used to minimize these problems. Other ingredients are used to tailor the finished fabric to specific end use requirements. The formulated nonwoven binders are then ready to be applied using the chemical binder application technologies outlined earlier.

With the variety of vinyl monomers, functional groups and surface active systems available, nonwoven binders can be engineered for various end use applications. By understanding the technology associated with chemical binders, one can develop nonwovens with improved properties. Binder design must be considered in order to develop optimum nonwoven products. Current effort in binder development is on upgrading the performance of today's binders to meet the requirements of the nonwoven applications of the future.

About the Author:

Ronald Pangrazi is project supervision-polymer emulsions group for the Resins and Specialty Chemicals Division of National Starch and Chemical, Bridgewater, NJ. He has been with the company since 1984 and holds a B. S. in Chemistry from St. Vincent College, Latrobe, PA. Mr. Pangrazi has been engaged in the development of emulsion polymers for nonwovens and paper saturation applications, specifically ethylene vinyl acetate copolymers and high temperature heat resistant polymers and he holds five patents in that area.
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Title Annotation:Nonwoven Bonding Technologies: There's More Than One way to Bond a Web; experts evaluate emulsion polymerization and fiber bonding
Author:Pangrazi, Ronald
Publication:Nonwovens Industry
Date:Oct 1, 1992
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