Latex Binders for Nonwoven Fabrics

In the early stages of nonwoven fabric development, different types of natural resins and glues were used as binders. These included modified starches, cellulose derivatives, and natural gums. While these early binders provided some integrity to the web, they also exhibited several glaring shortcomings. Common deficiencies with early nonwovens included stiffness or poor "hand", inadequate absorbency, and poor laundering and dry cleaning ability. Historically the binder has been the "weakest link" in the nonwoven fabric and has limited the performance of the fibers.

Eventually, synthetic latex binders were developed to meet the performance characteristics required of modern nonwoven fabrics. Polyvinyl acetate was the first successful synthetic binder used in substantial volume, and as a result, many of the early problems associated with natural binders were eliminated. However, several problems still remained. The most important of these is the required compromise necessary between strength (generally achieved by increasing the amount of binder that is applied) and softness or flexibility of the fabric.

Modern latex adhesive binders, despite their somewhat higher cost, provide the industry with a substantial improvement in this trade-off as well as other special properties. They are also the primary reason that nonwoven fabrics can compete with more traditional textiles in many applications. These modern latex adhesive binders are described in the section that follows.

The primary properties that are desired in a modern binder are summarized in Table 2. The actual combination of final properties, of course, must be varied depending on the specific end-use of the nonwoven.

Table 2: Properties Desired in a Binder for Nonwoven Fabrics³

Binders are generally made of polymer dispersions (lattices) in water. These can include polymers and copolymers that are produced by the reaction of monomers in the presence of initiators or catalysts. The general benefits of latex binders in nonwoven webs have been the subject of a previous SpecialChem4Adhesives article ⁴; whereas this article will concentrate on the chemical types of latex that are commonly used.

Common latex emulsions such as the following are the most popular binders used in the manufacture of non-woven fabrics.

These latex polymers are generally produced via free radical emulsion polymerization in water, where a monomer or comonomers are converted into a high molecular weight polymer.

Thermoplastic fusible adhesives may also be used in the production of nonwovens. These can be applied as water dispersible raw materials or as solid powders. Linear, amorphous polyesters and polyolefins having a low glass transition temperature are often used for these types of applications. They are primarily used in specialty products due to their somewhat higher cost than the conventional latex adhesives.

The binder is typically a formulated product consisting of a straight polymer emulsion along with additives to provide for specific processing and end-use physical properties. The formulation ingredients used in latex binders for nonwovens are summarized in Table 3. A typical formulation for an acrylic binder that produces firm, resilient, and durable nonwoven products is shown in Table 4. The order of addition for the various ingredients is also indicated.

Table 3: Additives and Modifiers Commonly Used in Latex Binder Formulations

Table 4: Binder Formulation For a General Purpose Disposable Rayon Nonwoven⁵

The binder formulation fundamentally consists mainly of the finely divided and dispersed polymer or copolymer particles, water, and additives such as surfactants, wetting agents, plasticizers, etc. The additives are generally used to convey practical application properties to the latex. Surfactants are an important component in all water-dispersed adhesives since they are used to stabilize the polymer particles in water during emulsification. Crosslinkers are sometimes added to functional polymers to provide crosslinking and improved heat and chemical resistance.

The many types of chemical binders available for nonwoven fabrics can be classified by their chemical type. Table 5 provides a comparison of the characteristics and relative performance of the various types. Advantages and disadvantages are also described. However, some caution must be exerted in using such a table because properties within a specific type of binder family can be significantly changed by formulation with copolymers, modifiers, and additives.

Table 5: Common Latex Polymers Used as the Base for Nonwoven Binders

Generally these polymers are available in different molecular weights and particle sizes, and the final properties of the binder will depend greatly on these parameters as well.

Most binders have good strength characteristics in nonwoven fabrics under dry conditions. The choice of binder is then most often determined by heat or chemical resistance required of the end product as well as application characteristics and costs.

As with other adhesive systems, the binder properties are quite dependent on the glass transition temperature (T_g) of the monomer unit selected to form the polymer. Higher T_g (e.g., vinyl acetate) polymers will provide greater strength and reduced flexibility. Binders made from polymers with a lower T_g (e.g., acrylic) will provide greater flexibility or softness.

The monomers selected for forming the polymer latex will also have a considerable influence on the hydrophilic (polar) or hydrophobic (non-polar) nature of the nonwoven fabric. Thus, the polarity of the polymer within the binder will effect the water absorbency and wet strength of the final nonwoven product.

Nonwoven fabrics that will commonly be exposed to elevated temperatures and harsh chemicals should be manufactured from a binder capable of crosslinking preferably at a low curing temperature. The thermosetting structure produced by the crosslinking especially provides for wash and dry cleaning resistance. A low curing temperature is generally necessary because many fibers (e.g., polyethylene) can be damaged by high processing temperatures.

The polymeric binder may be self-crosslinking or a reactive additive may be utilized. In this case, the polymer that is utilized incorporates functional monomers. Many of the binder polymers are available as carboxylated modifications, which incorporate varying amounts of organic acid groups pendant to the polymer backbone. These groups provide a site for crosslinking reactions as well as for improving adhesion, particularly to cellulosic fibers. The carboxyl containing polymers may be ionically crosslinked with polyvalent metals such as aluminum, calcium, or zinc. Copolymers containing the amide or hydroxyl functions may be blended with aminoplasts for crosslinking. Similarly, the epoxy function, via glycidyl methacrylate, for example, can be incorporated to effect crosslinking by reaction with pendant carboxyl or hydroxyl groups or by self-reaction.

Acrylics are arguably the best all-around binders, having good colorfastness and good dry/wet strength. They also provide good durability and a wide range of fabric hand properties. They are available as latex dispersions with glass transition temperatures ranging from very soft (T_g equal to -40°C) to very hard (T_g of +105°C). They can also be made to crosslink for substantial improvements in heat and chemical resistance and durability.

Synthetic elastomer dispersions, such as those based on styrene butadiene or nitrile rubber, are often used where soft fabrics with a high degree of elongation are required. Nitrile rubber based latex systems also provide good resistance to dry cleaning processes and to solvents and oils.

It is possible to obtain the advantageous properties of two or more emulsion systems by blending different polymer lattices before application. However, care must be taken not to blend incompatible materials (i.e., those having surfactants of different charges or significantly different drying rates). Such "polyblending" enables the nonwoven manufacturers to combine the solvent resistance of a nitrile copolymer with the softness of an acrylic. It is also possible to blend a halogen-containing polymer into the binder for the purpose of flame retardancy. One can also obtain loft or bounce from a styrene butadiene copolymer with the relative softness of an acrylic.

The process of manufacturing a nonwoven fabric with structural integrity consists of three steps.

1. Formation of the fibers and web
2. Binder application to the nonwoven web
3. Removal of moisture from the binder

Fiber and web formation is crucial in the production of nonwoven. Without proper fiber material and web formation processes it is almost impossible to realize optimum performance of the nonwoven structure. Good web formation is characterized by macro- and microscopic uniformity of fiber deposition. The science of fiber and web forming is very detailed and outside the scope of this article.

Bonding can be carried out as a separate and distinct operation. Generally it is a sequential operation performed along with web formation. The binders may be applied by saturating, spraying, printing, or foaming techniques. Chemical binders are applied to webs in amounts generally ranging from about 5% to as much as 60% by weight.

Saturation bonding essentially encapsulates the fiber by totally immersing the web in a binder bath or by flooding the web as it nears the nip point of a set of pressure rolls. Excess binder is removed by vacuum or roll pressure. Binder addition levels for saturation bonding range from 20% to 60% by weight. Advantages of these methods are simplicity, controllable tensile strength, and softness by the choice and amount of binders. The major disadvantages are the great reduction in softness of the fabric due to the high concentration of binder that is present in the final product.

Foam bonding applies the binder at higher binders solids concentration level. The process uses air as well as water as the diluent and carrier for the binder. Foam bonded nonwovens require less energy in drying, since less water is used. The foam is generated by introducing air into the formulated latex while mechanical agitating the binder solution. The air : latex ratio is approximately 5 : 25 but can vary significantly depending on the specific product being manufactured. Stabilizing agents are added to the binder solution to prevent the foam from collapsing during application and drying. The final fabric made with foam-applied binders will exhibit greater softness, hand, and resilience. The primary difficulties in this method are the control of the foaming and application processes.

In spray bonding, binders are sprayed onto moving webs. Spray bonding is used for fabric products that require high bulk such as fiber fill. The binder is atomized by air pressure, hydraulic pressure, or centrifugal forces and is applied to the upper surfaces of the web in fine droplets through a system of nozzles. The binder can also be sprayed on the lower web surface by reversing web direction and orientation on a secondary conveyor. After spraying, the web is passed through a heating zone to remove water and the binder is dried or crosslinked in a secondary heating zone if the binder system can be crosslinked.

Print bonding is a process that applies binder only in predetermined areas. It is used for fabric applications that require a part of the fabric to be binder-free. Many lightweight nonwovens are also print bonded because of the minimization of bonding agent. The printing pattern is engineered so that specific properties can be maintained. Print bonding is most often carried out with engraved gravure or rotary screen rolls where the pattern as well as the thickness of the binder application can be controlled.

Thermoplastic polymers can also be applied to the nonwoven web either as free powders or as finely divided particles dispersed in water. Adhesion is achieved by the application of heat and pressure. Polyesters and polyolefins having low T_g and molecular weight are the most common types of polymer used for nonwoven powders.

When using any of the methods of binder application described above, the selection of heating or drying equipment becomes vitally important. An important consideration is to select equipment and operating conditions that will minimize binder migration. This problem occurs during the early stages of drying as water moves to the heat surface carrying binder particles with it. As a result the exterior surfaces tend to be rich in binder and the interior surface will be binder-poor. To minimize this problem, the initial temperature oven should be no higher than 95-130°C until at least 50% of the water has been evaporated. At this point, temperature can be raised to the recommended final temperature to complete drying or curing reactions.