This article will deal with chemical bonding as opposed to the other two broad categories of nonwovens-bonding: mechanical bonding and thermal bonding.
Latex binders have been in existence at least as long as most modern nonwovens themselves. In some people’s opinion, they represent “old” technology, with little in the way of new applications in the future.
This is undoubtedly an over-simplification. The great benefit of latex binders is their overall versatility and utility.
By experimentation with latex composition, formulation, binder, additives, process and binder-application methods, the possibilities in end use are unlimited.
Strength enhancers are added to increase crosslinking, whereas surface modifiers are added to make the surface of the cured latex more or less wettable. Surfactants can improve the wetting properties as well as the processing, but most are water-soluble and so the improvement in wetting characteristics is transient. Hydrophobes in the form of fluorochemicals, polyolefins or silicone derivatives are added to reduce the wetting characteristics. Low-cost fillers can be used to reduce the cost of the product, and some fillers are functional as well, such as activated carbon black.
Emulsion polymers readily accept a broad range of additives that can increase their versatility and use.
From a functional viewpoint, binder performance can be separated into effects due to the fiber, cohesive strength and adhesion. In order to gauge the contribution that each of these factors makes to overall performance, they may be studied independently of one another.
Physical attractive forces refer to the interactions such as hydrogen bonding (e.g., polyvinyl alcohol), which serve to link polymer chains together but are usually not very strong bonds.
Chemical crosslinking ties the polymer chains together through the formation of irreversible covalent bonds, which yield a continuous, three-dimensional network (e.g., N-methylol acrylamide).
Decreasing the immobility of the polymer chains serves to enhance the cohesive strength by preventing the slippage of polymer chains past one another. However, as a byproduct of this, the stiffness of the polymer film is increased.
In addition to these functions, film formation of the polymer is a necessary condition for developing good cohesive strength. If there are cracks in the polymer film, they will reduce its strength and render chemical crosslinking ineffective. For a binder to form a continuous film, the polymer particles must coalesce.
Capillary forces arising during the evaporation of the water are the driving forces for coalescence (Figure 1). The greater the surface tension of the water phase, the greater the capillary forces. Reducing the size of the capillary by reducing the particle size increases the capillary forces and enhances coalescence. In general, smaller particles will lead to better film formation.
Liquids with a high surface tension normally will not wet a substrate well. Substrates of a low surface energy are not easily wetted, and vice versa. For example, pure water, having a high surface tension, often requires the addition of a surfactant to reduce surface tension and enhance wetting. The surfactant used in the polymerization process will function in this manner.
On the substrate side, synthetic fibers such as polypropylene have a low surface energy and as a consequence are much more difficult to wet and adhere to, unlike cellulose, which has a higher surface energy. For this reason, most synthetic fibers are treated (sized) to improve wetting.
Once wetting has been achieved, specific interactions between fiber and binder will yield a strong bond. Examples of these are hydrogen bonding, Van der Waals forces or even chemical reaction. Cellulose, which has hydroxyl groups available for hydrogen bonding and possible chemical reaction, will form stronger bonds than the relatively inert surfaces of polypropylene and polyester.
It is unclear whether the optimum binder distribution consists of an even binder coverage over the whole fiber or spot welding of the web by concentrating the binder at the fiber cross-points. Many researchers, however, feel that the latter case makes most efficient use of the binder.
1. Low viscosity – therefore easy to apply;
2. High molecular weight – toughness;
3. Wide variety of binders; and
4. Versatile range of binders.
1. Ease of handling;
2. Ease of application;
3. Simple machinery required;
4. Inexpensive vehicle for the polymer; and
5. Polymer dispersions can readily be applied in combination with other additives.
Disadvantages of using aqueous-type binders include:
1. Entrainment of at least small amounts of surfactants into the end product;
2. High temperatures are required for evaporating the water vehicle;
3. Migration of polymer and additives to the surface of the substrate during drying at elevated temperatures; and
4. Environmental problems can be encountered due to
residual latex in the process-cleanup stage and from venting to atmosphere in the drying/curing process.
National Starch and Chemical Co., Bridgewater, N.J., has recently introduced a new family of EVA emulsion binders that are hydrophilic but strong when wet with water or a variety of solvents. They have low odor, do not use alkyl phenol ethoxylate as surfactants, have low levels of volatile organic compounds and formaldehyde, and do not contain a biocide.
The new binders, which differ from each other in their ethylene to vinyl acetate ratio, are water-based, surfactant-stabilized, self-linking copolymers of ethylene and vinyl acetate. They have been found to be very suitable for short-fiber, air-formed nonwovens used in such applications as hygiene and incontinence products, wipes and toweling, and table-top items. These are all areas that have enjoyed significant recent growth and where multibonding systems (thermal bonding plus latex bonding) are now commonly used.
This article is based on “Web Bonding Technologies,” which will be presented as part of the Association of Nonwoven Fabrics Industry (INDA) Nonwovens Training Course, March 15-17, 2000, in Charlotte, N.C. The course material was originally printed in INDA’s Nonwovens Training Course book. Copyright is held by INDA who acknowledges MCW Technologies as the developer of the text. For more information on the course or other INDA activities, contact the association at 919-233-1210, fax: 919-233-1282, or visit the Web site: www.inda.org.
Additional information on DUR-O-SET® EVA emulsion binders is available from National Starch and Chemical Co. Information Center, One Matrix Dr., Monroe Township, NJ 08831; phone: 800-797-4992, fax: 609-409-5699 or e-mail: firstname.lastname@example.org.