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Latex bonded nonwovens.

one of the older bonding technologies, latex bonding is now branching out into other areas; lowering formaldehyde levels remain a major issue

The use of vinyl acetate based emulsion polymers as binders in the manufacture of highloft nonwovens has had a long history. Emulsion polymer technology has continued to evolve as new binders are required to meet the changing requirements of highloft nonwovens manufacturers.

Vinyl acetate based emulsion polymers were first introduced as highloft nonwoven binders in the mid to late 1960's. Many of these early products had not been designed for highloft nonwoven end uses; the technologies were "borrowed" from the paper and paint markets. The products provided a stiff batt at low cost but were generally considered deficient in performance characteristics such as color, compression recovery, processability and durability. These predominantly noncrosslinking polymers gained only minor market acceptance, but provided the manufacturers of vinyl acetate based emulsion polymers an impetus to improve low cost binder technology by focusing on the needs of the highloft nonwoven market.

Industry demand for low cost/high performance binders for highloft nonwovens continued to grow during the early 1970's. Suppliers of vinyl acetate binders developed the first products specifically designed to meet the needs of the highloft nonwovens industry. These second generation products incorporated N-methylol acrylamide (NMA) monomer to impart self-crosslinking functionality. This class of binders using this technology is referred to as the conventional crosslinking binder. These products addressed many of the performance deficiencies of the earlier noncrosslinking products while still providing the manufacturers of highloft nonwovens significant economic advantages over acrylic based technologies.

Conventional crosslinking binder technology had been improved by the mid to late 1970's to the point of providing excellent performance in all commercially important characteristics. These third generation conventional binders, now in use for about 15 years, have met and continue to meet the requirements of many highloft nonwoven manufacturers. The performance characteristics of the first generation noncrosslinking binders as well as the second and third generation conventional crosslinking hi loft nonwoven binders are summarized in Table 1.
Table 1

Performance Industrial Rating(*)
Property Excellent V. Good/Good Fair/Poor
Stiffness 3 1,2
Recovery 3 2 1
Processability 3 2 1
Economics 1,2,3
Durability 3 2 1
Bondability 2,3 1
Water Resistance 3 2 1
Cure Rate 3 2 1

(*) 1 indicates first generation noncrosslinking, 2 indicates
second generation conventional, 3 indicates third
generation conventional

The advent of crosslinking vinyl acetate binders for highloft nonwovens required the nonwoven manufacturers to develop the process methodology to "cure" (i.e., carry out the crosslinking reaction) the binders after application to the highloft batt. Most nonwoven manufacturers at that time had little knowledge of the mechanism by which the NMA in the polymer crosslinks the binders or the conditions needed to take full advantage of the crosslinking functionality on their nonwoven lines. Many users felt that a dry, firm-hand batt would perform well, although it is now known that the temperature reached by a batt during the drying step is the critical parameter for a full, durable cure.

The manufacturers of crosslinking vinyl acetate binders worked with the nonwoven manufacturers to define the conditions needed for proper cure/crosslinking, to provide chemical analysis of binders and to perform physical analysis of finished nonwoven products. Highloft nonwoven manufacturers now routinely monitor in-process the degree of cure on their batting, the binder add-on, binder distribution and other critical parameters.

Understanding Formaldehyde Levels

The NMA monomer from the crosslinking is actually an equilibrium composition resulting from the addition reaction of acrylamide to formaldehyde. This equilibrium results in low levels of free formaldehyde being present in the emulsion polymers containing NMA as the crosslinking functionality. Formaldehyde may also be released during the crosslinking reaction that creates the methylene bridge crosslinks. Finally, there are trace quantities of formaldehyde present in highloft nonwoven webs prepared with NMA crosslinking emulsion polymer binders. Thus, the formaldehyde content of a binder for highloft nonwovens must be considered in three aspects: latex formaldehyde content, formaldehyde emissions upon cure and final web formaldehyde content.

Web formaldehyde values obtained in the manufacture of highloft nonwovens will be impacted by application, drying and curing conditions. For example, higher binder add-on may be expected to result in higher web formaldehyde values. It should be noted that the emitted formaldehyde analytical method does not predict actual workplace formaldehyde concentrations observed in use on a nonwoven line, but gives values useful for the comparison of different emulsion polymer products. Actual workplace formaldehyde levels will be impacted by adequacy of ventilation (among other factors). Existing conventional crosslinking binders do meet workplace formaldehyde requirements for many highloft nonwoven manufacturers.

In recent years, the Occupational Safety and Health Administration (OSHA) has been reducing the permissible exposure limit (PEL) and short-term exposure limit (STEL) for formaldehyde at an accelerated pace. It is important not to confuse latex, emitted and web formaldehyde measurements (which express formaldehyde as ppm based on polymer weight or web weight) with PEL and STEL measurements (which express formaldehyde in ppm in air).

The 1971 PEL standard was 3 ppm with a 5 ppm STEL. This standard remained in effect until 1987 when OSHA revised the regulations to require a PEL standard of 1 ppm with an STEL of 2 ppm. The OSHA standard was again revised in 1992 to lower the PEL standard to 0.75 ppm while maintaining the STEL of 2 ppm. Some highloft nonwoven manufacturers may find it difficult to meet the current and future OSHA formaldehyde PEL and STEL requirements without upgrading the ventilation capabilities in their manufacturing plants. Further reductions in the OSHA formaldehyde PEL and STEL requirements might reasonably be expected to occur within the next few years.

Reduced Formaldehyde Binder Technology

Recognizing the need for high performance binders with reduced levels of formaldehyde, several binder companies began to introduce reduced formaldehyde binder technology around 1990. This technology is capable of reducing the latex formaldehyde content to approximately 20 ppm and the emitted formaldehyde level to approximately 850 ppm, a much lower latex level and a level of emitted formaldehyde comparable to conventional crosslinking vinyl acetate based emulsion binders. Web formaldehyde is essentially equal for both conventional and reduced formaldehyde binders at 10-12 ppm. It is generally recognized that insolubles of 80-85% are required for acceptable chemical cure. The ultimate goal is to provide formaldehyde levels that approach zero while maintaining high performance standards.
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Article Details
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Author:Leighton, John
Publication:Nonwovens Industry
Article Type:Cover Story
Date:Oct 1, 1993
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