REVIEW ARTICLE
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Wet-Laid Nonwovens Manufacture – Chemical Approaches Using Synthetic and Cellulosic Fibers Martin A. Hubbe,a,* and Alexander A. Koukoulas b Wet-laid forming, which can be regarded as being analogous to conventional papermaking processes but with use of chopped synthetic or staple fibers, continues to draw attention as an advantageous way to prepare advanced nonwoven textile products. This review of the literature considers scientific advances in the field, with emphasis placed on applications involving cellulosic fibers as a significant component of the product. Some primary challenges with respect to wet-laid processing concern the dispersion of the synthetic fibers in aqueous media and methods for avoiding their subsequent entanglement. Both mechanical and chemical strategies have been employed in order to achieve wellformed sheets of high uniformity and binding among the fibers to meet a variety of end-use specifications. The incorporation of cellulosic fibers has been shown to facilitate fiber dispersion and to impart certain beneficial characteristics and properties to wet-laid fabrics. The contrasting attributes of synthetic and cellulosic fibers contribute to some unique challenges during the processing of their mixtures during wet-laid forming. Keywords: Chopped synthetic fibers; Staple fibers; Dispersion; Colloidal stability; Entanglement; Cellulosic fibers; Uniformity of formation; Product performance Contact information: a: Department of Forest Biomaterials, North Carolina State University, Box 8005, Raleigh, NC 27695-8005 USA; b: ANL Consultants LLC, P.O. Box 2981, Savannah, GA 31408-2981 USA; * Corresponding author:
[email protected]
Contents of Article Introduction . . . . . . . . . . . . . . . . . . 5501 Motivations . . . . . . . . . . . . . . . . 5502 Fiber types . . . . . . . . . . . . . . . . 5503 Challenges . . . . . . . . . . . . . . . . 5505 Process overview . . . . . . . . . . . 5506 Wetting . . . . . . . . . . . . . . . . . . . . . 5507 Contact angles . . . . . . . . . . . . . 5508 Capillarity . . . . . . . . . . . . . . . . . 5509 Surfactants . . . . . . . . . . . . . . . . 5509 Foam and defoamers . . . . . . . . 5512 Dispersion of fibers . . . . . . . . . . . . 5513 Critical force concept . . . . . . . . 5514 Chemical aspects . . . . . . . . . . . 5517 Avoidance of roping . . . . . . . . . . . 5519 Crowding factor concepts . . . . 5519 Vortex effects . . . . . . . . . . . . . . 5521 Thickeners . . . . . . . . . . . . . . . . .5522 . Formation aid mechanisms. . . . 5525 Cellulose fiber effects . . . . . . . . 5529 Foam forming . . . . . . . . . . . . . . 5529
Strength development . . . . . . . . . . . . 5530 Hydroentanglement . . . . . . . . . . . 5530 Binder application to web . . . . . . . 5531 Binder types . . . . . . . . . . . . . . . . . 5532 Meltable fibers . . . . . . . . . . . . . . . 5533 Semisoluble fibers . . . . . . . . . . . . 5534 Wet-end binders . . . . . . . . . . . . . 5534 Blending with cellulosic fibers . . . 5536 Curing processes . . . . . . . . . . . . . 5538 Summary & future prospects . . . . 5538
Hubbe & Koukoulas (2016). “Wet-laid nonwovens,” BioResources 11(2), 5500-5552.
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REVIEW ARTICLE
INTRODUCTION This article reviews chemical aspects of the intriguing field of wet-laid nonwoven fabric manufacture. The wet-laid process – in its various forms – makes it possible to achieve cloth-like characteristics without weaving and without use of continuous filaments or threads. It offers high production speeds, great flexibility regarding the blending of different fiber types, and opportunities to achieve uniformity in the product. There are strong similarities between wet-laid technology and conventional papermaking. According to the Association of Nonwovens Fabrics Industry (INDA), a nonwoven product is a sheet of fibers that are bonded without weaving and without hydrogen bonding being the primary bonding mechanism (Kozak 1991). A key point of differentiation between papermaking and wetlaid nonwoven forming is that wet-laid systems employ a substantial content of fibers having lengths much greater than 3 mm, the typical length of fibers in softwood pulp (Nanko et al. 2005). Also, most nonwoven fibers are made mainly from chopped synthetic fibers or staple fibers, such as glass, polypropylene, or polyester, among others. A wide variety of cellulosic fibers are used in various wet-laid products, with particular scholarly interest having been shown for flax (Fages et al. 2012, 2013; Maity et al. 2014), cotton (Hervey et al. 1975; Farer et al. 1998), regenerated cellulose fibers (Wilke 1989; Johnson 1996; Maity et al. 2014), and wood pulp fibers (Gill et al. 1972; Schon 1977; Yang and Dean 1978a; Inagaki 1979). In wet-laid processing the fibers are initially suspended in an aqueous solution and then formed into a sheet on a moving screen, where the water is removed. To put things into perspective, it is much more common to prepare nonwoven fabric products – especially bulky nonwoven fabrics – without the use of water, using so-called “air-laid” or “dry-formed” processes; such approaches have been reviewed by others (Lichstein 1988; Turbak 1993; Russell 2007). There also have been some useful reviews of various aspects of the wet-laid nonwoven process (Fredericks 1976; Schoffmann 1980; Lichstein 1988; Turbak 1993; Williamson 1993). Table 1 lays out some typical ranges of fiber length, machine width, line speed, and basis weight for some of the most common product types. Table 1. Typical Fiber Length, Machine Parameters, and Basis Weights for Wet-Laid and Dry-Laid Production Methods (Russell 2007) Machine Parameters Fiber Length (mm)
Width (m)
Line Speed (m/min)
Typical Basis Weight (g/m2)
2000 (Perini J. 2014)
< 5.3
< 550 (Voith Paper 2008)
< 60
